Seismic Brace Design

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Design and

Detailing of

Seismic

Connections for

Braced Frame

Structures

Terry R. Lundeen

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 air-craft 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

1985.

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

A

s a result of lessons learned 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 build-ing, this paper presents the design of braced frame connec-tions 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

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

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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 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.

Table 1 General Criteria Code: Structure: Material Specifications: Loads:

• AISC Seismic Provisions for Structural Steel Buildings • AISC Manual for Load &

Resistance Factor Design

• 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

• Steel framing A572, Grade 50

• High-strength A325/A490 bolts

• Welding Electrodes: E70 • Roof Dead Load = 20psf • Roof Live Load = 25psf • Floor Dead Load = 80psf • Floor Live Load = 80psf

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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.

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.

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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. 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 2 - SCBF Elevation

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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.

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

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

Brace-to-Gusset Weld

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

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 5 — Gusset-to-Beam and Column Weld Forces

• 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.

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Weld of gusset-to-beam flanges

Use ½" weld for gusset to beam flanges. Weld of gusset to column

Use ¼" weld for gusset to column.

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:

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

Table 2 Criteria Block Shear Tension Yielding Plate Buckling Shear Yielding at Fillet Welds

Required Gusset Plate Thickness (in)

.42

.41 .54

.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.

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WELDED PIPE BRACE-TO-BEAM

CONNECTION (Fig. 6)

• 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.

The required strength is the same as for the pipe-to-column connection.

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

Gusset-to-Beam Weld

Figure 7 - Gusset-to-Beam Weld Forces

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

Check minimum thickness of gusset

From pipe to gusset:

From gusset to beam:

• 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-to-beam weld.

• A stiffener plate has been added at the

center of the gusset plate to help develop

the yield line.

Figure 6- Welded Pipe Brace-to-Beam Connection

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

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

connection.

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

Distribute brace force in proportion to web and flange areas

Force in flange

Force in web

• 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).

Brace-to-Gusset Connection

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

Table 3 Item

Single shear of brace

flange bolts

Flange plate gross section yielding

Flange plate net section rupture

Flange plate block shear Bearing of bolts in brace flange

Single shear of brace web bolts

Web plate gross section

yielding

Web plate net section rupture

Web plate block shear Bearing of bolts in brace web 308 308 308 308 308 176 176 176 176 176 354 405 356 397 524 265 276 203 367 239

Note that the flange and web are sized to have a

slightly higher sections than the brace flanges and

web are therefore acceptable by inspection.

The flange plate-to-gusset weld follows the same

procedures as for the pipe-to-column connection. Assume 15" weld along all (4) edges of the plate.

Use 15" of ¼" weld for the flange

plate-to-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.

• 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

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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.

• This connection has been configured for shop

welding the gusset plate to the beam and

field bolting the beam/gusset to the column.

• For the weak axis column connection,

stiffeners have been added at the top and

bottom of the gusset to preclude local

buckling.

Weld of gusset to beam flange

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

Use ¼" weld for shear tab to column.

Figure 10 — Gusset-to-Column Bolt Forces From LFRD Vol II, Table 8-19

From table

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

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 Criteria Tension Yielding Plate Buckling Shear Yielding @ Fillet Welds

.60 .73

1.09

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

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

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.

Gusset-to-Beam Weld

The gusset-to-beam weld follows the same procedures for welded pipe brace-to-beam connection.

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

Use 1" gusset plate

WIDE FLANGE COLUMN SPLICE (Fig. 13)

Figure 13 - Wide Flange Column Splice

Web Plate and Weld

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

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Size weld of plate-to-column web using LRFD Table 8-42.

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.

• 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.

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

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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

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

• 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

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

• This connection is arranged with the brace

terminating close to the beam flange, resulting

in a smaller gusset plate.

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.

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

Use 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

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 pipe-to-column connection. Table 5 Criteria Block Shear Tension Yielding Plate Buckling Shear Yielding @ Fillet Welds Required Gusset Plate Thickness (in)

.32in .33in .42in

1.92in

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

The required strength is the same as for the tube-to-column connection.

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

Gusset-to-Beam Connection

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

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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

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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• 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.

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

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

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.

Resultant

Use fillet weld for gusset-to-beam flange Weld of gusset to column

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.

Table 6 Criteria Block Shear Tension Yielding Plate Buckling Shear Yielding @ Fillet Welds

.33in .21in .31 in

.55in

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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.

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

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

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.

Table 7 Criteria Block Shear Tension Yielding Plate Buckling Shear Yielding @ Fillet Welds

.33in .29in .31in

.63in

Use gusset plate

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BEAM LINK (Fig. 23) Link Stiffener Welds

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

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

for connection of flange to stiffener. Weld For Web

Choose a weld Weld for Flange

Choose a weld Lateral support of link

Per AISC Sec 15.5, lateral support is to be provided at both the top and bottom of the link flanges at each end.

End Link Stiffeners

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

• Thickness or 3/8" whichever is greater (2) sided, full beam width & depth

Use plate thick

Link stiffener requirements are prescriptive.

Intermediate Link Stiffeners 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 <25" deep, stiffener is required on one side only.

- Thickness of 1 sided stiffeners > or whichever is greater.

-Width

Space intermediate web stiffeners at maximum

Web stiffeners full depth/width Only required on one side

• Because of the short link and high link rotation, the intermediate stiffeners must

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Design support for 6% of flange strength Lateral support of beam links @ ends of W18x40

Choose 3 x 3 x ¼

• The composite metal deck and concrete slab provide lateral support of the top flange.

BEAM-TO-COLUMN CONNECTIONS

Figure 26 - Beam-to-Column Connections

Per AISC Sec. 15.7, these connections shall have the strength to resist (2) equal and opposite forces equal to 2% of flange capacity - acting laterally on the beam flanges.

• The Provisions require nominal torsional

restraint of the beam away from the link. This requirement is met by adding stiffener

plates to a typical bolted shear connection. Plates and Welds

Weld at Column:

Choose a weld

Weld at Beam

Choose a weld with a plate ¼" x 4" x 4"

WIDE FLANGE COLUMN SPLICE @ EBF

Figure 27 — EBF Column Splice

• The column splice for the EBF is essentially

the same as for the OCBF. Required Strength

Per AISC Sec 8.3 the design strength of column splices shall meet or exceed the required strength of Sec. 8.2:

Eqn. 4-1 Eqn. 4-2 But need not exceed:

a. the maximum load transferred to the column considering times the nominal strength of the member

b. limit as determined form the resistance of the foundation to overturning uplift For this splice, Eqn 4-2 governs

However, for EBF also check axial tension when nominal shear strength of links reached,

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Flange Welds

Per AISC Sec. 8.3a and 8.3b:

Column splices made with fillet and partial joint penetration groove welds shall not be located within 4' nor half the column clear height of beam to column connections, whichever is less.

If subjected to a tensile stress per load combination 4-2 filler metal shall meet requirements of CVN toughness as required by Sec. 7.3b, and

1.) The design strength of partial joint penetration welds shall be at least equal to 200% of required strength.

2.) The minimum required strength for each flange shall be

Beveled transitions are not required when changes in thickness and width of flanges and web occur. Initially try a partial penetration groove weld that will be at least equal to 200% of the required strength.

Use a complete penetration weld at each flange, this will satisfy strength requirements of Sec 8.2 and Sec. 8.3a.

Locate splice @ 4' from floor or 14/2 - 7/2 = 3'-6"; 4' from floor governs

*provide shear plate to web for erection

CONCLUSION

Properly designed and detailed connections are critical to achieve the expected performance of braced frames in earthquakes. As can be seen in the design examples, there are numerous building code provisions that address connection design. These provisions have evolved over the years as new braced frame systems have been introduced and as more experience has been gained from the behavior of buildings in actual earthquakes.

ACKNOWLEDGEMENTS

The author wishes to acknowledge the considerable efforts of Garo Pehlivanian, Kristie Fromhold, Steve Curran and Michael Townsend in assisting in the development of this paper.