Horizontal Bracing

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MODERN STEEL CONSTRUCTION july 2010

Horizontl Brcin

steelwise

An overview of lateral load resisting systems and how to implement them.

IN MOST COMMERCIaL BUILDINgS, oorand roo diaphragms are used to distribute loadsin the horizontal plane o the structure to the lat-eral load resisting system. Due to the open natureo most industrial structures, diaphragms arenot present, and horizontal bracing is oten usedto distribute the loads in the horizontal plane.Horizontal bracing is also used in heavily-loadedcommercial structures, where a diaphragm is not present, or where the strength or stiness o thediaphragm is not adequate.

When horizontal bracing is used, the beamsat that elevation become members in a horizontaltruss system, carrying axial loads in addition to thenormal bending and shear gravity loads. Careulattention should be paid to the beam end connec-tions within the truss system because the axial loadstranserring through the connections can be large.

Selection o Structurl Shpes The most common shapes used or horizontalbracing are single angles and WT-shapes. Singleangles are the most economical shape or resist-ing small and medium loads, because WT-shapesmust be split rom W-shapes and straightened by the abricator. WT-shapes can be used to resist larger loads and where long spans are required.

Double-angles can be used in lieu o WT-shapes,but corrosion o the steel in the area between theangles can be a problem or exposed structures. I the original protective coating ails, it is difcult to recoat the area between the angles. In exposed

By Bo DoWsWell, P.e., Allen Brice, AnD BriAn BlAin

Bo Dowswell, P.E., (let) has been in the steel industry since 1985, spending the frst nine o those years as a steel detailer. He is co-ounder and principal o SDS Resources LLC, Birmingham, Ala., which specializes in designing steel connections and industrial struc-tures. Allen Brice (right) is senior designer drats-man with more than 20 years experience in structural drating and design, and Brian Blain is senior account manager. Both are also with SDS Resources.

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july 2010 MODERN STEEL CONSTRUCTION

structures, stitch plates in double-angle braces should bebolted, due to limited access or welding between the angles.

With bolted stitch plates, the angles are bolted together aterthey have been coated (painted or galvanized).

For very large loads, W-shapes can be used. For buildings with very light loads, rod bracing is commonly used. Some-times, architects preer the use o HSS when the bracing isexposed to view.

In open structures, the member should be rotated tothe position that is least likely to collect dust, debris, and/ or rainwater. Angle legs and WT stems usually are orienteddownward, and W-shapes normally are designed with the web oriented vertically.

Desin Considertions

Although the dominant load in a horizontal brace is axial

load, the eect o bending moments should be accountedor in the design. The moment due to sel weight can besignifcant or long, slender braces.

The details o the connections also may result in moments.It is preerable to bolt single-angle braces through the hori-zontal leg and WT braces through the ange as shown inFigure 2. Because there is an oset between the centroid o the brace and the gusset plate, the eccentric moment, M = Pe, must be accounted or in the member design, and this cansignifcantly lower the member strength. For preliminary design o WT braces connected through the ange, a goodstarting assumption is that the moment consumes hal thestrength o a concentrically loaded brace.

Because the length-to-depth ratio or horizontal brac-ing members can be very high, large deections under sel

weight are possible. This can lead to large second ordermoments, and serviceability problems due to sagging o thebrace as shown in exaggerated ashion in Figure 3. Also, inindustrial structures with vibrating machinery, large length-to-depth ratios can lead to vibration problems. A rule o thumb that can be used to determine an approximate pre-liminary brace depth is that the depth in inches should equalor exceed 0.006 F y times the span in t.

Brce Confurtions

Bracing is most efcient when placed at angles between30° and 60°. With steeper bevels, the end connections canbe cumbersome. Any brace confguration can be used to

make up a horizontal bracing system, as long as a completeload path is available to transer the load into the lateral loadresisting system. In most structures, the confguration is asecondary consideration, determined ater the location o the main gravity load-carrying members is set.

Under most conditions, a single brace system is adequate;however, an x-bracing system can be used to reduce thebuckling length o the compression brace in systems withlong brace spans. I the braces are designed as a tension-compression system and are connected at the intersection,the intersection can be considered a brace point or both in-plane and out-o-plane buckling. I the braces are designed

using a tension-only philosophy, it is recommended that they be connected at the intersection to reduce the possibility o serviceability problems due to the exibility o the braces.

Minus Dimension

Horizontal braces typically are not located at the top o steel, because the connections can interere with grating anddecking. To indicate the elevation o the brace, a “minusdimension” is usually shown on the design drawings. Theminus dimension is the distance rom the reerence eleva-tion (top o steel, top o grating, top o concrete, etc.) tothe top o the brace. Theoretically, the best location or thebracing is at the centroid o the beams, because this is where

it is located in the structural analysis model (unless osetsare used in the model). However, this usually is not practical,because most structures are built with multiple beam depths

within each bay. The minus dimensions are usually deter-mined by the connection details and the depth o fller beams.In practice, the moments caused by the osets between thebrace centroid and beam centroid usually are neglected. Thebest practice or design drawings is to list the nominal minusdimension, which will allow some exibility or the detailerto make minor adjustments where beam depths are slightly dierent within the brace span.

A simple example is illustrated in Figure 4 (ollowingpage). I the brace is placed 6 in. below the top o steel as

shown in Figure 4a, gusset plates are required at the W14 web. The more economical solution is shown in Figure 4b, where the nominal minus dimension is 14 in., and the bracepasses under the W14. The W14 fller beam in Figure 4a ismore expensive to abricate due to the gusset plates shop

welded to the web. The extra bolts and extra shipping pieceor the system in Figure 4a also make the arrangement moreexpensive to erect. Where braces pass under fller beams,they are usually connected to the bottom ange o the beam

with two bolts as shown in Figure 4b. This reduces the buck-ling length o the brace and acts as a support to reduce thedead load moment and deection.

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MODERN STEEL CONSTRUCTION july 2010

Connection Detils

Figure 5a shows a typical horizontal bracing connectionat a beam-to-beam intersection. Where horizontal bracingis located at a beam-to-column intersection, the gusset platemust be cut out around the column as shown in Figure 5b.

These are called wrap-around gusset plates. At locations with large columns and heavy beam connection angles, alarge area o the gusset plate must be cut out as shown inFigure 6 (opposite page). It is more economical or shops tocut square openings, but the optional diagonal cut shown at

the inside corner o the plate can signifcantly increase thebending strength o the gusset plate legs. The optional cut at the outside corner is used to ensure that the maximum edgedistance requirements in the AISC Specifcation are met.

Connections at the gusset-to-beam interace typically

are made with single clip angles on one side o the gusset.Usually the feld bolts are easier to access when the clipangles are placed on the top side o the gusset. For largerloads, double clip-angles (one on each side o the gusset)are common. Figure 7 shows a detail where the nominalminus dimension is 14 in. A single clip angle is used toconnect the gusset plate to the W18 beam. On the oppositeinterace, the gusset plate is bolted directly to the ange o the W14 beam. In practice, fller plates are common wherethe gusset is bolted directly to the beam ange, due to the

varying depths o the beams within a braced bay.Gusset plates usually are shop bolted to the brace orshipping to the feld. This is more efcient or erection,because the single assembly can be located and orientedeasier than i the brace were shipped separately rom the

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july 2010 MODERN STEEL CONSTRUCTION

gussets. On rare occasions, such as where shipping space isat a premium, it may be better to ship the pieces separately so they can be nested.

Connection Desin

Although the orce distribution in horizontal brace con-nections is indeterminate, the gusset-to-beam interaceorces can be determined easily. Figure 8 shows a plan view o a horizontal brace connection at a beam-to-beam inter-section. The engineer must assign a orce path to distributethe axial load in the brace to the gusset-plate to beam-webinteraces. Using a deection compatibility approach, theinterace orces shown in Figure 8a are incorrect because

the stiness parallel to the beam webs is much greater than

the stiness perpendicular to the beam webs. The smallload entering the web in the transverse direction can beneglected, which results in the interace orces shown inFigure 8b. Additionally, the orce distribution in Figure 8buses the simplest and most direct path to get the compo-nents o the brace orce directly into the beams.

Gusset plates are designed so the stresses on any crosssection o the plate are lower than the design stresses. Theselection o the most highly stressed section is at the discre-tion o the Engineer-o-Record and is based on judgment andexperience. Traditionally, beam equations have been used todetermine the stresses at the critical sections, and the normalstress and shear stress are usually considered separately.

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Horizontal Bracing

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