4 Load Combination

Section:

DESIGN PROCEDURES LOADS & CODES

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IBC: Load Combinations

A.

DP 1.2.1

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1 of 7 6 Oct2013

GENERAL

Section 1605 of IBC lists one set of load combinations for use with LRFD method (1605.2) and two sets of load combinations for use with ASD design method (1605.3). BlueScope allows either design method to be used; however, for the ASD method only the “basic” set of load combinations from section 1605.3.1 is available in VISION.1 IBC also invokes the load combinations with overstrength factor o specified in ASCE7, intended only for seismic design of some members or systems, when specifically called for2 by the referenced standard. This document lists all load combinations required for ASD and LRFD design. For each design method, the standard set of load combinations is presented first, followed by seismic combinations that use the overstrength factor. These are followed by additional combinations which are required when floor live loads or crane live loads are considered. The following references are used throughout this document: Table 1 Limit State / Design Consideration

References

Seismic Loads (ASCE 7)

DP 1.4.6

AISC 341 (Seismic Provisions)

DP 1.5

Serviceability

IBC 1604.3 DP 6.3

Crane load combinations with seismic loads

AISE Technical Report No. 13, 2003 Ed. 3.9 and 3.10

1

The “Alternate” set of load combinations in IBC 1605.3.2 is not compatible with BlueScope design. It is intended primarily for construction where material standards still allow one-third stress increase. 2 IBC requires load combinations with the overstrength factor for the design of collectors and elements supporting discontinuous walls or frames, when in Seismic Design Category C or above. Other use of load combinations with overstrength comes from AISC 341 (Seismic Provisions), which is mandatory for all buildings in high seismic applications, i.e., those assigned to Seismic Design Category D, E or F. For specific requirements, such as frame columns, anchor rods, and brace connections, please refer to other sections of this DP. When printed, this document becomes uncontrolled. Verify current revision number with controlled, on-line document.

Author:

Igor Marinovic

Section:

DESIGN PROCEDURES LOADS & CODES

Revision & Date

IBC: Load Combinations

B.

LRFD LOAD COMBINATIONS

B1.

Basic Load Combinations (a) (b) (c) (d) (e) (f) (g) (h) (i) (j) (k) (l) (m) (n)

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1.4 (D + Cg + Dc + Dp) 1.2 (D + Cg + Dc + Dp) + 1.6 (Lr or S or R) 1.2 (D + Cg + Dc + Dp) + 1.6 (Lr or S or R) + 0.5 W 1.2 (D + Cg + Dc + Dp) + 1.6 Su 1.2 (D + Cg + Dc + Dp) + 1.6 Sp 1.2 (D + Cg + Dc + Dp) + 1.6 S + 1.6 Sd 1.2 (D + Cg + Dc + Dp) + 1.6 S + 1.6 Ss 1.2 (D + Cg + Dc + Dp) + 1.6 S + 1.6 Sr 1.2 (D + Cg + Dc + Dp) + 1.0 W 1.0 MW 1.2 (D + Cg + Dc + Dp) + 1.0 W + 0.5 (Lr or S or R) (1.2 + 0.2 SDS) (D + Cg + Dc + Dp) ± QE + f2 S 0.9 (D + Cu + Dc + Dp) + 1.0 W (0.9 - 0.2 SDS) (D + Cu + Dc + Dp) ± QE f2

= 0.7 for roof configurations (such as saw tooth) that do not shed snow off the structure, = 0.2 for all other roof configurations

B1.1 Seismic load combinations with overstrength (o) (p) B2.

(1.2 + 0.2 SDS) (D + Cg + Dc + Dp) ± (0.9 - 0.2 SDS) (D + Cu + Dc + Dp) ±

QE Q o E o

Additional Load Combinations Involving Floor Live Load (LF) (q) (r) (s) (t) (u) (v)

1.2 (D + Cg + Dc + Dp) + 1.6 LF 1.2 (D + Cg + Dc + Dp) + 1.6 LF + 0.5 (Lr or S or R) 1.2 (D + Cg + Dc + Dp) + 1.6 (Lr or S or R) + f1 LF 1.2 (D + Cg + Dc + Dp) + 1.0 W + 0.5 (Lr or S or R) + f1 LF (1.2 + 0.2 SDS) (D + Cg + Dc + Dp) ± QE + f1 LF + f2 S (1.2 + 0.2 SDS) (D + Cg + Dc + Dp) ± o QE + f1 LF f 1 = 0.5. Exception: When floor live load > 100 psf, in places of public assembly, and for parking garages use f 1 = 1.0.

B3.

Additional Load Combinations Involving Crane Live Loads (Lc) (w) (x) (y) (z)

1.2 (D + Cg + Dc + Dp) + 1.6 Lc 1.2 (D + Cg + Dc + Dp) + 1.6 Lc + 0.5 (S or R or LF) 1.2 (D + Cg + Dc + Dp) + 1.6 (S or R or LF) + 0.5 Lc 1.2 (D + Cg + Dc + Dp) + 1.0 W + 0.5 Lc

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Author:

Igor Marinovic

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DESIGN PROCEDURES LOADS & CODES

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IBC: Load Combinations

C.

ASD LOAD COMBINATIONS

C1.

Basic Load Combinations (a) (b) (c) (d) (e) (f) (g) (h) (i) (j) (k) (l) (m) (n)

DP 1.2.1

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D + Cg + Dc + Dp D + Cg + Dc + Dp + (Lr or S or R) D + Cg + Dc + Dp + Su D + Cg + Dc + Dp + Sp D + Cg + Dc + Dp + S + Sd D + Cg + Dc + Dp + S + Ss D + Cg + Dc + Dp + S + Sr D + Cg + Dc + Dp + 0.6 W 0.6 MW (1.0+ 0.14 SDS) (D + Cg + Dc + Dp) ± 0.7 QE D + Cg + Dc + Dp + 0.45 W + 0.75 (Lr or S or R) (1.0+ 0.105 SDS) (D + Cg + Dc + Dp) ± 0.525 QE + (f3 S or f4 Lr) 0.6 (D + Cu + Dc + Dp) + 0.6 W (0.6 – 0.14 SDS) (D + Cu + Dc + Dp) ± 0.7 QE f 3 = 0.15 when flat roof snow load > 30 psf, otherwise zero f 4 = 0.75 when roof live load > 30 psf, otherwise zero Commentary: For IBC editions prior to 2009 VISION uses f 4 = 0 in all cases

C1.1 Seismic Load Combinations with Overstrength (o) (p) (q) C2.

C3.

(1.0+ 0.14 SDS) (D + Cg + Dc + Dp) ± 0.7 o QE (1.0+ 0.105 SDS) (D + Cg + Dc + Dp) ± 0.525 o QE + (f3 S or f4 Lr) (0.6 – 0.14 SDS) (D + Cu + Dc + Dp) ± 0.7 o QE

Additional Load Combinations Involving Floor Load (LF) (r) (s) (t)

D + Cg + Dc + Dp + LF D + Cg + Dc + Dp + 0.75 LF + 0.75 (Lr or S or R) D + Cg + Dc + Dp + 0.45 W + 0.75 LF + 0.75 (Lr or S or R)

(u) (v)

(1.0+ 0.105 SDS) (D + Cg + Dc + Dp) ± 0.525 (1.0+ 0.105 SDS) (D + Cg + Dc + Dp) ± 0.525

QE + 0.75 LF + (f3 S or f4 Lr or 0.75 R) o QE + 0.75 LF + (f 3 S or f 4 Lr or 0.75 R)

Additional Load Combinations Involving Crane Load (Lc) (w) (x) (y) (z)

D + Cg + Dc + Dp + Lc D + Cg + Dc + Dp + 0.75 Lc + 0.5625 S D + Cg + Dc + Dp + 0.75 Lc + 0.225 W D + Cg + Dc + Dp + 0.75 Lc + 0.75 LF

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Author:

Igor Marinovic

Section:

DESIGN PROCEDURES LOADS & CODES

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IBC: Load Combinations

D.

DP 1.2.1

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4 of 7 6 Oct2013

NOTES ABOUT LOAD COMBINATIONS IBC makes a distinction between Roof Live Load (Lr) and Live Load (L), that includes floor live load, cranes etc. Note that VISION uses the symbol L for Roof Live Load (instead of Lr). Unlike IBC which lists load combination using the generic term E for seismic loads, ASCE 7 section 12.4 resolves the seismic load effect E = QE 0.2 SDS D into horizontal and vertical components, and shows all seismic load combination in its final form. This ASCE 7 approach is adopted by BlueScope, as outlined in this document. Snow and wind load factor in ASD crane combinations (x) and (y) are the product of ASD combination factor (0.75), ASD basic wind load factor (0.6), and ASD exception for crane loads. The seismic mass of cranes and trolleys that lift a suspended load need include only the empty weight of the equipment (see the AISE ref. in Table 1). IBC 1605.1 also requires that each load combinations be investigated with one or more of the load factors set to zero, which is already included in the previously shown load combinations for LRFD and ASD. ASCE7 makes a distinction, as explained in the commentary, between minimum design wind loads (MW) and wind loads derived using coefficients determined by the provisions of the code based on wind tunnel research (W). “MW” and “W” are separate load types. “MW” is applied independently of any other transient loads. “W” is combined with other load types. By experience, self-straining loads, i.e., temperature loads (T) are not critical in BlueScope buildings construction; therefore, are not shown in the combinations.

D1.

Notations

D

= dead load of steel framing system furnished by BlueScope (actual steel weight), crane runway systems, and dead weight of floor systems Cg, Cu = user specified collateral load including dead weight of ceilings, sprinklers, permanent equipment, piping, ductwork, HVAC systems, etc. Dc = dead weight of the crane system: runway, bridge and trolley, as applicable (see D5) Dp = dead weight of partitions Lc = live load due to crane lifted loads Lr = roof live load due to use & occupancy LF =uniform floor live load due to use and occupancy MW = Minimum wind load per ASCE7 (16 psf). S = uniformly distributed snow load (see D3) Sd = drifting snow load Sp = partial loading snow Sr = rain-on-snow surcharge snow load Ss = sliding snow load Su = unbalanced roof snow load When printed, this document becomes uncontrolled. Verify current revision number with controlled, on-line document.

Author:

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DESIGN PROCEDURES LOADS & CODES

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IBC: Load Combinations R W QE SDS o

D2.

DP 1.2.1

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= rain accumulation load (not rain on snow surcharge) = wind load, based on code required coefficients and ultimate wind speed, Vult = earthquake load, base shear (V) or component force (Fp) = seismic spectral response acceleration parameter at short periods = redundancy coefficient (=1.3, except where specific condition satisfied use 1.0) = overstrength factor (between 2.0 and 3.0, except 1.25 for cantilevered systems) Stress increase

Strength based steel design, whether using the LRFD or ASD method, does not leave room for any kind of stress increases or adjustments to strength values. Conversion from ASD to LRFD strength is neither necessary nor allowed since all design equations are provided in a dual, LRFD/ASD format. D3.

Snow in load combinations

IBC (ASCE 7) defines two types of snow loads: a) the nominal snow load, which is the representative values for the 50-year mean recurrence interval, and b) the “arbitrary-point-intime” snow load which corresponds to an extreme and rare snow event. The first type of snow load is often referred to as “the companion load” as it is intended to be combined and evaluated in combination with other transient and non-transient loads. The other type of snow load represents an extreme load whose occurrence is not likely to coincide with the peak value of any other transient loads; hence, this load is considered acting alone, in combination with dead loads only. The resulting rules for use of snow load in LRFD and ASD load combinations are summarized as follows: (a) Uniform (balanced) snow load (S) is used in all applicable LRFD and ASD load combinations, as shown in sections B1 and C1. (b) If any of the extreme snow load effects are required for the job under consideration, i.e., the unbalanced snow, partial snow, drifting snow, sliding snow, or the rain-on-snow surcharge load, separate load combinations would be required for each applicable snow effect, but not in combination with any other transient loads. D4.

Two-bay buildings

For two-bay buildings, the interior support load (reaction) increase due to system continuity is utilized only with the sustained gravity loads that are uniformly applied over the whole roof surface, or for rectangular and line loads that are applied along the entire length of the roof, if supported by continuous purlins. This includes dead loads (self weight and collateral) and uniform snow loads, such as the unbalanced snow, the drifting snow (longitudinally only) and the specified minimum snow load (SMS).

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Author:

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DP 1.2.1

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LOADS & CODES

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IBC: Load Combinations

6 of 7 6 Oct2013

There is no increase for live, roof live, wind and seismic loads, or various non-uniform snow load applications. The Building Code prescribed loads already include the effect of time and special distribution for these loads; hence, no further adjustment or increase is required, regardless of one, two, three, or multi-bay condition.

D5.

Crane dead loads

The magnitude of crane dead loads, although represented here by a single symbol, Dc, depends on the load case under consideration. For wind uplift cases it includes the weight of the unmovable parts of the crane system only, such as runways. For the downward load cases the load Dc includes the weight of the unmovable parts and the weight of the crane bridge and trolley. Similarly, in seismic load cases the dead load Dc is the minimum load effect (runways only) when considering the subtractive load cases, and the maximum load effect (runways, bridge and trolley) for the additive load cases. Commentary: At this time VISION includes the dead weight of the crane bridge and trolley under the crane live loads, together with the lifted loads (live loads). This approach produces no adverse load effects. Using the previously listed state of the art LRFD load combinations, the VISION approach is conservative. For ASD design, due to non-linear LRFD-to-ASD conversion the VISION approach may produce somewhat unconservative results. As verified by numerous cases studies, this discrepancy has negligible impact on final design, since VISION effectively captures the extreme load cases, and the actual member design is normally governed by other more severe load combinations.

E.

SERVICEABILITY LOAD COMBINATIONS

E1.

General

IBC Section 1604.3 requires that structural systems and members have adequate stiffness to limit deflection and lateral drift. Buildings and components shall be checked for serviceability limit states under the effect of “service loads” (i.e.- NOT factored up as ultimate strength limit states are). The serviceability load combinations and BlueScope limits summarized in DP 6.1 comply with the recommendations of IBC Table 1604.3. Additional criteria applicable to metal building systems are also listed. Magnitude of loads to be used in serviceability checks are NOT the same as those used in LRFD or ASD strength checks, and are explained in the appropriate loading sections of this document.

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Author:

Igor Marinovic

Section:

DESIGN PROCEDURES LOADS & CODES

Revision & Date

IBC: Load Combinations

Document and Revision History REV. # DATE NAME

DP 1.2.1

Page

7 of 7 6 Oct2013

DESCRIPTION

0

10/01/2009

Igor Marinovic Original document

1

10/31/2009

Igor Marinovic Section D4 added – describes BBNA procedure for two-bay buildings.

2

11/24/2009

Igor Marinovic Section D5 added - clarify application of crane dead loads. Comment box added to explain VISION approach.

3

07/01/2010

Igor Marinovic Updated to reflect the requirements of the 2009 IBC; ASD load cases k, o, t and u updated (added +f4 Lr)

4

09/20/2010

Igor Marinovic Revised D4 – two-bay condition extended to other loads applied uniformly along the length

5

03/08/2012

Al Harrold

6

Oct 2013

S.Hyder

Update for ASCE7-10 / IBC 2012 Clarified the “MW” and “W” definitions and combinations. Also see DP 1.4.5 R5 related.

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Author:

Igor Marinovic