Seismic Brace Design

Design and Detailing of Seismic Connections for Braced Frame Structures

Author

T

erry Lundeen is a principal with the structural engineering firm of Coughlin Porter Lundeen, Inc., in Seattle. His experience over the past 20 years includes the design of numerous building structures as well as deep water offshore platforms and large aircraft assembly facilities. He received his bachelor of science in civil engineering from Bradley University in 1980 and his master of science in civil engineering from the University of Houston in

Frames and Eccentrically Braced Frames. The seismic design approach and details are based on practical implementation of the current provisions on numerous commercial, industrial, educational and residential buildings.

1985.

Terry R. Lundeen

Mr. Lundeen has a special interest in seismic design and retrofit of structures, he is active in the development of seismic design provisions for the Uniform Building Code through the Structural Engineers Association of Washington and for the federal NEHRP documents through the Building Seismic Safety Council and the American Society of Civil Engineers. He contributes to the preparation of the Western States Structural Engineers Exam and lectures on the seismic design of steel structures at the University of Washington. He is a registered structural engineer in California, Washington and British Columbia.

Summary s a result of lessons learned A from recent earthquakes (Loma Prieta, Northridge, Kobe) as well as on-going research, the seismic design and detailing of braced frame connections has evolved significantly over the past ten years. Using an example office building, this paper presents the design of braced frame connections according to the recently released 1997 Edition of the Seismic Provisions for Structural Steel Buildings by AISC. The examples include various types of brace connections and column splices for Specially Concentrically Braced Frames, Ordinary Concentrically Braced

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DESIGN AND DETAILING OF SEISMIC CONNECTIONS FOR BRACED FRAME STRUCTURES TERRY R. LUNDEEN

INTRODUCTION

This paper presents the design and detailing of braced frame connections for seismic loading. A prototype 4-story office building in Seismic Zone 3 is used as the basis for the examples. A typical floor framing plan with braced frame locations is given in Figure 1.

The overall forces on the structure are based on the 1997 Edition of the Uniform Building Code. The design of steel members and connections is based on the AISC Seismic Provisions for Steel Buildings, dated April 17, 1997. A list of the general design criteria is given in Table 1.

The examples include the three basic braced frame types: Special Concentrically Braced Frames (SCBF), Ordinary Concentrically Braced Frames (OCBF), and Eccentrically Braced Frames (EBF). A variety of brace types are provided including pipes, structural tubes, and wide flanges. Additionally, both welded and bolted connections are provided for reference.

While most of the new code provisions are similar to those of older versions, there have been some changes and updates. These changes include explicit consideration of material overstrength and more direct integration of the AISC Seismic Provisions into the model building codes. Additional, more detailed, revisions are also presented in this paper. While the subject of the paper is connection design, brace and column member issues that directly effect the connections are discussed. The detailed design of these members, however, is not provided.

Table 1 General Criteria Code:

• AISC Seismic Provisions for Structural Steel Buildings • AISC Manual for Load & Resistance Factor Design

Structure:

• Office building

• Located in Seismic Zone 3 • Soil profile type Sc • The frame configuration are as follows: 1. Special Concentrically Braced Frame; R = 6.4 2. Ordinary Concentrically Braced Frame; R = 5.6 3. Eccentrically Braced Frame; R = 7 Material Specifications:

• Steel framing A572, Grade 50 • High-strength A325/A490 bolts • Welding Electrodes: E70

Loads:

• • • •

Roof Dead Load = 20psf Roof Live Load = 25psf Floor Dead Load = 80psf Floor Live Load = 80psf (reducible)

Figure 1 - Typical Floor Plan

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SPECIAL CONCENTRICALLY BRACED FRAME (SCBF) CONNECTION DESIGN

For this system, a frame consisting of welded pipe braces and a frame consisting of bolted wide flange braces are provided. Frame elevations for both configurations are given in Figures 2 and 3. The braces are arranged in a Chevron pattern both because it represents the most commonly used arrangement and because of the additional design considerations given in the Provisions. For a building of this size, the welded pipe configuration is preferable both from a design and construction perspective. The bolted wide flange configuration is given as a reference for large structures with brace forces that cannot be accommodated with pipes. Similarly, the strong axis column orientation given in the first frame is desirable; however, a weak axis column arrangement is also provided for reference.

The SCBF is a newer version of the traditional

steel braced frame. This system was developed to provide documented ductility, both analytically and through testing. In general, yielding and column buckling of the braces provide this ductility. In order for this behavior to be achieved, local buckling in the braces or connections cannot occur.

Figure 2 - SCBF Elevation

Another requirement to guarantee the desirable behavior of this system is to preclude plastic hinge formation in Chevron beams under unbalanced brace buckling and yielding forces. Also, the beam flanges at Chevron connections must be braced out-of-plane.

The connections in SCBF's must be stronger than the yielding members. For this system, the connections must also have either the strength to develop a strong axis plastic hinge or be arranged to allow a weak axis yield line to form under the cyclic yielding and buckling of the braces. A final consideration for this system is with the

columns. In addition to having the strength to resist axial forces from the amplified earthquake load combinations, the columns and splices are designed for a nominal shear force in the column. This shear strength requirement is provided because plastic hinges formed in the columns at large story drifts in some of the initial analytical analyses of the system.

Figure 3 - Frame Elevation

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Brace-to-Gusset Weld The required weld thickness for the brace-togusset, assuming 12 in. of weld along (4) edges:

WELDED PIPE BRACE-TO-WIDE FLANGE COLUMN CONNECTION (Fig. 4)

Required Strength

The required strength of bracing connections, per AISC Sec. 13.3.a, is determined from the least of the following equations: 1. Bracing member's nominal axial tensile strength:

where

equals 1.1 per AISC

Sec. 6.2.

Use 12" of ½" weld on (4) edges

• The weld thicknesses are relatively large to limit the extension of the gusset plates beyond the yield line. Gusset-to-Beam and Column Welds

Using the Uniform Force Method as recommended per LRFD Vol. II Part 11, the axial force from the brace is resolved into the corresponding moment, horizontal, and vertical forces on the gusset plate. This is shown on the free body diagram of the gusset plate Fig. 5.

• As can be seen, the connection force to the beam is much larger than that to the column. As such, larger welds are used at the beam flange to control the size of the gusset plate.

Figure 4 - Welded Pipe Brace-to-Wide Flange Column Connection

2. Maximum force, transferred to brace by system as determined by analysis

• Case 1 is normally used in design since Case 2 basically requires static push-over analysis or non-linear time history analysis to establish the maximum system force. • This connection was designed with a "yield line" a distance of 2t from the brace in lieu of the flexural strength requirements of Section 13.3c. Figure 5 — Gusset-to-Beam and Column Weld Forces

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Weld of gusset-to-beam flanges

Gusset Plate Thickness

Per AISC Sec. 13.3.b: The design tensile strength, determined from the limit states of tension rupture and block shear rupture strength per LRFD Chapter D, shall be greater than or equal to the required strength, as determined from above.

Also, the design compression strength, determined from the plate buckling limit state, shall be greater than the buckling strength of the brace which is given from the following:

Use ½" weld for gusset to beam flanges.

Finally, the plate must have adequate shear yielding strength for the designed fillet weld sizes.

Weld of gusset to column

Criteria

Table 2 Required Gusset Plate Thickness (in)

Block Shear

.42

Tension Yielding

.41

Plate Buckling

.54

Shear Yielding at Fillet Welds

.71

Use ¾" gusset plate

• Once the overall dimensions of the gusset plate are established by the welds and yield line, the thickness is determined from the various remaining criteria.

Use ¼" weld for gusset to column.

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Gusset Plate Thickness

WELDED PIPE BRACE-TO-BEAM CONNECTION (Fig. 6)

The minimum gusset plate thickness follows the same procedures as for the pipe-to-column connection.

• The beam flanges of this connection must be braced out-of-plane per AISC Sec. 13.4a.4. Perpendicular floor beams or angle bracing similar to that shown in the EBF section can be used to provide this bracing.

Check minimum thickness of gusset

From pipe to gusset:

Required Strength

The required strength is the same as for the pipe-tocolumn connection. From gusset to beam: Brace-to-Gusset Weld

The brace-to-gusset weld is the same as for the pipe-to-column Connection.

Gusset-to-Beam Weld

• The Chevron beam is quite deep to provide the required strength for the unbalanced brace loads. This depth results in a relatively long gusset plate with large bending stresses. • The angle between the brace and the gusset plate has been limited to 30° to

recognize shear lag effects at the plate-tobeam weld. • A stiffener plate has been added at the center of the gusset plate to help develop the yield line.

Figure 7 - Gusset-to-Beam Weld Forces

Weld size required

Figure 6- Welded Pipe Brace-to-Beam Connection

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Brace-to-Gusset Connection

BOLTED WIDE FLANGE BRACE-TO-WIDE FLANGE COLUMN CONNECTION (Fig. 8)

Using the connection layout shown, the following basic LRFD requirements are checked:

Required Strength

Table 3

The required strength follows the same provisions and procedures as for the pipe-to-column

Item

connection.

Single shear of brace flange bolts

308

354

Flange plate gross section yielding

308

405

Flange plate net section rupture

308

356

Flange plate block shear

308

397

Bearing of bolts in brace flange

308

524

Single shear of brace web bolts

176

265

Web plate gross section yielding

176

276

Web plate net section rupture

176

203

Web plate block shear

176

367

Bearing of bolts in brace web

176

239

Note that the flange and web are sized to have a slightly higher sections than the brace flanges and

Figure 8 - Bolted Wide Flange Brace-to-Wide Flange Column Connection

web are therefore acceptable by inspection.

Distribute brace force in proportion to web and flange areas

The flange plate-to-gusset weld follows the same procedures as for the pipe-to-column connection.

Force in flange

Assume 15" weld along all (4) edges of the plate.

Use 15" of ¼" weld for the flange plate-to-

Force in web

gusset connection on (4) edges.

• While potentially easier to erect, the bolted connection requires a much more extensive design effort as well as increased fabrication cost.

• While the strength requirements are the same as for the welded pipe, the bolted wide flange produces much higher connection forces due to lower buckling-to-yield ratios (brace design based on buckling and connection design based on yielding).

• For a bolted connection such as this, the net section of the brace will by definition be the weak link in the connection. This situation occurs because the Provisions require the remaining portions of the connection to be sized for 110% of the tensile yield of the brace gross section.

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Gusset-to-Beam Welds

Gusset-to-Column Bolts

The gusset-to-beam welds follows the same procedures as for the pipe-to-column connection.

However, since the column is bending about its weak axis, is taken as approximately zero resulting in the moment and horizontal component of the column being approximately zero. The forces are shown on the free body diagram of the gusset plate in Fig. 9. Figure 10 — Gusset-to-Column Bolt Forces

From LFRD Vol II, Table 8-19

Figure 9 - Gusset-to-Beam Weld Forces

• This connection has been configured for shop welding the gusset plate to the beam and field bolting the beam/gusset to the column.

From table

• For the weak axis column connection, stiffeners have been added at the top and bottom of the gusset to preclude local buckling.

Use (5) 1" A490-x bolts in two vertical rows

Gusset Plate Thickness

Weld of gusset to beam flange

The minimum thickness of the gusset plate is determined following the same provisions and procedures discussed earlier for the pipe-to-column connection. Table 4

Required Gusset Plate Thickness (in)

Criteria

Use ¾" weld for gusset to beam flange. Weld of shear tab to column

Tension Yielding

.60

Plate Buckling

.73

Shear Yielding @ Fillet Welds

Use

1.09

gusset plate

Use ¼" weld for shear tab to column.

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BOLTED WIDE FLANGE BRACE-TO-BEAM CONNECTION (Fig. 11) Required Strength

The required strength is the same as for the bolted wide flange brace-to-weak axis wide flange column.

Gusset Plate Thickness The minimum thickness of the gusset plate follows the same procedures as for the pipe-to-column connection. Use 1" gusset plate

Brace-to-Gusset Connection The wide flange brace-to-gusset connection follows the same procedures as that for the bolted wide flange brace-to-weak axis wide flange column.

WIDE FLANGE COLUMN SPLICE (Fig. 13)

Gusset-to-Beam Weld

Figure 13 - Wide Flange Column Splice

Web Plate and Weld Figure 12 - Gusset-to-Beam Weld Forces The gusset-to-beam weld follows the same procedures for welded pipe brace-to-beam connection.

Per AISC Sec. 13.5.b: Splices shall be capable of developing nominal shear strength of smaller section.

Figure 11 — Bolted Wide Flange Brace-to-Beam Connection

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Size weld of plate-to-column web using LRFD

Per AISC Sec. 8.3a.2: The minimum required

Table 8-42.

strength for each flange shall be 0.5 times Partial penetration weld

Try complete penetration weld

Flexural Strength Check Per AISC Sec. 13.5.b: Splices shall develop 50 percent of the nominal flexural strength of the smaller section. Use

fillet weld

• Design the weld plate to resist the column shear and the flange welds to resist the axial tension force. • Load condition 4-2 becomes significant for taller, more slender frames.

Figure 14 — Splice Flexural Forces

• It is difficult for partial-penetration welds to comply with the column splice requirements.

• Although base plates have not been included in this paper, there is strong analogy between the strength and weld requirements of column splices and base plates. Flange Welds Per AISC Sec. 8.3a.1: If partial penetration weld used, the design strength of the joints must be at least 200 percent of the required strength per

equation 4-2. Equation 4-2 does not include the redundancy

factor.

Try partial joint weld

Since Equation 4-2 negligible, not applicable.

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ORDINARY CONCENTRICALLY BRACED FRAME (OCBF) CONNECTION DESIGN

This system is the basic steel braced frame that has been a part of seismic codes for many years. The

frame is configured with welded pipe braces (see Figure 15) for a direct comparison with the SCBF in the previous section. As opposed to the ductility approach for the SCBF, the design basis for the OCBF is primarily based on strength. The provisions require braces with greater stiffness (lower kl/r ratios) and greater strength (lower system R factor and 80% reduction of design strength). In addition to these requirements, new provisions have been added to preclude local buckling of the braces. The OCBF system also has special requirements for Chevron configurations. Instead of requiring

increased beam strength for unbalanced brace forces, the OCBF provisions amplify the design forces on the braces, resulting in even stronger, stiffer braces.

The connections have slightly lower demands than those of SCBF's. The design force can be based on the amplified seismic load combination if it is lower than the yielding of the brace. Also, until recently, there were no requirements for plastic hinge formation or out-of-plane yielding of the connection. These requirements were added to the current version of the Provisions. Even though the requirements are slightly less, the actual connections will be larger in the OCBF because of the larger forces in the stronger, stiffer braces. Column splices must be designed for the amplified earthquake load combinations, but have no special shear strength requirements. As for SCBF, the Provisions include special requirements for splices made with fillet welds or partial-penetration groove welds.

Figure 15 - OCBF Elevation

WELDED TUBE BRACE-TO-WIDE FLANGE COLUMN CONNECTION (Fig. 16)

Figure 16 — Welded Tube Brace-to-Wide Flange Column Connection

Required Strength

The required strength of bracing connections, per AISC Sec. 14.3.a, is determined from the least of the following equations:

Bracing member's nominal axial tensile strength: where

equals 1.1 per AISC Sec. 6.2

Force in the brace resulting from the following Load Combinations per AISC Sec. 4.1 Eqn. (4-1)

Eqn. (4-2)

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where for OCBF per UBC Table 16-N and does not include the redundancy factor

Weld of gusset-to-beam flanges

• The connection design for this OCBF is based on the amplified seismic forces instead of the brace yield force. Maximum force, transferred to brace by system as determined by analysis

Use

Brace-to-Gusset Weld

The required weld length for the brace to the gusset follows the same procedures as for the SCBF pipeto-column connection.

• This connection is arranged with the brace terminating close to the beam flange, resulting in a smaller gusset plate.

weld for gusset-to-beam flange

• Because the connection cannot rotate freely out-of-plane, the new version of the Provisions requires the welds to be designed for an additional force based on the plastic moment Strength of the brace. This additional requirement results in very large welds and a thick gusset plate. Weld of gusset-to-column

Assume 15in of weld along (4) edges. Use 15in of

weld on (4) edges

Gusset-to-Beam and Column Welds

The gusset-to-beam and column connections follow the same procedures used for the SCBF pipe-to-column connection. However, per AISC Sec. 14.3c, an additional plastic moment equal to will be included when the analysis indicates the brace will buckle.

Use 1¼" weld for gusset-to-beam column Gusset Plate Thickness

Determining the thickness of the gusset plate follows the same procedures as for the SCBF pipeto-column connection. Table 5 Criteria

Required Gusset Plate Thickness (in)

Block Shear

.32in

Tension Yielding

.33in

Plate Buckling

.42in

Shear Yielding @ Fillet Welds

Figure 17 — Gusset-to-Beam and Column Weld Forces

1.92in

Use 2" gusset plate

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WELDED TUBE BRACE-TO-BEAM CONNECTION (Fig. 18)

Required Strength

The required strength is the same as for the tube-tocolumn connection. Brace-to-Gusset Weld

The brace-to-gusset weld is the same as for the tube-to-column connection. Gusset-to-Beam Connection

Gusset Plate Thickness

The gusset-to-beam connection follows the same procedures for the SCBF pipe-to-column connection. Also included is the additional plastic

moment as discussed in the previous section.

The gusset plate thickness follows the same procedures as for the SCBF pipe-to-column connection.

use 1¼" gusset plate

Figure 19 — Gusset-to-Beam Connection

Figure 18 — Welded Tube Brace-to-Beam Connection

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ECCENTRICALLY BRACED FRAME (EBF) CONNECTION DESIGN The EBF system was introduced into the building codes in the late 1980's and has received moderate use in steel braced frame buildings since. The

frame in this example uses welded tube connections similar to the OCBF (see Figure 20 for a frame elevation). As for the SCBF and OCBF examples, a Chevron configuration with the links in the center was selected. The building codes currently also allow links to be placed adjacent to columns. For that configuration, the connection design criteria currently being developed for welded steel moment frame connections needs to be considered in addition to the topics presented in this paper. The ductility in the EBF system comes from the rotation and yielding of the link. The link in this example was configured for shear yielding (short link) rather than for flexural yielding (long link). The EBF provisions are based on a capacity design approach and therefore all members and connections must be stronger than the link. The brace design is based on buckling strength under the strain hardened link force. The required strength of the connection then needs to exceed the expected strength of the brace in compression. Additional connection issues with the EBF are associated with the design and detailing of the link. To assure stable yielding, web stiffeners are required at each end of the link and also at intermediate locations. In general, closer stiffener spacing is required for shear links than for flexural links. The Provisions do not allow web doubler plates or brace gusset plates extending into the link region. Finally, the Provisions require the flanges of the link to be braced out-of-plane.

Column splices must be designed for the amplified earthquake load combinations, but have no special shear strength requirements. As for SCBF, the Provisions include special requirements for splices made with fillet welds or partial-penetration groove welds.

Figure 20 – EBF Elevation WELDED TUBE BRACE-TO-WIDE FLANGE COLUMN CONNECTION (Fig. 21)

Figure 21 – Welded Tube Brace-to-Wide Flange Column Connection

Required Strength

The required strength of brace, per AISC Sec 15.6a is determined from the resulting forces generated by the expected nominal shear strength of the link increased by 125% to account for strain hardening. Next, per AISC Sec. 15.6d, the required strength of the connection shall be at least the expected nominal strength of the brace. For the TS 8 x 8 x

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Resultant

• The required connection strength of the EBF is the lowest of the various frames shown in this paper. The reason for this lower demand is that the EBF has the largest system R factor and that the connection force is based on brace compression strength rather than brace yielding.

Use

fillet weld for gusset-to-beam flange

Weld of gusset to column

Brace-to-Gusset Weld The required weld thickness for the brace to the gusset follows the same procedures as for the SCBF pipe-to-column connection. Assuming 14" of weld along (4) edges

Use 14" of

weld along (4) edges

Gusset-to-Beam and Column Welds Use weld (similar to weld along beam) for gusset to column

• As for the OCBF, the brace extends to the beam flange to minimize the size of the gusset plate. Gusset Plate Thickness Table 6

Figure 22 - Free Body Diagram of Brace to Beam/Column Connection

Required Gusset Plate Thickness (in)

Criteria

Uniform Force Method as recommended per LRFD Vol. II Part 11, the axial force from the brace is resolved into the corresponding moment, horizontal, and vertical forces on the gusset plate. This is shown on the free body diagram of the gusset plate Fig. 22. Weld of gusset-to-beam flanges

Block Shear

.33in

Tension Yielding

.21in

Plate Buckling

.31 in

Shear Yielding @ Fillet Welds

.55in

Use

gusset plate

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WELDED TUBE BRACE-TO-BEAM CONNECTION (Fig. 23)

Required Strength

The required strength is the same as the EBF welded tube brace-to-wide flange column connection. Brace-to-Gusset Weld

The required weld length for the brace to the gusset is the same as the EBF welded tube brace-to-wide flange column connection.

Choose

weld

• Since the gusset plate cannot extend into the link region, a stiffener is added at the end of the link to balance the loading on the welds. Gusset Plate Thickness Table 7 Required Gusset Plate Thickness (in)

Criteria

Figure 24 – Free Body Diagram of Brace-to-Beam/Column Connection

Elastic Vector Method

Block Shear

.33in

Tension Yielding

.29in

Plate Buckling

.31in

Shear Yielding @ Fillet Welds

.63in

Use

gusset plate

Figure 23 – Welded Tube Brace-to-Beam Connection

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BEAM LINK (Fig. 23)

Link Stiffener Welds

End Link Stiffeners

Per AISC Sec 15.3c, fillet welds connecting link stiffeners shall have a design strength:

Per AISC Sec. 15.3a, provide full depth web stiffeners on both sides of link at end of braces:

is area of stiffener) for connection of web to stiffener.

• Width

for connection of flange to stiffener.

• Thickness

or 3/8" whichever is greater

Weld For Web

(2) sided, full beam width & depth

Use plate

thick

Link stiffener requirements are prescriptive. Choose a

Intermediate Link Stiffeners

weld

Weld for Flange

Per AISC Sec. 15.3b: 1.) Provide intermediate web stiffeners spaced at; since link length and link rotation 2.) - Intermediate link web stiffeners shall be full depth. -If link depth