ASCE 7-05 Code Master

CMSeismicNoBullets.qxp

STEP 7

3/19/2008

8:55 AM

Page 1

STEP 8

DETERMINE SEISMIC BASE SHEAR, V

The following seismic base shear equation is given in ASCE 7-05 Section 12.8.1[2003 NEHRP Provisions Section 5.2.1]: V = CsW where Cs is the seismic response coefficient

DISTRIBUTE V OVER THE HEIGHT OF THE BUILDING

ASCE 7-05 Section 12.8.3 [2003 NEHRP Provisions Section 5.2.3] describes how the seismic base shear is distributed over the height of the structure. The story forces are computed as follows: Fx = Cvx V

W is the weight of the building plus that of any contents that could, with a high degree of probability, be attached to the structure at the time of the earthquake. In addition to the obvious dead load of the structure, ASCE 705 Section 12.7.2 [2003 NEHRP Provisions Section 5.2.1] requires that the following loads be included in the effective seismic weight, W: Description

n

Areas of storage (other than 25 percent of floor live load public garages and open parking garages) Building with partitions

10 psf or actual weight, whichever is greater

Buildings with roofs designed for snow

Where flat roof snow loads are greater than 30 psf, 20 percent of the design snow load needs to be included, regardless of actual roof slope. 100 percent of operating weight

∑w h i=1

An example of this distribution is shown in the figure below. A k exponent larger than 1 places a greater proportion of the base shear in the upper stories, compared with a linear distribution produced by a k value of 1, to account for higher modes of vibration in structures having fundamental periods exceeding 0.5 seconds. For a one- to three-story building, the period is less than 0.5 second; therefore, the distribution of seismic forces will be linear. Fn

R/I

Level i

Hn

Design Base Shear, V

V=

0.5S1W R/I

, where S1 > 0.6g

V=

2

(R/I)T

V = 0.01W Period, T

V Building, n stories high

SD1TLW

TL

The period TL is given in ASCE 7-05 Figures 22-15 through 22-20 [2003 NEHRP Provisions Figures 3.3-16 through 3.3-21]. The building site needs to be located on the applicable map to determine TL, which ranges between 4 and 16 seconds, depending upon the location. The following map is the TL map for the conterminous United States:

Distribution of Seismic Forces

STEP 9

DETERMINE REDUNDANCY COEFFICIENT, ρ

The redundancy coefficient reflects the multiple load path concept – that of providing more than one alternate path for every load to travel from its point of application to the ultimate point of resistance. Just as regular structures have proven themselves to outperform irregular structures in earthquakes, structures with redundant seismic force-resisting systems have performed better than those with little or no redundancy. The redundancy coefficient is applied as necessary to increase the effect of the horizontal earthquake ground motion to compensate for the lack of structural redundancy in the seismic force-resisting system. ASCE 7-05 Section 12.3.4 [2003 NEHRP Provisions Section 4.3.3] describes how to determine the redundancy coefficient, ρ. The redundancy coefficient does not apply (meaning that it may be taken equal to 1) in SDCs A, B, and C; seismic design forces for structures assigned to these seismic design categories are therefore unaffected by the redundancy of the seismic force-resisting system.

(For areas outside the conterminous United States, visit www.skghoshassociates.com/CMSDC)

The typical one- to three-story building addressed in this CodeMaster will qualify as a short-period building and, therefore, the seismic base shear is determined by the following equation: SDS W R/I SDS is determined in Steps 1 and 3; R is determined in Step 5; I is determined in Step 6; and W is the seismic weight of the building as described in this step. V=

E = ρQE ± 0.2 S DS D { 1 424 3 Effect of horizontal earthquake ground motion

For structures assigned to SDC D, E or F, the value of the redundancy coefficient equals 1.3, unless it can be shown that one of two described conditions is met. The first condition involves showing that the removal of an individual seismic force-resisting element will not cause: (1) the remaining structure to suffer a reduction in story strength of more than 33 percent, or (2) create an extreme torsional irregularity. The second condition applies only to a structure that is regular in plan at all levels and requires that the seismic force-resisting system consists of at least two bays of seismic forceresisting perimeter framing on each side of the structure in each orthogonal direction at each story resisting more than 35 percent of the base shear.

D: Design Dead Load

Effect of vertical earthquake ground motion

The structural effects of the earthquake forces, meaning the bending moments, shear forces and axial forces caused by them, must be combined with the effects of gravity (bending moments, shear forces, axial forces caused by the dead, live, snow loads, etc.) using the design load combinations set forth in 2006 IBC Section 1605 [ASCE 7-05 Section 2.0; no corresponding section in the 2003 NEHRP Provisions]. For strength design, the two load combinations applicable in seismic design are: (2006 IBC Eq. 16-5 – Additive) (2006 IBC Eq. 16-7 – Counteractive)

2006 IBC Eq. 16-5 is the additive load combination in which gravity effects add to earthquake effects. 2006 IBC Eq. 16-7 is the counteractive load combination in which gravity effects counteract earthquake effects (the plus sign includes the minus and the minus sign governs). With incorporation of the expression for E, the above load combinations become: (1.2 + 0.2SDS)D + f1L + f2S + ρQE (0.9 - 0.2SDS)D - ρQE + 1.6H

Hi V=

T1 = SD1/SDS

Wi

SD1W (R/I)T

SDS: Determined in Steps 1 and 3

1.2D + 1.0E + f1L + f2S 0.9D + 1.0E + 1.6H

Fi

SDSW

What is E? E is the combined effect of horizontal and vertical earthquake-induced forces and is quantified by the following equation: ρ: Determined in Step 9

Cs is calculated according to one of three equations depending on the period of the structure as illustrated in the following figure (there are also minimum base shear requirements for long-period structures): V=

ASCE 7-05 Sections 12.4.2 and 12.4.3 [2003 NEHRP Provisions Sections 4.2.2.1 and 4.2.2.2] address the determination of E and Em.

k i i

For structures with T < 0.5 sec, k=1 For structures with T > 2.5 sec, k = 2 For structures with 0.5 sec < T < 2.5 sec, k can be 2 or can be determined by linear interpolation between 1 and 2.

Include in Seismic Weight

Permanent equipment

Where: C vx =

w x hkx

STEP 10 DETERMINE SEISMIC LOAD EFFECTS, E AND EM

(2006 IBC Eq. 16-5 – Additive) (2006 IBC Eq. 16-7 – Counteractive)

In other words, the consideration of vertical earthquake ground motion increases the dead load factor in the additive load combination and decreases it in the counteractive load combination. For example, consider a fully redundant structure (ρ = 1.0) located where SDS = 1.0 with a bearing wall system consisting of shear walls used for the seismic forceresisting system and f1 =1.0. If the bending moments in a shear wall cross-section due to dead loads, live loads, snow loads and horizontal earthquake forces are 200 ft-kips, 60 ft-kips, 0 ft-kips and 150 ft-kips, respectively, the design moments (required flexural strengths) by the strength design load combinations (IBC Equations 16-5 and 16-7) are: Mu = [(1.2) + (0.2)(1.0)]( 200) + 60 + (1)(150) = 490 ft-kips Mu = [(0.9) - (0.2)(1.0)](200) - (1)(150) = -10 ft-kips The shear wall needs to be reinforced to carry these bending moments at the cross-section in question. What is Em? Em is the maximum seismic load effect and is required for the design of certain elements critical to the stability of the structure. This maximum load effect generated in a building can be much greater than those due to the designlevel force. Em= Ω0QE ± 0.2SDSD Ωo is the overstrength factor and increases the design-level internal forces to represent the actual forces that may be experienced by an element as a result of the design-level ground motion. Ωo is obtained from ASCE 7-05 Table 12.2-1 [2003 NEHRP Provisions Table 4.3-1]. Em is determined using the same procedure as for determining E. Em is used in the additive and the counteractive load combinations the same way as E, except that the factored snow load effect, f2S, is typically not included in the additive combination. Because Em is a strength-level force effect, adjustments need to be made if allowable stress design is used. The allowable stresses may be increased by a factor of 1.2 in accordance with ASCE 7-05 Section12.4.3.3.

The special seismic load combinations set forth in IBC Section 1605.4 are required for such elements as collectors; columns or other elements supporting reactions from discontinuous shear walls or frames; and batter piles and their connections.

STEP 11

CHECK DRIFT CONTROL REQUIREMENTS

CodeMaster SEISMIC DESIGN

The interstory drift expected to be caused by the design earthquake is limited by the code. Some reasons for limiting drift are: 1) to control member inelastic strain, 2) to minimize differential movement demand on the seismic safety elements, and 3) to limit damage to nonstructural elements. ASCE 7-05 Section 12.12.1 contains drift control requirements [2003 NEHRP Provisions Section 4.5.1]. Drift determination is addressed in ASCE 7-05 Section 12.8.6 [2003 NEHRP Provisions Section 5.2.6]. The first step is to determine δxe, the elastically computed lateral deflection at floor level x under code-prescribed seismic forces (the design base shear, V, distributed along the height of the structure in the manner prescribed by the code). Next, the deflections, δxe, are multiplied by the deflection amplification factor, Cd, (because the actual lateral deflections will be greater under the design earthquake excitation) and divided by I in accordance with the following equation: δx = Cd δxe/ I Cd is set forth in ASCE 7-05 Table 12.2-1 (2003 NEHRP Provisions Table 4.3-1). I is in the denominator of the equation to eliminate I from the drift computation (remember that the code-prescribed seismic forces that produced δxe were originally augmented by I). It is important and necessary to do this because the drift limits of ASCE 7-05 and the 2003 NEHRP Provisions are a function of the occupancy of a structure. The drift limit for a hospital is half that for an office building on the same site.

S EISMIC D ESIGN

This CodeMaster identifies the 11 steps involved in designing a typical one- to threestory building for seismic loads in accordance with the 2006 International Building Code (IBC), ASCE 7-05 Minimum Design Loads for Buildings and Other Structures, and the 2003 NEHRP Recommended Provisions for Seismic Regulations for New Buildings and Other Structures (known as 2003 NEHRP Provisions or FEMA 450-1*). Information will be presented on how these three documents work together. The NEHRP Provisions feed directly into the ASCE 7 development process; ASCE 7 in turn serves as a primary referenced standard in the IBC. The seismic design provisions of the 2006 IBC are based on those of ASCE 7-05 and make extensive reference to that standard. In fact, almost all of the seismic design provisions are adopted through reference to ASCE 7-05. Beginning with Step 4, only references to ASCE 7-05 and the 2003 NEHRP Provisions are made. The only seismic provisions included in the text of the 2006 IBC are related to ground motion, soil parameters, and determination of Seismic Design Category (SDC), as well as definitions of terms actually used within those provisions and the four exceptions under the scoping provisions. It is important to note that where this CodeMaster provides section references from the documents, the corresponding requirements often differ from one another. In some cases, these differences are subtle and an explanation of these differences is beyond the scope of this CodeMaster.

ASCE 7-05 TABLE 12.12-1 ALLOWABLE STORY DRIFT, Δa

a, b

Occupancy Category Structure Structures, other than masonry shear wall structures, 4 stories or less with interior walls, partitions, ceilings and exterior wall systems that have been designed to accommodate the story drifts. Masonry cantilever shear wall structures

d

I or II

III

IV

c 0.025hsx 0.020hsx 0.015hsx

0.010hsx 0.010hsx 0.010hsx

Other masonry shear wall structures

0.007hsx 0.007hsx 0.007hsx

All other structures

0.020hsx 0.015hsx 0.010hsx

a,b,c,d

See ASCE 7-05 Table 12.12-1 for footnotes.

CLOSING COMMENTS This CodeMaster has presented the step-by-step process required to complete seismic design as it relates to the seismic design demands. Many other code requirements need to be addressed when completing the entire seismic design of a building. These other code requirements cover: direction of loading, deformation compatibility, P-Δ effects, detailing, structural component load effects, nonstructural components, inspections, foundations, and material specific requirements.

2003

NEHRP

ASCE

7-0 05

obtaining seismic design parameters using the same data that was used to prepare the ground motion maps published in the 2006 IBC, ASCE 7-05, and the 2003 NEHRP Provisions. By inputting the longitude and latitude of the building location, this method provides for a more accurate and reliable determination of Ss and S1. The FEMA 450 CD also contains this calculation tool.

STEP 2

DETERMINE IF STRUCTURE IS EXEMPT FROM SEISMIC REQUIREMENTS

2006 IBC Section 1613.1 allows the following four exceptions from compliance with the 2006 IBC seismic design requirement: Exception Detached one- and two- family dwellings in SDC A, B, or C or No. 1 located where Ss is less than 0.4g.

The design story drift, Δ, is computed as the difference of the deflections δx at the centers of mass of the diaphragms at the top and the bottom of the story under consideration. For structures assigned to SDC C and higher, with horizontal irregularities 1a or 1b, the design story drift, Δ, is computed as the largest difference of the deflections along any of the edges of the diaphragms at the top and the bottom of the story under consideration. This accounts for torsional effects. Once the drift is computed, it is checked against the allowable story drift set forth in ASCE 7-05 Table 12.12-1 [2003 NEHRP Provisions Table 4.5-1]. The first and the last rows of the table apply to buildings other than masonry shear wall buildings. If such buildings are more than four stories tall, the last row applies. If, however, such buildings are four stories or less in height, the designer has a choice between two drift limits: (1) where nonstructural elements have been designed to accommodate the story drift (less stringent) and (2) all other structures (more stringent). This is consistent with the intent of the drift limit, which is to limit damage to drift-sensitive nonstructural elements.

2006 IBC

Areas of U.S. with Ss < 0.4 g (Shown in green) For areas outside the conterminous United States, visit www.skghoshassociates.com/CMSDC

At this stage, the SDC has not been determined; however, Ss has been determined in Step 1. After Step 3 is completed, this exception may be revisited. 2003 NEHRP Provisions

ASCE 7–05

2006 IBC

* The 2003 NEHRP Provisions (FEMA 450-1) is a resource document funded and published by the Federal Emergency Management Agency (FEMA). It is intended to capture research results and lessons learned and may contain information beyond that found in ASCE 7-05 or the IBC. The accompanying Commentary (FEMA 450-2) may assist the user in understanding the basis for code requirements. Copies of the 2003 NEHRP Provisions and the accompanying Commentary may be viewed or downloaded on the Building Seismic Safety Council's (BSSC) website: www.bssconline.org. The 2003 NEHRP Provisions also includes a CD that contains the two documents as well as the seismic design maps and a program to determine the mapped seismic design values. Hard copies or the CD may be obtained free-of-charge by contacting the FEMA Publication Distribution Facility at 1-800-480-2520.

STEP 1

Conventional light-frame wood construction complying with 2006 Exception IBC Section 2308 (see definition for "conventional light-frame wood No. 2 construction" in 2006 IBC Section 2302). Agricultural storage structures intended for incidental human Exception occupancy only (see definition for "agricultural building" in 2006 IBC No. 3 Section 202). Vehicular bridges, electrical transmission towers, hydraulic Exception structures, buried utility lines and their appurtenances, nuclear No. 4 reactors and other similarly described structures in the code. 2006 IBC Section 1613.5.1

Structures located in areas with Ss < 0.15g and S1 < 0.04g need only comply with SDC A requirements.

DETERMINE SS AND S1

The first step in seismic design is determining the mapped maximum considered earthquake (MCE) spectral response accelerations at short periods, Ss, and at 1second period, S1. These values can be determined using one of two methods: 1.

2006 IBC Figures 1613.5(1) through 1613.5(14) [ASCE 7-05 Figures 22-1 through 22-20; 2003 NEHRP Provisions Figures 3.3-1 through 3.3-14], or

2.

USGS website at http://earthquake.usgs.gov/research/hazmaps/. The U.S. Geological Survey (USGS) has prepared an Internet calculation tool for

Areas of U.S. with Ss < 0.15g and S1 < 0.04g (shown in green) For areas outside the conterminous United States, visit www.skghoshassociates.com/CMSDC

CodeMaster developed by:

S C I

Structures & Codes Institute

A subsidiary of S.K. Ghosh Associates Inc. www.skghoshassociates.com ISBN 978-0-9793084-1-3

Tel: (847) 991-2700 Fax: (847) 991-2702 [email protected]

Similar exceptions are found in ASCE 7-05 Sections 11.1.2 and 11.4.1 and 2003 NEHRP Provisions Section 1.1.2.1.

CMSeismicNoBullets.qxp

STEP 3

3/19/2008

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Page 2

DETERMINE SEISMIC DESIGN REQUIREMENTS (SDC)

The SDC assigned to a building is a classification based on its occupancy or use and the level of expected soil-modified seismic ground motion at its site. In order to determine the SDC, the following items first need to be determined: 1. Soil Classification. The soil needs to be classified as Site Class A, B, C, D, E, or F in accordance with 2006 IBC Section 1613.5.2 and Table 1613.5.2 [ASCE 7-05 Sections 11.4.2, 20.1, 20.3 and 20.4; 2003 NEHRP Provisions Section 3.5]. Site class definitions are dependent on soil parameters such as shear wave velocity, standard penetration resistance, undrained shear strength, and soil profile descriptions.

SDC if certain conditions are met. The conditions that a structure must satisfy for this relaxation to be applicable are: • S1 < 0.75g at site of structure. • Ta < 0.8Ts where Ta is the approximate fundamental period of the structure and Ts = SD1/SDS. • Upper-bound design base shear is used in design. • T used to calculate story drift < Ts. • Diaphragms are rigid, or for diaphragms that are flexible, vertical elements of seismic force-resisting system spaced at < 40 ft.

3.

SDS and SD1. SDS is the 5-percent-damped design spectral response acceleration at short periods and is calculated as follows: SDS = (2/3)(Fa)(Ss). The Fa value is obtained from 2006 IBC Table 1613.5.3(1) [ASCE 7-05 Table 11.4-1; 2003 NEHRP Provisions Table 3.3-1] and is a function of the site class and Ss. SD1 is the 5-percent-damped design spectral response acceleration at 1second period and is calculated as follows: SD1 = (2/3)(Fv)(S1). The Fv value is obtained from 2006 IBC Table 1613.5.3(2) [ASCE 7-05 Table 11.4-2; 2003 NEHRP Provisions Table 3.3-2] and is a function of the site class and S1. Occupancy Category. Occupancy Category is a term used to describe the category of structures based on occupancy or use. Use 2006 IBC Table 1604.5 to determine the Occupancy Category [ASCE 7-05 Table 1-1; 2003 NEHRP Provisions uses Seismic Use Group in accordance with Section 1.2]. The following table summarizes Occupancy Category assignments:

Occupancy Category

Nature of Category

I

Occupancy Category I is assigned to agricultural facilities, temporary facilities and minor storage facilities that represent a low hazard to human life in the event of failure.

II

Occupancy Category II is assigned to most buildings; it is assigned to buildings not otherwise classified as Occupancy Category I, III, or IV.

III

IV

Occupancy Category III is for buildings with large numbers of persons such as: • Schools with more than 250 students, • Assembly uses with more than 300 people, and • Buildings with total occupancy greater than 5000 people. Occupancy Category III is also assigned to: • Nonessential utility facilities, and • Jails and detention facilities. Occupancy Category IV includes hospitals and acute care facilities; fire, police and emergency response stations; structures containing highly toxic materials; aviation control towers; and utilities required for essential facilities.

Once SDS, SD1 and the Occupancy Category have been determined, 2006 IBC Tables 1613.5.6(1) and 1613.5.6(2) should be used for the SDC determination [ASCE 7-05 Tables 11.6-1 and 11.6-2; 2003 NEHRP Provisions Tables 1.4-1 and 1.4-2], unless the structure is located where S1 > 0.75g. If that is the case, Occupancy Category I, II or III structures are assigned to SDC E, and Occupancy Category IV structures are assigned to SDC F. Although 2006 IBC Section 1613.5.6 [ASCE 7-05 Section 11.6 and 2003 NEHRP Provisions Section 1.4.1] indicates that the building is to be assigned the more severe SDC in accordance with the two tables, there is an exception in this section that allows only Table 1613.5.6(1) to be used to determine

The fundamental period, T, of a building may be taken equal to Ta, as given in ASCE 7-05 Section 12.8.2.1 [2003 NEHRP Provisions Section 5.2.2.1]: Ta = Cthxn where hn is the height in feet above the base to the highest level of the structure and the parameters Ct and x are determined from ASCE 7-05 Table 12.8-2 [2003 NEHRP Provisions Table 5.2-2]. Note: For a three-story building with hn equal to 30 feet, depending on the structural system, the approximate period can vary from 0.26 second to 0.43 second.

SECRETS OF THE CODEMASTER: 2006 IBC Section 1613.5.2 [ASCE 7-05 Section 20.1; 2003 NEHRP Provisions Section 3.5] makes the following allowance for situations where soil properties are not known: When the soil properties are not known in sufficient detail to determine the site class, Site Class D can be used unless the building official determines that Site Class E or F soil is likely to be present at the site. ASCE 7-05 Section 20.1 includes the following statement: Where site-specific data are not available to a depth of 100 feet, appropriate soil properties are permitted to be estimated by the registered design professional preparing the soils report based on known geologic conditions. 2.

How to Determine the Fundamental Period, T, of a Building

Once the design using base shear computed from T=Ta has progressed to a certain stage, the value of the fundamental period may be refined through rational analysis. However, the rationally computed T is still limited (except in drift computations) to no more than CuTa, where Cu is a coefficient given in ASCE 7-05 Table 12.8-1 [2003 NEHRP Provisions Table 5.2-1].

Areas of U.S. with S1 > 0.75g (shown in red) For areas outside the conterminous United States, visit www.skghoshassociates.com/CMSDC

Once the SDC is determined, it is important to understand the impact such a classification has on the seismic design of the building. If a building is assigned SDC A, this means that the building has a minimal seismic vulnerability. All of the design requirements applicable to such a building are found in ASCE 7-05 Section 11.7 [2003 NEHRP Provisions Section 1.5].

How to Determine Ts

Ts is the period at which the flat-top portion of the response spectrum transitions to the descending (period-dependent) branch. Ts is shown in ASCE 7-05 Figure 11.4-1 [2003 NEHRP Provisions Figure 3.3-15] and is illustrated as follows:

TORSIONAL IRREGULARITY

Irregular structures with T < 3.5Ts and having D, E, F horizontal irregularities Type 2, 3, 4, or 5 of ASCE 7-05 Table 12.3-1 or vertical irregularities Type 4 or 5 of ASCE 7-05 Table 12.3-2. All other structures P indicates permitted; NP indicates not permitted.

2

⎛ > 1.2 ⎜⎜ ⎜ ⎝

Δ +Δ 1

2

2

⎞ ⎟ ⎟⎟ ⎠

• Torsional irregularity is to be considered only when diaphragms are not flexible.

Δ2

Δ1

• Extreme torsional irregularity exists when

Δ

EXTREME TORSIONAL IRREGULARITY

Horizontal Irregularity Type 2:

Δ +Δ 1

2

2

⎞ ⎟ ⎟⎟ ⎠

Re-entrant corner irregularity exists when both projection b > 0.15a and projection d > 0.15.c.

a b

REENTRANT CORNER

2

⎛ > 1.4 ⎜⎜ ⎜ ⎝

• Extreme torsional irregularity is to be considered only when diaphragms are not flexible.

re-entrant corner d

c

Equivalent Lateral Force Procedure P

Dynamic Analysis Procedure P

P

P

How to Determine if Building is Irregular?

What is important to note is that if a building is SDC D, E or F and has a T > 3.5 Ts, it must be designed using a dynamic analysis procedure. Also, if a building meets all of the following conditions, it must be designed using a dynamic analysis procedure:

b

P

NP

P

Extreme soft story

• Contains one of the following irregularities: horizontal irregularity type 1a or 1b or vertical irregularity type 1a, 1b, 2 or 3.

Extreme soft story irregularity exists when soft story stiffness < 60% story stiffness above or < 70% of the average stiffness of 3 stories above.

The R-value represents a relative rating of the ability of a structural system to resist severe earthquake ground motion without collapse. It is also the reduction in seismic force demand in proportion to the perceived ductility of a given structural system (ductility is the ability of a structure to continue to carry gravity loads as it deforms laterally beyond the stage of elastic response). The following table illustrates the different types of seismic force-resisting systems addressed in ASCE 7-05 Table 12.2-1, which sets forth the R-values [2003 NEHRP Provisions Table 4.3-1].

Out-of-plane offset irregularity exists when there are discontinuities in the vertical elements of the lateral forceresisting system.

Horizontal Irregularity Type 5:

Nonparallel systems irregularity exists where the vertical lateral force-resisting elements are not parallel to or symmetric about the major orthogonal axes of the seismic force-resisting system.

NONPARALLEL SYSTEMS

Vertical Irregularity Type 2:

Vertical Irregularity Type 1a: Stiff resisting elements

a

b > 1.3a

Vertical Irregularity Type 4:

Vertical Irregularity Type 5a:

Vertical geometric irregularity exists when horizontal dimension of lateral-force-resisting system in story > 130% of that in adjacent story.

Soft story irregularity exists when soft story stiffness < 70% story stiffness above or < 80% of the average stiffness of 3 stories above.

a

a

a

In-plane discontinuity in Stiff resisting vertical lateralforce-resisting elements elements exists In plane when the in-plane offset = 2a. offset is greater Length of than the lengths of lateralthose elements or forcethere exists a resisting reduction in element = a stiffness of resisting elements in the story below.

Stiff resisting elements Weak story

Weak story irregularity exists when the story lateral strength < 80% lateral strength of story above.

Vertical Irregularity Type 5b: EXTREME WEAK STORY

soft story

Weight irregularity exists when story mass > 150% adjacent story mass (a roof that is lighter than the floor below need not be considered).

Vertical Irregularity Type 3:

VERTICAL STRUCTURAL IRREGULARITIES

SOFT STORY

Lateral Forces

WEIGHT (MASS) IRREGULARITY

VERTICAL DISCONTINUITY IN VERTICAL LATERALFORCE RESISTING ELEMENTS

Lateral Forces

Gravity Loads

Heavy mass Stiff Resisting Elements (Shear Walls or Braced Frames) Bearing Wall System

Stiff resisting elements Weak story

Extreme weak story irregularity exists when the story lateral strength < 65% lateral strength of story above.

Further discussion of the simplified design procedure and discussion of the dynamic analysis procedures are beyond the scope of this CodeMaster. The equivalent lateral force procedure is discussed in the following steps.

Stiff Resisting Elements (Shear Walls or Braced Frames) Building Wall System Gravity Loads

Gravity Loads

(supported by frames)

Lateral Forces

Lateral Forces

Moment-Resisting Frame System

b a

• SDC D, E or F, and • Not of light-frame construction, and

P

EXTREME SOFT STORY

WEAK STORY ASCE 7-05 Tables 12.3-1 and 12.3-2 define the different horizontal and vertical structural irregularities [2003 NEHRP Provisions Tables 4.3-2 and 4.3-3].

Stiff resisting elements

VERTICAL GEOMETRIC IRREGULARITY

opening

OUT-OF-PLANE OFFSETS

A typical value of Ts is 0.5 second. For any building one to three stories in height, T will always be less than 3.5 Ts. It is not until a building is in the 17- to 20-story height range that T may be greater than 3.5 Ts.

STEP 5 DETERMINE R, RESPONSE MODIFICATION COEFFICIENT

Vertical Irregularity Type 1b:

Gravity Loads

Horizontal Irregularity Type 1b:

Horizontal Irregularity Type 4:

ASCE 7-05 Table 12.6-1 (Summarized)

All structures Regular structures with T < 3.5 Ts and all structures of light-frame construction

• Torsional irregularity exists when

Diaphragm discontinuity exists when area of opening > 0.5(a)(b) or effective diaphragm stiffness changes more than 50% from one story to the next.

Permissible analysis procedures for buildings not qualifying for the simplified design procedure are set forth in ASCE 7-05 Section 12.6 [2003 NEHRP Provisions Section 4.4.1]. The following table summarizes the permissible analysis procedures. In order to use this table, a building's fundamental period needs to be determined, as does Ts, and whether or not it is regular or irregular – all of which are explained below.

Structural Characteristics

Δ1

Δ

DETERMINE ANALYSIS PROCEDURES

The simplified design procedure is in stand-alone ASCE 7-05 Section 12.14 (2003 NEHRP Provisions Alternative Simplified Chapter 4). It is a conservative method of determining design forces for certain simple buildings. It is optional for these simple buildings, but it should be kept in mind that the design forces will be higher than those calculated using one of the other two methods. The procedure is limited in its applicability to simple and redundant Occupancy Category I and II structures not exceeding 3 stories where the seismic force-resisting elements are arranged in a torsion-resistant, regular layout. Furthermore, only bearing wall and building frame systems qualify to use the procedure. See ASCE 7-05 Section 12.14.1.1 for 12 limitations that must be met in order for the simplified design procedure to be used [2003 NEHRP Provisions Section Alt. 4.1.1].

B, C

Δ2

DIAPHRAGM DISCONTINUITY

Three types of analysis procedures can be used in the seismic design of a building according to ASCE 7-05: 1) simplified design procedure, 2) equivalent lateral force procedure, and 3) dynamic analysis procedure.

SDC

Horizontal Irregularity Type 1a:

Horizontal Irregularity Type 3:

Each subsequent SDC letter assignment (B through F) means an increase in seismic performance requirements. Among other code requirements, the SDC establishes permissible structural systems, height limits, restrictions on irregular buildings, permitted analysis procedures, detailing requirements, and requirements for nonstructural components.

STEP 4

HORIZONTAL STRUCTURAL IRREGULARITIES

Gravity Loads

Lateral Forces

Stiff Resisting Elements (Shear Walls or Braced Frames) Dual Systems with Moment Frames (Moment frames resist at least 25% of the design seismic forces) Gravity Loads

Stiff Resisting Elements (Shear Walls)

Lateral Forces

Ordinary Moment Frame

Shear Wall-Frame Interactive System

Cantilevered Column System A system in which lateral forces are resisted entirely by columns acting as cantilevers from the base

The following table provides sections indicating how to determine R-values for different combinations. Combination Description

ASCE 7-05

2003 NEHRP

Framing Systems in Different Directions

Section 12.2.2

Section 4.3.1.2.1

Framing Systems in Same Horizontal Direction

Section 12.2.3

Section 4.3.1.2.1

Vertical Combinations of Framing Systems

Section 12.2.3.1

Section 4.3.1.2.1

Horizontal Combinations of Framing Systems

Section 12.2.3.2

Section 4.3.1.2.2

STEP 6

DETERMINE SEISMIC IMPORTANCE FACTOR, I

The seismic importance factor represents an attempt to control the seismic performance capabilities of buildings in different occupancy categories. The importance factor modifies the minimum base shear forces and reflects the relative importance assigned to the occupancy during and following an earthquake. The seismic importance factor is related to the Occupancy Category. An Occupancy Category I or II structure is assigned I = 1.0; an Occupancy Category III structure is assigned I = 1.25; and an Occupancy Category IV structure is assigned I = 1.5. As will be seen in Step 7, I = 1.25 results in increasing the design seismic force by 25 percent and I = 1.50 results in increasing the design seismic force by 50 percent. (See ASCE 7-05 Table 11.5-1 and 2003 NEHRP Provisions Table 1.3-1 for importance factor assignments).

CMSeismicNoBullets.qxp

STEP 3

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8:55 AM

Page 2

DETERMINE SEISMIC DESIGN REQUIREMENTS (SDC)

The SDC assigned to a building is a classification based on its occupancy or use and the level of expected soil-modified seismic ground motion at its site. In order to determine the SDC, the following items first need to be determined: 1. Soil Classification. The soil needs to be classified as Site Class A, B, C, D, E, or F in accordance with 2006 IBC Section 1613.5.2 and Table 1613.5.2 [ASCE 7-05 Sections 11.4.2, 20.1, 20.3 and 20.4; 2003 NEHRP Provisions Section 3.5]. Site class definitions are dependent on soil parameters such as shear wave velocity, standard penetration resistance, undrained shear strength, and soil profile descriptions.

SDC if certain conditions are met. The conditions that a structure must satisfy for this relaxation to be applicable are: • S1 < 0.75g at site of structure. • Ta < 0.8Ts where Ta is the approximate fundamental period of the structure and Ts = SD1/SDS. • Upper-bound design base shear is used in design. • T used to calculate story drift < Ts. • Diaphragms are rigid, or for diaphragms that are flexible, vertical elements of seismic force-resisting system spaced at < 40 ft.

3.

SDS and SD1. SDS is the 5-percent-damped design spectral response acceleration at short periods and is calculated as follows: SDS = (2/3)(Fa)(Ss). The Fa value is obtained from 2006 IBC Table 1613.5.3(1) [ASCE 7-05 Table 11.4-1; 2003 NEHRP Provisions Table 3.3-1] and is a function of the site class and Ss. SD1 is the 5-percent-damped design spectral response acceleration at 1second period and is calculated as follows: SD1 = (2/3)(Fv)(S1). The Fv value is obtained from 2006 IBC Table 1613.5.3(2) [ASCE 7-05 Table 11.4-2; 2003 NEHRP Provisions Table 3.3-2] and is a function of the site class and S1. Occupancy Category. Occupancy Category is a term used to describe the category of structures based on occupancy or use. Use 2006 IBC Table 1604.5 to determine the Occupancy Category [ASCE 7-05 Table 1-1; 2003 NEHRP Provisions uses Seismic Use Group in accordance with Section 1.2]. The following table summarizes Occupancy Category assignments:

Occupancy Category

Nature of Category

I

Occupancy Category I is assigned to agricultural facilities, temporary facilities and minor storage facilities that represent a low hazard to human life in the event of failure.

II

Occupancy Category II is assigned to most buildings; it is assigned to buildings not otherwise classified as Occupancy Category I, III, or IV.

III

IV

Occupancy Category III is for buildings with large numbers of persons such as: • Schools with more than 250 students, • Assembly uses with more than 300 people, and • Buildings with total occupancy greater than 5000 people. Occupancy Category III is also assigned to: • Nonessential utility facilities, and • Jails and detention facilities. Occupancy Category IV includes hospitals and acute care facilities; fire, police and emergency response stations; structures containing highly toxic materials; aviation control towers; and utilities required for essential facilities.

Once SDS, SD1 and the Occupancy Category have been determined, 2006 IBC Tables 1613.5.6(1) and 1613.5.6(2) should be used for the SDC determination [ASCE 7-05 Tables 11.6-1 and 11.6-2; 2003 NEHRP Provisions Tables 1.4-1 and 1.4-2], unless the structure is located where S1 > 0.75g. If that is the case, Occupancy Category I, II or III structures are assigned to SDC E, and Occupancy Category IV structures are assigned to SDC F. Although 2006 IBC Section 1613.5.6 [ASCE 7-05 Section 11.6 and 2003 NEHRP Provisions Section 1.4.1] indicates that the building is to be assigned the more severe SDC in accordance with the two tables, there is an exception in this section that allows only Table 1613.5.6(1) to be used to determine

The fundamental period, T, of a building may be taken equal to Ta, as given in ASCE 7-05 Section 12.8.2.1 [2003 NEHRP Provisions Section 5.2.2.1]: Ta = Cthxn where hn is the height in feet above the base to the highest level of the structure and the parameters Ct and x are determined from ASCE 7-05 Table 12.8-2 [2003 NEHRP Provisions Table 5.2-2]. Note: For a three-story building with hn equal to 30 feet, depending on the structural system, the approximate period can vary from 0.26 second to 0.43 second.

SECRETS OF THE CODEMASTER: 2006 IBC Section 1613.5.2 [ASCE 7-05 Section 20.1; 2003 NEHRP Provisions Section 3.5] makes the following allowance for situations where soil properties are not known: When the soil properties are not known in sufficient detail to determine the site class, Site Class D can be used unless the building official determines that Site Class E or F soil is likely to be present at the site. ASCE 7-05 Section 20.1 includes the following statement: Where site-specific data are not available to a depth of 100 feet, appropriate soil properties are permitted to be estimated by the registered design professional preparing the soils report based on known geologic conditions. 2.

How to Determine the Fundamental Period, T, of a Building

Once the design using base shear computed from T=Ta has progressed to a certain stage, the value of the fundamental period may be refined through rational analysis. However, the rationally computed T is still limited (except in drift computations) to no more than CuTa, where Cu is a coefficient given in ASCE 7-05 Table 12.8-1 [2003 NEHRP Provisions Table 5.2-1].

Areas of U.S. with S1 > 0.75g (shown in red) For areas outside the conterminous United States, visit www.skghoshassociates.com/CMSDC

Once the SDC is determined, it is important to understand the impact such a classification has on the seismic design of the building. If a building is assigned SDC A, this means that the building has a minimal seismic vulnerability. All of the design requirements applicable to such a building are found in ASCE 7-05 Section 11.7 [2003 NEHRP Provisions Section 1.5].

How to Determine Ts

Ts is the period at which the flat-top portion of the response spectrum transitions to the descending (period-dependent) branch. Ts is shown in ASCE 7-05 Figure 11.4-1 [2003 NEHRP Provisions Figure 3.3-15] and is illustrated as follows:

TORSIONAL IRREGULARITY

Irregular structures with T < 3.5Ts and having D, E, F horizontal irregularities Type 2, 3, 4, or 5 of ASCE 7-05 Table 12.3-1 or vertical irregularities Type 4 or 5 of ASCE 7-05 Table 12.3-2. All other structures P indicates permitted; NP indicates not permitted.

2

⎛ > 1.2 ⎜⎜ ⎜ ⎝

Δ +Δ 1

2

2

⎞ ⎟ ⎟⎟ ⎠

• Torsional irregularity is to be considered only when diaphragms are not flexible.

Δ2

Δ1

• Extreme torsional irregularity exists when

Δ

EXTREME TORSIONAL IRREGULARITY

Horizontal Irregularity Type 2:

Δ +Δ 1

2

2

⎞ ⎟ ⎟⎟ ⎠

Re-entrant corner irregularity exists when both projection b > 0.15a and projection d > 0.15.c.

a b

REENTRANT CORNER

2

⎛ > 1.4 ⎜⎜ ⎜ ⎝

• Extreme torsional irregularity is to be considered only when diaphragms are not flexible.

re-entrant corner d

c

Equivalent Lateral Force Procedure P

Dynamic Analysis Procedure P

P

P

How to Determine if Building is Irregular?

What is important to note is that if a building is SDC D, E or F and has a T > 3.5 Ts, it must be designed using a dynamic analysis procedure. Also, if a building meets all of the following conditions, it must be designed using a dynamic analysis procedure:

b

P

NP

P

Extreme soft story

• Contains one of the following irregularities: horizontal irregularity type 1a or 1b or vertical irregularity type 1a, 1b, 2 or 3.

Extreme soft story irregularity exists when soft story stiffness < 60% story stiffness above or < 70% of the average stiffness of 3 stories above.

The R-value represents a relative rating of the ability of a structural system to resist severe earthquake ground motion without collapse. It is also the reduction in seismic force demand in proportion to the perceived ductility of a given structural system (ductility is the ability of a structure to continue to carry gravity loads as it deforms laterally beyond the stage of elastic response). The following table illustrates the different types of seismic force-resisting systems addressed in ASCE 7-05 Table 12.2-1, which sets forth the R-values [2003 NEHRP Provisions Table 4.3-1].

Out-of-plane offset irregularity exists when there are discontinuities in the vertical elements of the lateral forceresisting system.

Horizontal Irregularity Type 5:

Nonparallel systems irregularity exists where the vertical lateral force-resisting elements are not parallel to or symmetric about the major orthogonal axes of the seismic force-resisting system.

NONPARALLEL SYSTEMS

Vertical Irregularity Type 2:

Vertical Irregularity Type 1a: Stiff resisting elements

a

b > 1.3a

Vertical Irregularity Type 4:

Vertical Irregularity Type 5a:

Vertical geometric irregularity exists when horizontal dimension of lateral-force-resisting system in story > 130% of that in adjacent story.

Soft story irregularity exists when soft story stiffness < 70% story stiffness above or < 80% of the average stiffness of 3 stories above.

a

a

a

In-plane discontinuity in Stiff resisting vertical lateralforce-resisting elements elements exists In plane when the in-plane offset = 2a. offset is greater Length of than the lengths of lateralthose elements or forcethere exists a resisting reduction in element = a stiffness of resisting elements in the story below.

Stiff resisting elements Weak story

Weak story irregularity exists when the story lateral strength < 80% lateral strength of story above.

Vertical Irregularity Type 5b: EXTREME WEAK STORY

soft story

Weight irregularity exists when story mass > 150% adjacent story mass (a roof that is lighter than the floor below need not be considered).

Vertical Irregularity Type 3:

VERTICAL STRUCTURAL IRREGULARITIES

SOFT STORY

Lateral Forces

WEIGHT (MASS) IRREGULARITY

VERTICAL DISCONTINUITY IN VERTICAL LATERALFORCE RESISTING ELEMENTS

Lateral Forces

Gravity Loads

Heavy mass Stiff Resisting Elements (Shear Walls or Braced Frames) Bearing Wall System

Stiff resisting elements Weak story

Extreme weak story irregularity exists when the story lateral strength < 65% lateral strength of story above.

Further discussion of the simplified design procedure and discussion of the dynamic analysis procedures are beyond the scope of this CodeMaster. The equivalent lateral force procedure is discussed in the following steps.

Stiff Resisting Elements (Shear Walls or Braced Frames) Building Wall System Gravity Loads

Gravity Loads

(supported by frames)

Lateral Forces

Lateral Forces

Moment-Resisting Frame System

b a

• SDC D, E or F, and • Not of light-frame construction, and

P

EXTREME SOFT STORY

WEAK STORY ASCE 7-05 Tables 12.3-1 and 12.3-2 define the different horizontal and vertical structural irregularities [2003 NEHRP Provisions Tables 4.3-2 and 4.3-3].

Stiff resisting elements

VERTICAL GEOMETRIC IRREGULARITY

opening

OUT-OF-PLANE OFFSETS

A typical value of Ts is 0.5 second. For any building one to three stories in height, T will always be less than 3.5 Ts. It is not until a building is in the 17- to 20-story height range that T may be greater than 3.5 Ts.

STEP 5 DETERMINE R, RESPONSE MODIFICATION COEFFICIENT

Vertical Irregularity Type 1b:

Gravity Loads

Horizontal Irregularity Type 1b:

Horizontal Irregularity Type 4:

ASCE 7-05 Table 12.6-1 (Summarized)

All structures Regular structures with T < 3.5 Ts and all structures of light-frame construction

• Torsional irregularity exists when

Diaphragm discontinuity exists when area of opening > 0.5(a)(b) or effective diaphragm stiffness changes more than 50% from one story to the next.

Permissible analysis procedures for buildings not qualifying for the simplified design procedure are set forth in ASCE 7-05 Section 12.6 [2003 NEHRP Provisions Section 4.4.1]. The following table summarizes the permissible analysis procedures. In order to use this table, a building's fundamental period needs to be determined, as does Ts, and whether or not it is regular or irregular – all of which are explained below.

Structural Characteristics

Δ1

Δ

DETERMINE ANALYSIS PROCEDURES

The simplified design procedure is in stand-alone ASCE 7-05 Section 12.14 (2003 NEHRP Provisions Alternative Simplified Chapter 4). It is a conservative method of determining design forces for certain simple buildings. It is optional for these simple buildings, but it should be kept in mind that the design forces will be higher than those calculated using one of the other two methods. The procedure is limited in its applicability to simple and redundant Occupancy Category I and II structures not exceeding 3 stories where the seismic force-resisting elements are arranged in a torsion-resistant, regular layout. Furthermore, only bearing wall and building frame systems qualify to use the procedure. See ASCE 7-05 Section 12.14.1.1 for 12 limitations that must be met in order for the simplified design procedure to be used [2003 NEHRP Provisions Section Alt. 4.1.1].

B, C

Δ2

DIAPHRAGM DISCONTINUITY

Three types of analysis procedures can be used in the seismic design of a building according to ASCE 7-05: 1) simplified design procedure, 2) equivalent lateral force procedure, and 3) dynamic analysis procedure.

SDC

Horizontal Irregularity Type 1a:

Horizontal Irregularity Type 3:

Each subsequent SDC letter assignment (B through F) means an increase in seismic performance requirements. Among other code requirements, the SDC establishes permissible structural systems, height limits, restrictions on irregular buildings, permitted analysis procedures, detailing requirements, and requirements for nonstructural components.

STEP 4

HORIZONTAL STRUCTURAL IRREGULARITIES

Gravity Loads

Lateral Forces

Stiff Resisting Elements (Shear Walls or Braced Frames) Dual Systems with Moment Frames (Moment frames resist at least 25% of the design seismic forces) Gravity Loads

Stiff Resisting Elements (Shear Walls)

Lateral Forces

Ordinary Moment Frame

Shear Wall-Frame Interactive System

Cantilevered Column System A system in which lateral forces are resisted entirely by columns acting as cantilevers from the base

The following table provides sections indicating how to determine R-values for different combinations. Combination Description

ASCE 7-05

2003 NEHRP

Framing Systems in Different Directions

Section 12.2.2

Section 4.3.1.2.1

Framing Systems in Same Horizontal Direction

Section 12.2.3

Section 4.3.1.2.1

Vertical Combinations of Framing Systems

Section 12.2.3.1

Section 4.3.1.2.1

Horizontal Combinations of Framing Systems

Section 12.2.3.2

Section 4.3.1.2.2

STEP 6

DETERMINE SEISMIC IMPORTANCE FACTOR, I

The seismic importance factor represents an attempt to control the seismic performance capabilities of buildings in different occupancy categories. The importance factor modifies the minimum base shear forces and reflects the relative importance assigned to the occupancy during and following an earthquake. The seismic importance factor is related to the Occupancy Category. An Occupancy Category I or II structure is assigned I = 1.0; an Occupancy Category III structure is assigned I = 1.25; and an Occupancy Category IV structure is assigned I = 1.5. As will be seen in Step 7, I = 1.25 results in increasing the design seismic force by 25 percent and I = 1.50 results in increasing the design seismic force by 50 percent. (See ASCE 7-05 Table 11.5-1 and 2003 NEHRP Provisions Table 1.3-1 for importance factor assignments).

CMSeismicNoBullets.qxp

STEP 3

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Page 2

DETERMINE SEISMIC DESIGN REQUIREMENTS (SDC)

The SDC assigned to a building is a classification based on its occupancy or use and the level of expected soil-modified seismic ground motion at its site. In order to determine the SDC, the following items first need to be determined: 1. Soil Classification. The soil needs to be classified as Site Class A, B, C, D, E, or F in accordance with 2006 IBC Section 1613.5.2 and Table 1613.5.2 [ASCE 7-05 Sections 11.4.2, 20.1, 20.3 and 20.4; 2003 NEHRP Provisions Section 3.5]. Site class definitions are dependent on soil parameters such as shear wave velocity, standard penetration resistance, undrained shear strength, and soil profile descriptions.

SDC if certain conditions are met. The conditions that a structure must satisfy for this relaxation to be applicable are: • S1 < 0.75g at site of structure. • Ta < 0.8Ts where Ta is the approximate fundamental period of the structure and Ts = SD1/SDS. • Upper-bound design base shear is used in design. • T used to calculate story drift < Ts. • Diaphragms are rigid, or for diaphragms that are flexible, vertical elements of seismic force-resisting system spaced at < 40 ft.

3.

SDS and SD1. SDS is the 5-percent-damped design spectral response acceleration at short periods and is calculated as follows: SDS = (2/3)(Fa)(Ss). The Fa value is obtained from 2006 IBC Table 1613.5.3(1) [ASCE 7-05 Table 11.4-1; 2003 NEHRP Provisions Table 3.3-1] and is a function of the site class and Ss. SD1 is the 5-percent-damped design spectral response acceleration at 1second period and is calculated as follows: SD1 = (2/3)(Fv)(S1). The Fv value is obtained from 2006 IBC Table 1613.5.3(2) [ASCE 7-05 Table 11.4-2; 2003 NEHRP Provisions Table 3.3-2] and is a function of the site class and S1. Occupancy Category. Occupancy Category is a term used to describe the category of structures based on occupancy or use. Use 2006 IBC Table 1604.5 to determine the Occupancy Category [ASCE 7-05 Table 1-1; 2003 NEHRP Provisions uses Seismic Use Group in accordance with Section 1.2]. The following table summarizes Occupancy Category assignments:

Occupancy Category

Nature of Category

I

Occupancy Category I is assigned to agricultural facilities, temporary facilities and minor storage facilities that represent a low hazard to human life in the event of failure.

II

Occupancy Category II is assigned to most buildings; it is assigned to buildings not otherwise classified as Occupancy Category I, III, or IV.

III

IV

Occupancy Category III is for buildings with large numbers of persons such as: • Schools with more than 250 students, • Assembly uses with more than 300 people, and • Buildings with total occupancy greater than 5000 people. Occupancy Category III is also assigned to: • Nonessential utility facilities, and • Jails and detention facilities. Occupancy Category IV includes hospitals and acute care facilities; fire, police and emergency response stations; structures containing highly toxic materials; aviation control towers; and utilities required for essential facilities.

Once SDS, SD1 and the Occupancy Category have been determined, 2006 IBC Tables 1613.5.6(1) and 1613.5.6(2) should be used for the SDC determination [ASCE 7-05 Tables 11.6-1 and 11.6-2; 2003 NEHRP Provisions Tables 1.4-1 and 1.4-2], unless the structure is located where S1 > 0.75g. If that is the case, Occupancy Category I, II or III structures are assigned to SDC E, and Occupancy Category IV structures are assigned to SDC F. Although 2006 IBC Section 1613.5.6 [ASCE 7-05 Section 11.6 and 2003 NEHRP Provisions Section 1.4.1] indicates that the building is to be assigned the more severe SDC in accordance with the two tables, there is an exception in this section that allows only Table 1613.5.6(1) to be used to determine

The fundamental period, T, of a building may be taken equal to Ta, as given in ASCE 7-05 Section 12.8.2.1 [2003 NEHRP Provisions Section 5.2.2.1]: Ta = Cthxn where hn is the height in feet above the base to the highest level of the structure and the parameters Ct and x are determined from ASCE 7-05 Table 12.8-2 [2003 NEHRP Provisions Table 5.2-2]. Note: For a three-story building with hn equal to 30 feet, depending on the structural system, the approximate period can vary from 0.26 second to 0.43 second.

SECRETS OF THE CODEMASTER: 2006 IBC Section 1613.5.2 [ASCE 7-05 Section 20.1; 2003 NEHRP Provisions Section 3.5] makes the following allowance for situations where soil properties are not known: When the soil properties are not known in sufficient detail to determine the site class, Site Class D can be used unless the building official determines that Site Class E or F soil is likely to be present at the site. ASCE 7-05 Section 20.1 includes the following statement: Where site-specific data are not available to a depth of 100 feet, appropriate soil properties are permitted to be estimated by the registered design professional preparing the soils report based on known geologic conditions. 2.

How to Determine the Fundamental Period, T, of a Building

Once the design using base shear computed from T=Ta has progressed to a certain stage, the value of the fundamental period may be refined through rational analysis. However, the rationally computed T is still limited (except in drift computations) to no more than CuTa, where Cu is a coefficient given in ASCE 7-05 Table 12.8-1 [2003 NEHRP Provisions Table 5.2-1].

Areas of U.S. with S1 > 0.75g (shown in red) For areas outside the conterminous United States, visit www.skghoshassociates.com/CMSDC

Once the SDC is determined, it is important to understand the impact such a classification has on the seismic design of the building. If a building is assigned SDC A, this means that the building has a minimal seismic vulnerability. All of the design requirements applicable to such a building are found in ASCE 7-05 Section 11.7 [2003 NEHRP Provisions Section 1.5].

How to Determine Ts

Ts is the period at which the flat-top portion of the response spectrum transitions to the descending (period-dependent) branch. Ts is shown in ASCE 7-05 Figure 11.4-1 [2003 NEHRP Provisions Figure 3.3-15] and is illustrated as follows:

TORSIONAL IRREGULARITY

Irregular structures with T < 3.5Ts and having D, E, F horizontal irregularities Type 2, 3, 4, or 5 of ASCE 7-05 Table 12.3-1 or vertical irregularities Type 4 or 5 of ASCE 7-05 Table 12.3-2. All other structures P indicates permitted; NP indicates not permitted.

2

⎛ > 1.2 ⎜⎜ ⎜ ⎝

Δ +Δ 1

2

2

⎞ ⎟ ⎟⎟ ⎠

• Torsional irregularity is to be considered only when diaphragms are not flexible.

Δ2

Δ1

• Extreme torsional irregularity exists when

Δ

EXTREME TORSIONAL IRREGULARITY

Horizontal Irregularity Type 2:

Δ +Δ 1

2

2

⎞ ⎟ ⎟⎟ ⎠

Re-entrant corner irregularity exists when both projection b > 0.15a and projection d > 0.15.c.

a b

REENTRANT CORNER

2

⎛ > 1.4 ⎜⎜ ⎜ ⎝

• Extreme torsional irregularity is to be considered only when diaphragms are not flexible.

re-entrant corner d

c

Equivalent Lateral Force Procedure P

Dynamic Analysis Procedure P

P

P

How to Determine if Building is Irregular?

What is important to note is that if a building is SDC D, E or F and has a T > 3.5 Ts, it must be designed using a dynamic analysis procedure. Also, if a building meets all of the following conditions, it must be designed using a dynamic analysis procedure:

b

P

NP

P

Extreme soft story

• Contains one of the following irregularities: horizontal irregularity type 1a or 1b or vertical irregularity type 1a, 1b, 2 or 3.

Extreme soft story irregularity exists when soft story stiffness < 60% story stiffness above or < 70% of the average stiffness of 3 stories above.

The R-value represents a relative rating of the ability of a structural system to resist severe earthquake ground motion without collapse. It is also the reduction in seismic force demand in proportion to the perceived ductility of a given structural system (ductility is the ability of a structure to continue to carry gravity loads as it deforms laterally beyond the stage of elastic response). The following table illustrates the different types of seismic force-resisting systems addressed in ASCE 7-05 Table 12.2-1, which sets forth the R-values [2003 NEHRP Provisions Table 4.3-1].

Out-of-plane offset irregularity exists when there are discontinuities in the vertical elements of the lateral forceresisting system.

Horizontal Irregularity Type 5:

Nonparallel systems irregularity exists where the vertical lateral force-resisting elements are not parallel to or symmetric about the major orthogonal axes of the seismic force-resisting system.

NONPARALLEL SYSTEMS

Vertical Irregularity Type 2:

Vertical Irregularity Type 1a: Stiff resisting elements

a

b > 1.3a

Vertical Irregularity Type 4:

Vertical Irregularity Type 5a:

Vertical geometric irregularity exists when horizontal dimension of lateral-force-resisting system in story > 130% of that in adjacent story.

Soft story irregularity exists when soft story stiffness < 70% story stiffness above or < 80% of the average stiffness of 3 stories above.

a

a

a

In-plane discontinuity in Stiff resisting vertical lateralforce-resisting elements elements exists In plane when the in-plane offset = 2a. offset is greater Length of than the lengths of lateralthose elements or forcethere exists a resisting reduction in element = a stiffness of resisting elements in the story below.

Stiff resisting elements Weak story

Weak story irregularity exists when the story lateral strength < 80% lateral strength of story above.

Vertical Irregularity Type 5b: EXTREME WEAK STORY

soft story

Weight irregularity exists when story mass > 150% adjacent story mass (a roof that is lighter than the floor below need not be considered).

Vertical Irregularity Type 3:

VERTICAL STRUCTURAL IRREGULARITIES

SOFT STORY

Lateral Forces

WEIGHT (MASS) IRREGULARITY

VERTICAL DISCONTINUITY IN VERTICAL LATERALFORCE RESISTING ELEMENTS

Lateral Forces

Gravity Loads

Heavy mass Stiff Resisting Elements (Shear Walls or Braced Frames) Bearing Wall System

Stiff resisting elements Weak story

Extreme weak story irregularity exists when the story lateral strength < 65% lateral strength of story above.

Further discussion of the simplified design procedure and discussion of the dynamic analysis procedures are beyond the scope of this CodeMaster. The equivalent lateral force procedure is discussed in the following steps.

Stiff Resisting Elements (Shear Walls or Braced Frames) Building Wall System Gravity Loads

Gravity Loads

(supported by frames)

Lateral Forces

Lateral Forces

Moment-Resisting Frame System

b a

• SDC D, E or F, and • Not of light-frame construction, and

P

EXTREME SOFT STORY

WEAK STORY ASCE 7-05 Tables 12.3-1 and 12.3-2 define the different horizontal and vertical structural irregularities [2003 NEHRP Provisions Tables 4.3-2 and 4.3-3].

Stiff resisting elements

VERTICAL GEOMETRIC IRREGULARITY

opening

OUT-OF-PLANE OFFSETS

A typical value of Ts is 0.5 second. For any building one to three stories in height, T will always be less than 3.5 Ts. It is not until a building is in the 17- to 20-story height range that T may be greater than 3.5 Ts.

STEP 5 DETERMINE R, RESPONSE MODIFICATION COEFFICIENT

Vertical Irregularity Type 1b:

Gravity Loads

Horizontal Irregularity Type 1b:

Horizontal Irregularity Type 4:

ASCE 7-05 Table 12.6-1 (Summarized)

All structures Regular structures with T < 3.5 Ts and all structures of light-frame construction

• Torsional irregularity exists when

Diaphragm discontinuity exists when area of opening > 0.5(a)(b) or effective diaphragm stiffness changes more than 50% from one story to the next.

Permissible analysis procedures for buildings not qualifying for the simplified design procedure are set forth in ASCE 7-05 Section 12.6 [2003 NEHRP Provisions Section 4.4.1]. The following table summarizes the permissible analysis procedures. In order to use this table, a building's fundamental period needs to be determined, as does Ts, and whether or not it is regular or irregular – all of which are explained below.

Structural Characteristics

Δ1

Δ

DETERMINE ANALYSIS PROCEDURES

The simplified design procedure is in stand-alone ASCE 7-05 Section 12.14 (2003 NEHRP Provisions Alternative Simplified Chapter 4). It is a conservative method of determining design forces for certain simple buildings. It is optional for these simple buildings, but it should be kept in mind that the design forces will be higher than those calculated using one of the other two methods. The procedure is limited in its applicability to simple and redundant Occupancy Category I and II structures not exceeding 3 stories where the seismic force-resisting elements are arranged in a torsion-resistant, regular layout. Furthermore, only bearing wall and building frame systems qualify to use the procedure. See ASCE 7-05 Section 12.14.1.1 for 12 limitations that must be met in order for the simplified design procedure to be used [2003 NEHRP Provisions Section Alt. 4.1.1].

B, C

Δ2

DIAPHRAGM DISCONTINUITY

Three types of analysis procedures can be used in the seismic design of a building according to ASCE 7-05: 1) simplified design procedure, 2) equivalent lateral force procedure, and 3) dynamic analysis procedure.

SDC

Horizontal Irregularity Type 1a:

Horizontal Irregularity Type 3:

Each subsequent SDC letter assignment (B through F) means an increase in seismic performance requirements. Among other code requirements, the SDC establishes permissible structural systems, height limits, restrictions on irregular buildings, permitted analysis procedures, detailing requirements, and requirements for nonstructural components.

STEP 4

HORIZONTAL STRUCTURAL IRREGULARITIES

Gravity Loads

Lateral Forces

Stiff Resisting Elements (Shear Walls or Braced Frames) Dual Systems with Moment Frames (Moment frames resist at least 25% of the design seismic forces) Gravity Loads

Stiff Resisting Elements (Shear Walls)

Lateral Forces

Ordinary Moment Frame

Shear Wall-Frame Interactive System

Cantilevered Column System A system in which lateral forces are resisted entirely by columns acting as cantilevers from the base

The following table provides sections indicating how to determine R-values for different combinations. Combination Description

ASCE 7-05

2003 NEHRP

Framing Systems in Different Directions

Section 12.2.2

Section 4.3.1.2.1

Framing Systems in Same Horizontal Direction

Section 12.2.3

Section 4.3.1.2.1

Vertical Combinations of Framing Systems

Section 12.2.3.1

Section 4.3.1.2.1

Horizontal Combinations of Framing Systems

Section 12.2.3.2

Section 4.3.1.2.2

STEP 6

DETERMINE SEISMIC IMPORTANCE FACTOR, I

The seismic importance factor represents an attempt to control the seismic performance capabilities of buildings in different occupancy categories. The importance factor modifies the minimum base shear forces and reflects the relative importance assigned to the occupancy during and following an earthquake. The seismic importance factor is related to the Occupancy Category. An Occupancy Category I or II structure is assigned I = 1.0; an Occupancy Category III structure is assigned I = 1.25; and an Occupancy Category IV structure is assigned I = 1.5. As will be seen in Step 7, I = 1.25 results in increasing the design seismic force by 25 percent and I = 1.50 results in increasing the design seismic force by 50 percent. (See ASCE 7-05 Table 11.5-1 and 2003 NEHRP Provisions Table 1.3-1 for importance factor assignments).

CMSeismicNoBullets.qxp

STEP 7

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Page 1

STEP 8

DETERMINE SEISMIC BASE SHEAR, V

The following seismic base shear equation is given in ASCE 7-05 Section 12.8.1[2003 NEHRP Provisions Section 5.2.1]: V = CsW where Cs is the seismic response coefficient

DISTRIBUTE V OVER THE HEIGHT OF THE BUILDING

ASCE 7-05 Section 12.8.3 [2003 NEHRP Provisions Section 5.2.3] describes how the seismic base shear is distributed over the height of the structure. The story forces are computed as follows: Fx = Cvx V

W is the weight of the building plus that of any contents that could, with a high degree of probability, be attached to the structure at the time of the earthquake. In addition to the obvious dead load of the structure, ASCE 705 Section 12.7.2 [2003 NEHRP Provisions Section 5.2.1] requires that the following loads be included in the effective seismic weight, W: Description

n

Areas of storage (other than 25 percent of floor live load public garages and open parking garages) Building with partitions

10 psf or actual weight, whichever is greater

Buildings with roofs designed for snow

Where flat roof snow loads are greater than 30 psf, 20 percent of the design snow load needs to be included, regardless of actual roof slope. 100 percent of operating weight

∑w h i=1

An example of this distribution is shown in the figure below. A k exponent larger than 1 places a greater proportion of the base shear in the upper stories, compared with a linear distribution produced by a k value of 1, to account for higher modes of vibration in structures having fundamental periods exceeding 0.5 seconds. For a one- to three-story building, the period is less than 0.5 second; therefore, the distribution of seismic forces will be linear. Fn

R/I

Level i

Hn

Design Base Shear, V

V=

0.5S1W R/I

, where S1 > 0.6g

V=

2

(R/I)T

V = 0.01W Period, T

V Building, n stories high

SD1TLW

TL

The period TL is given in ASCE 7-05 Figures 22-15 through 22-20 [2003 NEHRP Provisions Figures 3.3-16 through 3.3-21]. The building site needs to be located on the applicable map to determine TL, which ranges between 4 and 16 seconds, depending upon the location. The following map is the TL map for the conterminous United States:

Distribution of Seismic Forces

STEP 9

DETERMINE REDUNDANCY COEFFICIENT, ρ

The redundancy coefficient reflects the multiple load path concept – that of providing more than one alternate path for every load to travel from its point of application to the ultimate point of resistance. Just as regular structures have proven themselves to outperform irregular structures in earthquakes, structures with redundant seismic force-resisting systems have performed better than those with little or no redundancy. The redundancy coefficient is applied as necessary to increase the effect of the horizontal earthquake ground motion to compensate for the lack of structural redundancy in the seismic force-resisting system. ASCE 7-05 Section 12.3.4 [2003 NEHRP Provisions Section 4.3.3] describes how to determine the redundancy coefficient, ρ. The redundancy coefficient does not apply (meaning that it may be taken equal to 1) in SDCs A, B, and C; seismic design forces for structures assigned to these seismic design categories are therefore unaffected by the redundancy of the seismic force-resisting system.

(For areas outside the conterminous United States, visit www.skghoshassociates.com/CMSDC)

The typical one- to three-story building addressed in this CodeMaster will qualify as a short-period building and, therefore, the seismic base shear is determined by the following equation: SDS W R/I SDS is determined in Steps 1 and 3; R is determined in Step 5; I is determined in Step 6; and W is the seismic weight of the building as described in this step. V=

E = ρQE ± 0.2 S DS D { 1 424 3 Effect of horizontal earthquake ground motion

For structures assigned to SDC D, E or F, the value of the redundancy coefficient equals 1.3, unless it can be shown that one of two described conditions is met. The first condition involves showing that the removal of an individual seismic force-resisting element will not cause: (1) the remaining structure to suffer a reduction in story strength of more than 33 percent, or (2) create an extreme torsional irregularity. The second condition applies only to a structure that is regular in plan at all levels and requires that the seismic force-resisting system consists of at least two bays of seismic forceresisting perimeter framing on each side of the structure in each orthogonal direction at each story resisting more than 35 percent of the base shear.

D: Design Dead Load

Effect of vertical earthquake ground motion

The structural effects of the earthquake forces, meaning the bending moments, shear forces and axial forces caused by them, must be combined with the effects of gravity (bending moments, shear forces, axial forces caused by the dead, live, snow loads, etc.) using the design load combinations set forth in 2006 IBC Section 1605 [ASCE 7-05 Section 2.0; no corresponding section in the 2003 NEHRP Provisions]. For strength design, the two load combinations applicable in seismic design are: (2006 IBC Eq. 16-5 – Additive) (2006 IBC Eq. 16-7 – Counteractive)

2006 IBC Eq. 16-5 is the additive load combination in which gravity effects add to earthquake effects. 2006 IBC Eq. 16-7 is the counteractive load combination in which gravity effects counteract earthquake effects (the plus sign includes the minus and the minus sign governs). With incorporation of the expression for E, the above load combinations become: (1.2 + 0.2SDS)D + f1L + f2S + ρQE (0.9 - 0.2SDS)D - ρQE + 1.6H

Hi V=

T1 = SD1/SDS

Wi

SD1W (R/I)T

SDS: Determined in Steps 1 and 3

1.2D + 1.0E + f1L + f2S 0.9D + 1.0E + 1.6H

Fi

SDSW

What is E? E is the combined effect of horizontal and vertical earthquake-induced forces and is quantified by the following equation: ρ: Determined in Step 9

Cs is calculated according to one of three equations depending on the period of the structure as illustrated in the following figure (there are also minimum base shear requirements for long-period structures): V=

ASCE 7-05 Sections 12.4.2 and 12.4.3 [2003 NEHRP Provisions Sections 4.2.2.1 and 4.2.2.2] address the determination of E and Em.

k i i

For structures with T < 0.5 sec, k=1 For structures with T > 2.5 sec, k = 2 For structures with 0.5 sec < T < 2.5 sec, k can be 2 or can be determined by linear interpolation between 1 and 2.

Include in Seismic Weight

Permanent equipment

Where: C vx =

w x hkx

STEP 10 DETERMINE SEISMIC LOAD EFFECTS, E AND EM

(2006 IBC Eq. 16-5 – Additive) (2006 IBC Eq. 16-7 – Counteractive)

In other words, the consideration of vertical earthquake ground motion increases the dead load factor in the additive load combination and decreases it in the counteractive load combination. For example, consider a fully redundant structure (ρ = 1.0) located where SDS = 1.0 with a bearing wall system consisting of shear walls used for the seismic forceresisting system and f1 =1.0. If the bending moments in a shear wall cross-section due to dead loads, live loads, snow loads and horizontal earthquake forces are 200 ft-kips, 60 ft-kips, 0 ft-kips and 150 ft-kips, respectively, the design moments (required flexural strengths) by the strength design load combinations (IBC Equations 16-5 and 16-7) are: Mu = [(1.2) + (0.2)(1.0)]( 200) + 60 + (1)(150) = 490 ft-kips Mu = [(0.9) - (0.2)(1.0)](200) - (1)(150) = -10 ft-kips The shear wall needs to be reinforced to carry these bending moments at the cross-section in question. What is Em? Em is the maximum seismic load effect and is required for the design of certain elements critical to the stability of the structure. This maximum load effect generated in a building can be much greater than those due to the designlevel force. Em= Ω0QE ± 0.2SDSD Ωo is the overstrength factor and increases the design-level internal forces to represent the actual forces that may be experienced by an element as a result of the design-level ground motion. Ωo is obtained from ASCE 7-05 Table 12.2-1 [2003 NEHRP Provisions Table 4.3-1]. Em is determined using the same procedure as for determining E. Em is used in the additive and the counteractive load combinations the same way as E, except that the factored snow load effect, f2S, is typically not included in the additive combination. Because Em is a strength-level force effect, adjustments need to be made if allowable stress design is used. The allowable stresses may be increased by a factor of 1.2 in accordance with ASCE 7-05 Section12.4.3.3.

The special seismic load combinations set forth in IBC Section 1605.4 are required for such elements as collectors; columns or other elements supporting reactions from discontinuous shear walls or frames; and batter piles and their connections.

STEP 11

CHECK DRIFT CONTROL REQUIREMENTS

CodeMaster SEISMIC DESIGN

The interstory drift expected to be caused by the design earthquake is limited by the code. Some reasons for limiting drift are: 1) to control member inelastic strain, 2) to minimize differential movement demand on the seismic safety elements, and 3) to limit damage to nonstructural elements. ASCE 7-05 Section 12.12.1 contains drift control requirements [2003 NEHRP Provisions Section 4.5.1]. Drift determination is addressed in ASCE 7-05 Section 12.8.6 [2003 NEHRP Provisions Section 5.2.6]. The first step is to determine δxe, the elastically computed lateral deflection at floor level x under code-prescribed seismic forces (the design base shear, V, distributed along the height of the structure in the manner prescribed by the code). Next, the deflections, δxe, are multiplied by the deflection amplification factor, Cd, (because the actual lateral deflections will be greater under the design earthquake excitation) and divided by I in accordance with the following equation: δx = Cd δxe/ I Cd is set forth in ASCE 7-05 Table 12.2-1 (2003 NEHRP Provisions Table 4.3-1). I is in the denominator of the equation to eliminate I from the drift computation (remember that the code-prescribed seismic forces that produced δxe were originally augmented by I). It is important and necessary to do this because the drift limits of ASCE 7-05 and the 2003 NEHRP Provisions are a function of the occupancy of a structure. The drift limit for a hospital is half that for an office building on the same site.

S EISMIC D ESIGN

This CodeMaster identifies the 11 steps involved in designing a typical one- to threestory building for seismic loads in accordance with the 2006 International Building Code (IBC), ASCE 7-05 Minimum Design Loads for Buildings and Other Structures, and the 2003 NEHRP Recommended Provisions for Seismic Regulations for New Buildings and Other Structures (known as 2003 NEHRP Provisions or FEMA 450-1*). Information will be presented on how these three documents work together. The NEHRP Provisions feed directly into the ASCE 7 development process; ASCE 7 in turn serves as a primary referenced standard in the IBC. The seismic design provisions of the 2006 IBC are based on those of ASCE 7-05 and make extensive reference to that standard. In fact, almost all of the seismic design provisions are adopted through reference to ASCE 7-05. Beginning with Step 4, only references to ASCE 7-05 and the 2003 NEHRP Provisions are made. The only seismic provisions included in the text of the 2006 IBC are related to ground motion, soil parameters, and determination of Seismic Design Category (SDC), as well as definitions of terms actually used within those provisions and the four exceptions under the scoping provisions. It is important to note that where this CodeMaster provides section references from the documents, the corresponding requirements often differ from one another. In some cases, these differences are subtle and an explanation of these differences is beyond the scope of this CodeMaster.

ASCE 7-05 TABLE 12.12-1 ALLOWABLE STORY DRIFT, Δa

a, b

Occupancy Category Structure Structures, other than masonry shear wall structures, 4 stories or less with interior walls, partitions, ceilings and exterior wall systems that have been designed to accommodate the story drifts. Masonry cantilever shear wall structures

d

I or II

III

IV

c 0.025hsx 0.020hsx 0.015hsx

0.010hsx 0.010hsx 0.010hsx

Other masonry shear wall structures

0.007hsx 0.007hsx 0.007hsx

All other structures

0.020hsx 0.015hsx 0.010hsx

a,b,c,d

See ASCE 7-05 Table 12.12-1 for footnotes.

CLOSING COMMENTS This CodeMaster has presented the step-by-step process required to complete seismic design as it relates to the seismic design demands. Many other code requirements need to be addressed when completing the entire seismic design of a building. These other code requirements cover: direction of loading, deformation compatibility, P-Δ effects, detailing, structural component load effects, nonstructural components, inspections, foundations, and material specific requirements.

2003

NEHRP

ASCE

7-0 05

obtaining seismic design parameters using the same data that was used to prepare the ground motion maps published in the 2006 IBC, ASCE 7-05, and the 2003 NEHRP Provisions. By inputting the longitude and latitude of the building location, this method provides for a more accurate and reliable determination of Ss and S1. The FEMA 450 CD also contains this calculation tool.

STEP 2

DETERMINE IF STRUCTURE IS EXEMPT FROM SEISMIC REQUIREMENTS

2006 IBC Section 1613.1 allows the following four exceptions from compliance with the 2006 IBC seismic design requirement: Exception Detached one- and two- family dwellings in SDC A, B, or C or No. 1 located where Ss is less than 0.4g.

The design story drift, Δ, is computed as the difference of the deflections δx at the centers of mass of the diaphragms at the top and the bottom of the story under consideration. For structures assigned to SDC C and higher, with horizontal irregularities 1a or 1b, the design story drift, Δ, is computed as the largest difference of the deflections along any of the edges of the diaphragms at the top and the bottom of the story under consideration. This accounts for torsional effects. Once the drift is computed, it is checked against the allowable story drift set forth in ASCE 7-05 Table 12.12-1 [2003 NEHRP Provisions Table 4.5-1]. The first and the last rows of the table apply to buildings other than masonry shear wall buildings. If such buildings are more than four stories tall, the last row applies. If, however, such buildings are four stories or less in height, the designer has a choice between two drift limits: (1) where nonstructural elements have been designed to accommodate the story drift (less stringent) and (2) all other structures (more stringent). This is consistent with the intent of the drift limit, which is to limit damage to drift-sensitive nonstructural elements.

2006 IBC

Areas of U.S. with Ss < 0.4 g (Shown in green) For areas outside the conterminous United States, visit www.skghoshassociates.com/CMSDC

At this stage, the SDC has not been determined; however, Ss has been determined in Step 1. After Step 3 is completed, this exception may be revisited. 2003 NEHRP Provisions

ASCE 7–05

2006 IBC

* The 2003 NEHRP Provisions (FEMA 450-1) is a resource document funded and published by the Federal Emergency Management Agency (FEMA). It is intended to capture research results and lessons learned and may contain information beyond that found in ASCE 7-05 or the IBC. The accompanying Commentary (FEMA 450-2) may assist the user in understanding the basis for code requirements. Copies of the 2003 NEHRP Provisions and the accompanying Commentary may be viewed or downloaded on the Building Seismic Safety Council's (BSSC) website: www.bssconline.org. The 2003 NEHRP Provisions also includes a CD that contains the two documents as well as the seismic design maps and a program to determine the mapped seismic design values. Hard copies or the CD may be obtained free-of-charge by contacting the FEMA Publication Distribution Facility at 1-800-480-2520.

STEP 1

Conventional light-frame wood construction complying with 2006 Exception IBC Section 2308 (see definition for "conventional light-frame wood No. 2 construction" in 2006 IBC Section 2302). Agricultural storage structures intended for incidental human Exception occupancy only (see definition for "agricultural building" in 2006 IBC No. 3 Section 202). Vehicular bridges, electrical transmission towers, hydraulic Exception structures, buried utility lines and their appurtenances, nuclear No. 4 reactors and other similarly described structures in the code. 2006 IBC Section 1613.5.1

Structures located in areas with Ss < 0.15g and S1 < 0.04g need only comply with SDC A requirements.

DETERMINE SS AND S1

The first step in seismic design is determining the mapped maximum considered earthquake (MCE) spectral response accelerations at short periods, Ss, and at 1second period, S1. These values can be determined using one of two methods: 1.

2006 IBC Figures 1613.5(1) through 1613.5(14) [ASCE 7-05 Figures 22-1 through 22-20; 2003 NEHRP Provisions Figures 3.3-1 through 3.3-14], or

2.

USGS website at http://earthquake.usgs.gov/research/hazmaps/. The U.S. Geological Survey (USGS) has prepared an Internet calculation tool for

Areas of U.S. with Ss < 0.15g and S1 < 0.04g (shown in green) For areas outside the conterminous United States, visit www.skghoshassociates.com/CMSDC

CodeMaster developed by:

S C I

Structures & Codes Institute

A subsidiary of S.K. Ghosh Associates Inc. www.skghoshassociates.com ISBN 978-0-9793084-1-3

Tel: (847) 991-2700 Fax: (847) 991-2702 [email protected]

Similar exceptions are found in ASCE 7-05 Sections 11.1.2 and 11.4.1 and 2003 NEHRP Provisions Section 1.1.2.1.

CMSeismicNoBullets.qxp

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STEP 8

DETERMINE SEISMIC BASE SHEAR, V

The following seismic base shear equation is given in ASCE 7-05 Section 12.8.1[2003 NEHRP Provisions Section 5.2.1]: V = CsW where Cs is the seismic response coefficient

DISTRIBUTE V OVER THE HEIGHT OF THE BUILDING

ASCE 7-05 Section 12.8.3 [2003 NEHRP Provisions Section 5.2.3] describes how the seismic base shear is distributed over the height of the structure. The story forces are computed as follows: Fx = Cvx V

W is the weight of the building plus that of any contents that could, with a high degree of probability, be attached to the structure at the time of the earthquake. In addition to the obvious dead load of the structure, ASCE 705 Section 12.7.2 [2003 NEHRP Provisions Section 5.2.1] requires that the following loads be included in the effective seismic weight, W: Description

n

Areas of storage (other than 25 percent of floor live load public garages and open parking garages) Building with partitions

10 psf or actual weight, whichever is greater

Buildings with roofs designed for snow

Where flat roof snow loads are greater than 30 psf, 20 percent of the design snow load needs to be included, regardless of actual roof slope. 100 percent of operating weight

∑w h i=1

An example of this distribution is shown in the figure below. A k exponent larger than 1 places a greater proportion of the base shear in the upper stories, compared with a linear distribution produced by a k value of 1, to account for higher modes of vibration in structures having fundamental periods exceeding 0.5 seconds. For a one- to three-story building, the period is less than 0.5 second; therefore, the distribution of seismic forces will be linear. Fn

R/I

Level i

Hn

Design Base Shear, V

V=

0.5S1W R/I

, where S1 > 0.6g

V=

2

(R/I)T

V = 0.01W Period, T

V Building, n stories high

SD1TLW

TL

The period TL is given in ASCE 7-05 Figures 22-15 through 22-20 [2003 NEHRP Provisions Figures 3.3-16 through 3.3-21]. The building site needs to be located on the applicable map to determine TL, which ranges between 4 and 16 seconds, depending upon the location. The following map is the TL map for the conterminous United States:

Distribution of Seismic Forces

STEP 9

DETERMINE REDUNDANCY COEFFICIENT, ρ

The redundancy coefficient reflects the multiple load path concept – that of providing more than one alternate path for every load to travel from its point of application to the ultimate point of resistance. Just as regular structures have proven themselves to outperform irregular structures in earthquakes, structures with redundant seismic force-resisting systems have performed better than those with little or no redundancy. The redundancy coefficient is applied as necessary to increase the effect of the horizontal earthquake ground motion to compensate for the lack of structural redundancy in the seismic force-resisting system. ASCE 7-05 Section 12.3.4 [2003 NEHRP Provisions Section 4.3.3] describes how to determine the redundancy coefficient, ρ. The redundancy coefficient does not apply (meaning that it may be taken equal to 1) in SDCs A, B, and C; seismic design forces for structures assigned to these seismic design categories are therefore unaffected by the redundancy of the seismic force-resisting system.

(For areas outside the conterminous United States, visit www.skghoshassociates.com/CMSDC)

The typical one- to three-story building addressed in this CodeMaster will qualify as a short-period building and, therefore, the seismic base shear is determined by the following equation: SDS W R/I SDS is determined in Steps 1 and 3; R is determined in Step 5; I is determined in Step 6; and W is the seismic weight of the building as described in this step. V=

E = ρQE ± 0.2 S DS D { 1 424 3 Effect of horizontal earthquake ground motion

For structures assigned to SDC D, E or F, the value of the redundancy coefficient equals 1.3, unless it can be shown that one of two described conditions is met. The first condition involves showing that the removal of an individual seismic force-resisting element will not cause: (1) the remaining structure to suffer a reduction in story strength of more than 33 percent, or (2) create an extreme torsional irregularity. The second condition applies only to a structure that is regular in plan at all levels and requires that the seismic force-resisting system consists of at least two bays of seismic forceresisting perimeter framing on each side of the structure in each orthogonal direction at each story resisting more than 35 percent of the base shear.

D: Design Dead Load

Effect of vertical earthquake ground motion

The structural effects of the earthquake forces, meaning the bending moments, shear forces and axial forces caused by them, must be combined with the effects of gravity (bending moments, shear forces, axial forces caused by the dead, live, snow loads, etc.) using the design load combinations set forth in 2006 IBC Section 1605 [ASCE 7-05 Section 2.0; no corresponding section in the 2003 NEHRP Provisions]. For strength design, the two load combinations applicable in seismic design are: (2006 IBC Eq. 16-5 – Additive) (2006 IBC Eq. 16-7 – Counteractive)

2006 IBC Eq. 16-5 is the additive load combination in which gravity effects add to earthquake effects. 2006 IBC Eq. 16-7 is the counteractive load combination in which gravity effects counteract earthquake effects (the plus sign includes the minus and the minus sign governs). With incorporation of the expression for E, the above load combinations become: (1.2 + 0.2SDS)D + f1L + f2S + ρQE (0.9 - 0.2SDS)D - ρQE + 1.6H

Hi V=

T1 = SD1/SDS

Wi

SD1W (R/I)T

SDS: Determined in Steps 1 and 3

1.2D + 1.0E + f1L + f2S 0.9D + 1.0E + 1.6H

Fi

SDSW

What is E? E is the combined effect of horizontal and vertical earthquake-induced forces and is quantified by the following equation: ρ: Determined in Step 9

Cs is calculated according to one of three equations depending on the period of the structure as illustrated in the following figure (there are also minimum base shear requirements for long-period structures): V=

ASCE 7-05 Sections 12.4.2 and 12.4.3 [2003 NEHRP Provisions Sections 4.2.2.1 and 4.2.2.2] address the determination of E and Em.

k i i

For structures with T < 0.5 sec, k=1 For structures with T > 2.5 sec, k = 2 For structures with 0.5 sec < T < 2.5 sec, k can be 2 or can be determined by linear interpolation between 1 and 2.

Include in Seismic Weight

Permanent equipment

Where: C vx =

w x hkx

STEP 10 DETERMINE SEISMIC LOAD EFFECTS, E AND EM

(2006 IBC Eq. 16-5 – Additive) (2006 IBC Eq. 16-7 – Counteractive)

In other words, the consideration of vertical earthquake ground motion increases the dead load factor in the additive load combination and decreases it in the counteractive load combination. For example, consider a fully redundant structure (ρ = 1.0) located where SDS = 1.0 with a bearing wall system consisting of shear walls used for the seismic forceresisting system and f1 =1.0. If the bending moments in a shear wall cross-section due to dead loads, live loads, snow loads and horizontal earthquake forces are 200 ft-kips, 60 ft-kips, 0 ft-kips and 150 ft-kips, respectively, the design moments (required flexural strengths) by the strength design load combinations (IBC Equations 16-5 and 16-7) are: Mu = [(1.2) + (0.2)(1.0)]( 200) + 60 + (1)(150) = 490 ft-kips Mu = [(0.9) - (0.2)(1.0)](200) - (1)(150) = -10 ft-kips The shear wall needs to be reinforced to carry these bending moments at the cross-section in question. What is Em? Em is the maximum seismic load effect and is required for the design of certain elements critical to the stability of the structure. This maximum load effect generated in a building can be much greater than those due to the designlevel force. Em= Ω0QE ± 0.2SDSD Ωo is the overstrength factor and increases the design-level internal forces to represent the actual forces that may be experienced by an element as a result of the design-level ground motion. Ωo is obtained from ASCE 7-05 Table 12.2-1 [2003 NEHRP Provisions Table 4.3-1]. Em is determined using the same procedure as for determining E. Em is used in the additive and the counteractive load combinations the same way as E, except that the factored snow load effect, f2S, is typically not included in the additive combination. Because Em is a strength-level force effect, adjustments need to be made if allowable stress design is used. The allowable stresses may be increased by a factor of 1.2 in accordance with ASCE 7-05 Section12.4.3.3.

The special seismic load combinations set forth in IBC Section 1605.4 are required for such elements as collectors; columns or other elements supporting reactions from discontinuous shear walls or frames; and batter piles and their connections.

STEP 11

CHECK DRIFT CONTROL REQUIREMENTS

CodeMaster SEISMIC DESIGN

The interstory drift expected to be caused by the design earthquake is limited by the code. Some reasons for limiting drift are: 1) to control member inelastic strain, 2) to minimize differential movement demand on the seismic safety elements, and 3) to limit damage to nonstructural elements. ASCE 7-05 Section 12.12.1 contains drift control requirements [2003 NEHRP Provisions Section 4.5.1]. Drift determination is addressed in ASCE 7-05 Section 12.8.6 [2003 NEHRP Provisions Section 5.2.6]. The first step is to determine δxe, the elastically computed lateral deflection at floor level x under code-prescribed seismic forces (the design base shear, V, distributed along the height of the structure in the manner prescribed by the code). Next, the deflections, δxe, are multiplied by the deflection amplification factor, Cd, (because the actual lateral deflections will be greater under the design earthquake excitation) and divided by I in accordance with the following equation: δx = Cd δxe/ I Cd is set forth in ASCE 7-05 Table 12.2-1 (2003 NEHRP Provisions Table 4.3-1). I is in the denominator of the equation to eliminate I from the drift computation (remember that the code-prescribed seismic forces that produced δxe were originally augmented by I). It is important and necessary to do this because the drift limits of ASCE 7-05 and the 2003 NEHRP Provisions are a function of the occupancy of a structure. The drift limit for a hospital is half that for an office building on the same site.

S EISMIC D ESIGN

This CodeMaster identifies the 11 steps involved in designing a typical one- to threestory building for seismic loads in accordance with the 2006 International Building Code (IBC), ASCE 7-05 Minimum Design Loads for Buildings and Other Structures, and the 2003 NEHRP Recommended Provisions for Seismic Regulations for New Buildings and Other Structures (known as 2003 NEHRP Provisions or FEMA 450-1*). Information will be presented on how these three documents work together. The NEHRP Provisions feed directly into the ASCE 7 development process; ASCE 7 in turn serves as a primary referenced standard in the IBC. The seismic design provisions of the 2006 IBC are based on those of ASCE 7-05 and make extensive reference to that standard. In fact, almost all of the seismic design provisions are adopted through reference to ASCE 7-05. Beginning with Step 4, only references to ASCE 7-05 and the 2003 NEHRP Provisions are made. The only seismic provisions included in the text of the 2006 IBC are related to ground motion, soil parameters, and determination of Seismic Design Category (SDC), as well as definitions of terms actually used within those provisions and the four exceptions under the scoping provisions. It is important to note that where this CodeMaster provides section references from the documents, the corresponding requirements often differ from one another. In some cases, these differences are subtle and an explanation of these differences is beyond the scope of this CodeMaster.

ASCE 7-05 TABLE 12.12-1 ALLOWABLE STORY DRIFT, Δa

a, b

Occupancy Category Structure Structures, other than masonry shear wall structures, 4 stories or less with interior walls, partitions, ceilings and exterior wall systems that have been designed to accommodate the story drifts. Masonry cantilever shear wall structures

d

I or II

III

IV

c 0.025hsx 0.020hsx 0.015hsx

0.010hsx 0.010hsx 0.010hsx

Other masonry shear wall structures

0.007hsx 0.007hsx 0.007hsx

All other structures

0.020hsx 0.015hsx 0.010hsx

a,b,c,d

See ASCE 7-05 Table 12.12-1 for footnotes.

CLOSING COMMENTS This CodeMaster has presented the step-by-step process required to complete seismic design as it relates to the seismic design demands. Many other code requirements need to be addressed when completing the entire seismic design of a building. These other code requirements cover: direction of loading, deformation compatibility, P-Δ effects, detailing, structural component load effects, nonstructural components, inspections, foundations, and material specific requirements.

2003

NEHRP

ASCE

7-0 05

obtaining seismic design parameters using the same data that was used to prepare the ground motion maps published in the 2006 IBC, ASCE 7-05, and the 2003 NEHRP Provisions. By inputting the longitude and latitude of the building location, this method provides for a more accurate and reliable determination of Ss and S1. The FEMA 450 CD also contains this calculation tool.

STEP 2

DETERMINE IF STRUCTURE IS EXEMPT FROM SEISMIC REQUIREMENTS

2006 IBC Section 1613.1 allows the following four exceptions from compliance with the 2006 IBC seismic design requirement: Exception Detached one- and two- family dwellings in SDC A, B, or C or No. 1 located where Ss is less than 0.4g.

The design story drift, Δ, is computed as the difference of the deflections δx at the centers of mass of the diaphragms at the top and the bottom of the story under consideration. For structures assigned to SDC C and higher, with horizontal irregularities 1a or 1b, the design story drift, Δ, is computed as the largest difference of the deflections along any of the edges of the diaphragms at the top and the bottom of the story under consideration. This accounts for torsional effects. Once the drift is computed, it is checked against the allowable story drift set forth in ASCE 7-05 Table 12.12-1 [2003 NEHRP Provisions Table 4.5-1]. The first and the last rows of the table apply to buildings other than masonry shear wall buildings. If such buildings are more than four stories tall, the last row applies. If, however, such buildings are four stories or less in height, the designer has a choice between two drift limits: (1) where nonstructural elements have been designed to accommodate the story drift (less stringent) and (2) all other structures (more stringent). This is consistent with the intent of the drift limit, which is to limit damage to drift-sensitive nonstructural elements.

2006 IBC

Areas of U.S. with Ss < 0.4 g (Shown in green) For areas outside the conterminous United States, visit www.skghoshassociates.com/CMSDC

At this stage, the SDC has not been determined; however, Ss has been determined in Step 1. After Step 3 is completed, this exception may be revisited. 2003 NEHRP Provisions

ASCE 7–05

2006 IBC

* The 2003 NEHRP Provisions (FEMA 450-1) is a resource document funded and published by the Federal Emergency Management Agency (FEMA). It is intended to capture research results and lessons learned and may contain information beyond that found in ASCE 7-05 or the IBC. The accompanying Commentary (FEMA 450-2) may assist the user in understanding the basis for code requirements. Copies of the 2003 NEHRP Provisions and the accompanying Commentary may be viewed or downloaded on the Building Seismic Safety Council's (BSSC) website: www.bssconline.org. The 2003 NEHRP Provisions also includes a CD that contains the two documents as well as the seismic design maps and a program to determine the mapped seismic design values. Hard copies or the CD may be obtained free-of-charge by contacting the FEMA Publication Distribution Facility at 1-800-480-2520.

STEP 1

Conventional light-frame wood construction complying with 2006 Exception IBC Section 2308 (see definition for "conventional light-frame wood No. 2 construction" in 2006 IBC Section 2302). Agricultural storage structures intended for incidental human Exception occupancy only (see definition for "agricultural building" in 2006 IBC No. 3 Section 202). Vehicular bridges, electrical transmission towers, hydraulic Exception structures, buried utility lines and their appurtenances, nuclear No. 4 reactors and other similarly described structures in the code. 2006 IBC Section 1613.5.1

Structures located in areas with Ss < 0.15g and S1 < 0.04g need only comply with SDC A requirements.

DETERMINE SS AND S1

The first step in seismic design is determining the mapped maximum considered earthquake (MCE) spectral response accelerations at short periods, Ss, and at 1second period, S1. These values can be determined using one of two methods: 1.

2006 IBC Figures 1613.5(1) through 1613.5(14) [ASCE 7-05 Figures 22-1 through 22-20; 2003 NEHRP Provisions Figures 3.3-1 through 3.3-14], or

2.

USGS website at http://earthquake.usgs.gov/research/hazmaps/. The U.S. Geological Survey (USGS) has prepared an Internet calculation tool for

Areas of U.S. with Ss < 0.15g and S1 < 0.04g (shown in green) For areas outside the conterminous United States, visit www.skghoshassociates.com/CMSDC

CodeMaster developed by:

S C I

Structures & Codes Institute

A subsidiary of S.K. Ghosh Associates Inc. www.skghoshassociates.com ISBN 978-0-9793084-1-3

Tel: (847) 991-2700 Fax: (847) 991-2702 [email protected]

Similar exceptions are found in ASCE 7-05 Sections 11.1.2 and 11.4.1 and 2003 NEHRP Provisions Section 1.1.2.1.