Builders nationwide are becoming increasinglyfamiliar with shear walls, especially along theearthquake-prone West Coast and in the wakeof recent hurricanes in the East. Theyre aware of the kinds of metalconnectors involved, andtheyre accustomed to hav-

ing the inspector check plywood nailing schedules andframing layouts.

But shear walls are just one element of earthquake-and wind-resistant construction. Less obvious, andsomewhat more difficult to build, are the collectors.Also known as drag-struts or drag-ties, collectorsgather the lateral earthquake loads from a large areaof a building a roof or floor diaphragm, for exam-ple and deliver them to a structural element, suchas a shear wall, that can resist the force. But unless thecollectors are properly built, the shear walls will beineffective.

The lateral forces from earthquakes or high windsare spread out over the entire area of the roof or floordiaphragm. And yet many contemporary customhomes have only small areas of shear walls compared

with roof and floor areas. A collector gathers the forcespread through the diaphragm and transfers, ordrags, it to the shear wall.

Tension and Compression ForcesFor a collector to work, it must meet the following

conditions: Forces must actually get to the collector. The collector must be continuous (or be composed

of elements joined together to act as one continuousmember).

It must have tensile capacity. It must be able to resist compression.

Seismic forces cycle back and forth, which is whythe collector is stressed alternately in compression andthen in tension. To design a collector that will resistboth tension and compression, engineers must con-sider that high winds may come from any direction.

The most common collector in a typical wood-framehouse is a wall top plate. But with more complexhouse configurations, theres not always a wall plateavailable to act as a collector. In such a case, you mayhave to use another member for the collector a truss

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LATERAL-FORCE COLLECTORSfor Seismic and

Wind-Resistant Framing

by Thor Matteson

P R A C T I C A L E N G I N E E R I N G

Without these vital structural links, even

perfectly constructed shear walls may not protect

a house from an earthquake or windstorm

Forces Carried by a Collector

Wind or earthquake force of4,800 lbs. acting in this direction

4,800-lb. forcein shear wall

Force collected from 32'of roof builds up intop plate to 3,840 lbs.at this point

Force in roof diaphragmof 120 lbs. per linear footShear

wall

4,800-lb. resistingforce in foundation

Window wall cannotresist lateral forces

32'

8'

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Figure 1. The top plate above the windows in the front wall of thishouse must be able to collect the cumulative lateral force from theroof diaphragm 120 pounds per foot in this example anddeliver it to the shear wall at the left end of the building. An ordinarydouble top plate, as commonly built, would not be up to the task.

or beam, for example or you may need to assembleone from blocking and metal strapping.

Top Plates as Collectors Look at the example in Figure 1 (previous page), a 40-

foot-long house with a long window wall along theeaves. Lateral forces transfer from the diaphragm sheath-ing in the roof to the eaves blocking, and from there intothe top plate. To ensure a complete load path, you mustnail the sheathing into the eaves (frieze) blocks, which inturn must be securely fastened to the top plate.

In a typical house, these forces might range from80 to 120 pounds per foot along the eaves. In thishouse, the designer left only 8 feet at the end of the

window wall for the engineer to use as a shear wall.We have 32 feet of windows that can resist no shear,so the top plate must collect all the force in the 32feet of roof diaphragm above the windows and dragit to the shear wall.

The diaphragm force in this example 120pounds per lineal foot of roof, acting parallel to theeaves wall builds up in the collector as we getcloser to the shear wall. At the right end of thehouse, the force is zero. Ten feet closer to the shearwall, the force is 1,200 pounds (10 feet x 120pounds/foot = 1,200 pounds); another 10 feet closer,the force is 2,400 pounds. Finally, when we have col-lected all 32 feet worth of diaphragm force, we have

a total of 3,840 pounds of force in the topplate. Note that were showing tensionforce in the illustration, but if the windwas blowing in the opposite direction, orthe earthquake forces were reversed, thecollector would act in compression alongthe top plate.

Top plate collectors can typically carrythe compression force, but they often needadditional tension capacity. A single 2x4top plate in a standard grade would not beable to carry 3,840 pounds of tension force.A double top plate might work if we rein-forced any splices in the plates. As a mini-mum, the IBC (International Building Code),the IRC (International Residential Code), andthe UBC (Uniform Building Code) all requireeight 16-penny nails at lap splices. Thisrequirement was new as of 1994 and essen-tially doubled the required nailing atsplices. Even so, using allowable code val-ues, this minimum nailing requirementwould resist only about 1,000 pounds,assuming you used sinkers. Because this isfar short of the 3,840 pounds we need inthe example, we need to splice across thejoints in the double plate with sheet metalstraps to carry the tension force.

Other members as collectors. Trusses,joists, rafters, and other such continuous

elements can easily serve as collectors. Figure 2 showstwo instances where a shear wall is connected by astrap to a collector element in line with it. The strapprovides tensile capacity. To resist compression, the

Figure 2. The strap in the photo above carries tension forces from thecollector truss to the shear wall beyond (lower left in the photo). In thephoto at top, straps running in both directions connect the wood I-joistsecond-floor framing to the shear walls below.

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Building a CollectorWhen Framing Runs Perpendicular to Shear Wall

Solid blocking transfers force fromthe floor diaphragm to the strap below,and carries compression forces

Shear wall

Floor framing above shear wall

When the force actsaway from shear wall,the steel strap collectsforce from each block andcarries it to the shear wall

Steel collector strap

When the diaphragm force actstowards the shear wall, the collectoris acting in compression and thestrap is not engaged at all

Collector strap;nailing from strap toblocks typically need notexceed nailing fromfloor sheathing to blocks

Solid blocking

Floor sheathingnailing to blocking

Floor framing

Floor sheathingnailing to blocking

Collector strap;nailing to shear wallmust handle tensionfrom row of blocking

Connect blocking toshear wall top platewith framing anchorsor toe-nails (resistscompression forces)

Solid blocking

Shear wall

4 nails each bay

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Figure 3. Blocking is a criticalcomponent of a collector when theframing runs perpendicular to theshear wall. The solid blocks in thephoto (next page) handle compres-sion force, while the strap collectstension force and carries it to anearby shear wall, as shown here.

truss must bear against eaves blocking, which in turnmust connect to the top of the shear wall.

Building Collectors From Scratch When you dont have a continuous member to act as

a collector, you have to build one. While its easy toreinforce the splices in a top plate, assembling a collec-tor that runs perpendicular to framing members takesgreater effort.

Figure 3 (previous page) shows a collector assembledbetween second-floor framing by installing blocks tiedtogether with a steel strap. In this case, the strap andblocks are collecting the force from the floordiaphragm and carrying it to a shear wall beyond.

Blocks. The nails from the diaphragm sheathing intothe blocks transfer a small amount of lateral force fromthe diaphragm into each block. How the force gets fromeach block to the shear wall depends on the direction inwhich the force acts. If the diaphragm force is pushingtoward the shear wall from the row of blocks (from rightto left), the collector is acting in compression and thesteel strap is not engaged at all. Starting with the blockfarthest from the shear wall, each block pushes againstthe next in the direction of the shear wall. The secondfarthest block from the shear wall has one other blockpushing against it; the next block has two blocks push-ing against it, and so on. By the time we reach the shear

wall, the last block in line pushes againstthe shear wall with the force collectedfrom the whole row of blocks.

Strap. When the force in Figure 3acts in the opposite direction, eachblock transfers a small amount of forceto the strap. The strap collects the forcefrom all the blocks and delivers it to theshear wall.

You can nail the strap either directly tothe blocks or through the sheathing intothe blocks. Figure 4 shows a strap nailedto the top of the blocks before the floorsheathing was installed. The carpenterrouted a 1/8-inch slot for the strap so itwould not create a lump in the sheath-ing. Note that this requires nailing thestrap to the blocking, then nailing thesheathing to the blocking. If a strap isinstalled on top of the sheathing, as in

Figure 4. This strap was installed from the top before the floor sheathing was naileddown. The carpenter routed out the tops of the framing members to avoid a bulge inthe carpeted floor above.

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Figure 5, the same nails can connect both the strap andthe sheathing to the blocks. Remember that the blocksare important not only because they provide backingfor the strap but also because they carry the compres-sion forces when the collector acts in compression.

Figure 6 shows a collector assembled from wood I-joist blocks. The strap nailed along the bottom of the

blocks collects the force fromeach block and carries it to ashear wall just outside thephoto. Note that because thereare no web stiffeners in the I-joists, the blocks bear solidlyonly at the joist flanges andhave gaps at the webs. Whenthe sheathing exerts a force inone direction at the top of theblock and the strap exerts aforce in the opposite directionat the bottom, the block willwant to roll. Connecting theends of the blocks to the joistswill prevent this. For a relativelysmall collector such as this one,I would usually consider that

the blocks are held tightly enough on three sides so thatthey will not tend to roll. But if this collector wasdesigned to carry a large load, the blocks would need tobe toe-nailed to web stiffeners installed on both sides ofevery I-joist. Obviously, compared with using solid lum-ber framing, this becomes very labor intensive.

Use only as many nails as needed. Believe it or notand contrary to common advice, you dont usuallyhave to fill every nail hole in a strap used for a collec-tor. Remember that what youre doing is transferringforces from the floor sheathing into the strap and thento the shear wall. Look at Figure 6 again. Notice thatthe strap has about ten nails connecting it to eachblock obviously installed by a carpenter intent onfilling the holes. Yet how many nails do you thinkconnect the sheathing to each block? If only three orfour nails connect the sheathing to the block, we cer-tainly dont need more than three or four nails fromthe block to the strap. Too many nails may actuallysplit the wood member, which would defeat the pur-pose of the strap. Some inspectors may insist that younail all the holes in a collector strap. If you want to

Figure 6. This collector was assembled from wood I-joist blocks and strapping.Ideally, the ends of the I-joist block webs would bear solidly against web fillers inthe floor joists.

Figure 5. This strap was nailed to blocking below throughthe floor sheathing.

Figure 7. This strap, which runs from the top of a shearwall, will connect to the bottom of a roof truss above. Notethat none of the trusses on the regular layout fell in linewith the shear wall, so the builder had to add an extra off-layout truss to catch the strap. The roof sheathing will alsohave to be solidly nailed to this truss so that it picks up lat-eral forces in the roof diaphragm.

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push your luck, you could try to explain to theinspector why straps do not always need completenailing. Otherwise, do what the inspector tells you,trying your best not to split the wood.

The strap in Figure 5 was designed and inspected bya structural engineer. It has just the right number ofnails to collect the forces along 45 feet of diaphragmand deliver them to a shear wall.

Follow the Load PathThe collector provides the load path to the shear

wall. But the forces we want to collect are in the flooror roof diaphragm. So it is critical to nail the sheathingto the collector truss or beam, or to the row of blocksacting as the collector.If the collector memberdoes not fall on the reg-ular framing layout, itspossible that nailingthe sheathing to thecollector will get over-looked and there willbe a gap in the load path.

Figure 7 (previouspage) shows a strapthat will connect to aroof truss. Becausenone of the roof trussesin the regular 24-inchon-center layout fell inline with the shearwall, the builder had to add an extra truss to pick up thestrap. In a case like this, the carpenter nailing off thesheathing must be careful to nail the roof sheathing tothat truss to complete the load path. The plans shouldclearly note this additional nailing requirement.Similarly, when collectors run perpendicular to theframing, it is easy to overlook nailing the sheathing tothe row of blocks unless the strap happens to land ontop of the sheathing to remind you. Again, the plansshould show these connections.

The photos in Figure 8 show a logical framingsequence. First, the straps get nailed to the top platesof adjoining support walls. Then, after the roof trussesare installed, the straps get nailed to the bottom of thegirder truss and to the blocking between the common

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Figure 8. In this framing sequence, straps attached to thetops of shear walls are left hanging (top) until the installa-tion of a blocked girder truss (above left), which will act asthe collector for a section of roof. Finally, the straps arenailed to the bottom of the girder truss (above).

trusses. The girder truss, blocks, and straps all act asthe collector. Nailing the main roof sheathing to theblocks completes the load path from the roofdiaphragm to the shear wall.

Common Collector PitfallsMost problems with collectors arise when the fram-

ing members run perpendicular to the force we need tocollect.

Many short straps do not equal one long strap. Theline of blocks shown in Figure 9 willserve as a collector in compression, butwill fail miserably in tension. At somepoint along the line, the collectedforces will overwhelm the individualstrap connections. Think of it in termsof a game of tug-of-war. If the membersof one team are each holding a sepa-rate length of rope, theyll have a hardtime beating a team whose membersare combining their grips on a singlelength of rope (Figure 10). The personat the very front of the line on the separate-rope team would be the onlyperson holding the rope that the other

team is pulling on, and his grip would have to holdthe combined force of his entire team.

Its important for building designers not only toaccount for the tension force that a collector mustcarry, but also to detail how to build the collector.

Figure 9. These photos illustrate a common mistake inbuilding collectors using many short straps instead ofone long one. Each short strap must carry the cumulativeload to that point in the collector, meaning that the strapnear the end of the collector is likely to fail.

Figure 10. Think of a collector strap in terms of the game of tug-of-war. The team holding many short ropes will have a tough timebeating the team whose force is combined in one continuous rope.

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Problems like the grossly overloaded collector inFigure 9 can arise when the plans use vague notes likedrag-tie or block and strap in line with wall.Engineers shouldnt expect carpenters to build some-thing that is not shown adequately on the plans.

Gaps in rows of blocks can lead to failure. In con-trast to the collector in Figure 9, Figure 11 shows a col-lector that will fail in compression. The gap in the rowof blocks will need to close before the collector candeliver much force to the shear wall to the right (out-side the photo). You can bet this gap would not closegently during an earthquake. While the floor sheath-ing would handle some level of lateral loads, therepeated back-and-forth movement of the housewould cause this gap to close and open many times,slamming the separate building segments against eachother with thousands of pounds of force.

Furthermore, the strap spanning the gap will buckleand then straighten out each time the gap closes andopens. After several cycles, the strap will fatigue andbreak, which could allow the gap to open up. Then thetwo building sections might separate completely orjust bash each other to bits.

Many commercial buildings collapsed or sufferedmajor damage in the 1994 Northridge earthquake dueto collector failures. Those failures prompted codechanges that reflect the importance of collectors.When sizing collectors in such buildings, designersmust now use an additional safety factor of about175% of what the codes previously required. Thesechanges do not yet affect buildings with wood-framedshear walls, but they do indicatean awareness of the importanceof collectors to structural safety.

Just because you can bendstraps doesnt mean you should.For a steel strap tie to work effec-tively, it must be installed with-out kinks, bends, or twists. Thestrap shown in Figure 12 has lostsome of its strength because ofthe twist pounded into it. It willalso tend to straighten out whenits put in tension, which in thiscase could displace the top plateor the joist that it connects to, aswell as rip nails out of thosemembers.

Figure 11. The missing block in this collector between thegirder and the I-joist to its left could prove costly in anearthquake: The back-and-forth cycling of the quake willsmash the two members together, folding and stretchingthe strap below until it fails.

Figure 12. Straps should not be bent: The kink in this strap weakens it.

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Watch out for plan changes. Figure 13shows what can happen when a stockplan gets changed without accounting forthe lateral load path. These two houses arelocated in the same tract and have identi-cal floor plans. Presumably to make thehouses look different, the front-facing gable of House Awas changed to a hip in House B (next page). The effectwas that the same strap running from the shear wall tothe collector truss in House A runs to nowhere in HouseB. The carpenters had nothing to connect the strap toin House B, so they added some blocks perpendicular tothe bottom chords of the first two trusses. But becausethe trusses have almost no strength in that direction,theres no way for the forces from the roof diaphragm toeven get to the strap and the shear wall beyond.Correcting this problem would involve installingsheathed frames between the trusses from the bottomchord to the roof sheathing, then nailing the strap

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Figure 13. These photos were taken of two nearly identicalhouses in the same tract and illustrate how a change in theengineered plans can render a collector useless. In House A,which was built according to the stamped plans, the sectionof roof in the small front-facing gable is tied by a strap run-ning from a shear wall at the corner of the house to a col-lector truss running in the same direction (see plan).

Gable rooftrusses

Collector truss(in line withshear wall)

Photo taken here Collector path

Shear wall

House A

A

along the entire row of frames at the bottom,and nailing the roof sheathing to the frames atthe top. As installed, the blocks shown inHouse B will not collect any force except fromthe ceiling; during an earthquake or wind-storm, they would do no more than rip out asmall area of ceiling drywall.

This is a common problem in tract home construc-tion. Too often the plans get changed by the architect,owner, or truss manufacturer after they have left theengineers office. That is one reason hiring the designengineer to observe construction progress is recom-mended almost universally by the experts who examinedestruction after hurricanes or earthquakes.

Thor Matteson, S.E., is a structural engineer inMariposa, Calif. This article was adapted with permissionfrom The Wood Framed Shear-Wall ConstructionGuide, available at www.shearwalls.com or through theInternational Code Council (888/699-0541).

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Hip trusses(perpendicularto collector andshear wall)

Photo taken here Collector path

Shearwall

House B

Blocking at trussbottom chordineffectiveas collector

BIn House B, the builder chose to modify the smallgable by making it a hip roof. That resulted in hiptrusses running perpendicular to the shear wall.The carpenters made an effort to connect thestrap to something solid by adding some blockingbetween the first two hip trusses. However, sincethe blocking doesnt extend to the roof, where itcould connect to the sheathing, it will do a poorjob of transferring diaphragm forces from the roofto the shear wall.