Wood Truss lateral bracing
Wood Truss lateral bracing
(OP)
I am having an argument with a contractor regarding the lateral bracing of roof trusses and would appreciate some feedback. In every other wood frame project I have worked on, the detailing of the lateral bracing of web members (where required) is specified in the truss shop drawings since the design of trusses is a design/build item. In this particular project, the truss manufacturer is saying that it is our responsibility as the EOR to design the lateral bracing. I am at a loss since this issue has never come up before in the dozens of such projects of this type that we have done. Has anyone else had any experience with this and can offer some advice? Thanks!






RE: Wood Truss lateral bracing
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Generally bracing of web members (bracing that is required to help the components of the trusses perform) are designed by the truss manufacturer's engineer...not the EOR.
Bracing that takes lateral building loads and transfers them to other parts of the structure, perhaps through the trusses, is the responsibility of the engineer of record.
This would include top chord truss bracing (roof sheathing) and bottom chord bracing (ceiling sheathing perhaps), and gable end-wall bracing, etc.
RE: Wood Truss lateral bracing
In fact, by some interpretations of IBC 2000, this code allowed the truss suppilier to shove this responsibility off on the EOR ( IBC 2000 Section 2303.4.1 Part 17).
IBC 2006 has tried to close this loop hole (IBC 2006 Section 2303.4.1.5).
In our specifications we specifically have closed this loop hole becasue it became such a hassle.
RE: Wood Truss lateral bracing
The truss manufacturer stated that it is their responsibility to indicate which webs required permanent lateral bracing, but the specification of the bracing was the responsibility of the EOR.
A heated group of engineers then started saying that we as engineers should be able to leave the design "from the double top plates on up" to the truss manufacturer, who has a truss engineer responsible for their design. The truss manufacturer indicated that this was not the case. They claim that they design "components" of the roof, not the "roof system" itself.
A Simpson Strong-Tie representative was also present stating that it is the responsibility of the EOR, and of course indicated that Simpson has a truss spacer/bracer that could be used as temporary and permanent lateral bracing (note that the main types of structures discussed were homes and small commercial buildings that would have the top and bottom chords permanently laterally braced by the roof sheathing and ceiling drywall respectively).
Building Officials and plan checkers of the surrounding area were also present and they indicated that some of them were already asking for the permanent lateral bracing designs from engineers. Those who hadn't been doing so said they'd soon start asking for this as well.
For a reference, they handed us the "BCSI" manual, which is the Building Component Safety Information manual, a "Guide to good practice for handling, installing, restraining & bracing of metal plate connected wood trusses"
One thing that I wish I would have thought of at the seminar was this: If there are homes out there that get built without the need of an engineer or other design professional, who will then be responsible for this permanent lateral bracing??
I know this will probably get most of this forum's engineers angry, as was the case with the engineers that were present at the seminar. I just wanted to inform you what I found, so please don't kill the messenger.
RE: Wood Truss lateral bracing
It is really nothing more than putting the requirements illustrated in Truss Plate Institue HIB91 publication on the drawings. Pretty much every set of truss shop drawings I receive has a copy of this publication included with the drawings, but my understanding is that the EOR is responsible for specifying it.
This is much different that required member bracing for design purposes within the truss. For example if a truss web member requires a continuous brace to meet the Le/d requirements, then this design is the responsibility of the truss manufacturer and NOT the EOR.
RE: Wood Truss lateral bracing
If there is no EOR, and the building is prescriptive, then the lateral bracing for the roof trusses should be generically spec'd by the IBC, BOAC, or whatever. If this is not spelled out by the appropriate building code, then it needs to be.
But, hey, more responsibility = more fees, right?
Mike McCann
McCann Engineering
RE: Wood Truss lateral bracing
We need to start requiring the truss manufacturer's to use the various products to reinforce those webs.
RE: Wood Truss lateral bracing
Truss manufacturer's are responsibile for indicating permanent web bracing. And they do that. I believe we are talking about permanent lateral bracing of the roof system, not the individual trusses.
RE: Wood Truss lateral bracing
Mike McCann
McCann Engineering
RE: Wood Truss lateral bracing
I definitely agree that the EOR is responsible for providing adequate bracing of top and bottom chords since they form part of the main structure. But I still think the truss designer needs to specify the bracing for the individual truss members. That seems to be the general consensus from what I've read here.
RE: Wood Truss lateral bracing
Assuming we are still talking about wood trusses, just use 2X4 nailed flat to the web member with (2) 12d at the brace point and be done with it.
Mike McCann
McCann Engineering
RE: Wood Truss lateral bracing
I am not sure if this is written in a standard somewhere or if this is what my company has adopted, but that is what we do.
RE: Wood Truss lateral bracing
RE: Wood Truss lateral bracing
For what it's worth, the bracing designs I've seen look very similar to the stuff shown in that BCSI guide.
I have a hunch that the reason TPI doesn't want to design the roof as a "system" is that every truss plant would need and engineer on staff locally to review each system, and then there would be alot more liability on the individual plants. As it is now, the engineers that "Stamp" the component drawings are not employed by the plants, but by the plate/software company and probably never even see the layouts.
RE: Wood Truss lateral bracing
For the most part, you are correct. Most plants do not employ a P.E. to "stamp" the component drawings. I, however, happen to be employed by a truss manufacturer, and "stamp" our truss design drawings. The issue really goes one step further. And that is, in residential construction, unless it is required by code you aren't going to get a truss manufacturing plant (or anyone else) to provide a lateral design for the roof system. The client will not pay for it, and we aren't going to do it for free.
RE: Wood Truss lateral bracing
However, in the above case the drawings and specifications require the truss supplier to design lateral bracing. The truss supplier then has a choice between supplying trusses that comply with the specifications or not supplying the trusses at all.
Before an engineer writes such a specification though, he or she should understand some of the othe issues that need to be considered in producing a final design. Cost, is one of the first issues. An engineer could write a specification that required all truss webs to carry their loads with out any out of plane bracing. The end result would be very expensive trusses.
Another problem with making a truss supplier responsible for everthing above the top plates, is that common building elements that typically brace the top and bottom chords, are not supplied by the truss supplier. And in most cases all the elements are installed by some one other than the truss supplier.
I design heavy timber truss which are often supported on concrete piers. It is not uncommon to see the drawings requiring the truss supplier to design the embeds. Embeds which are generally supplied by the misc. steel supplier.
I try to explain to the EOR, that first of all I am not an expert in embeds, I am an expert in wood design. Then I explain that if he or she wants me to design the embeds I am going to require 6,000 psi concret with #11 bars and the embed plate is going to be 2" thick with 24" long head studs.
The point I am trying to make above is that the EOR is trying to get me to design elements that I am not going to supply. Not being an expert on embed design I am going to make it large enough that I feel comfortable with the design.
RE: Wood Truss lateral bracing
Given that engineers typically do all their design prior to having the trusses designed the truss roof system design is left to be done “by others” (very inefficient design process by the way from an engineering economics perspective but that is another story that will be changing over time as well. For a look at the future see http://www.inteframe.com). Given this, the default roof system bracing is defined in BCSI B3 on permanent bracing. See http://www.sbcindustry.com/bracing.php for more information on industry bracing.
BCSI is referenced in the IRC for residential construction so B3 should be applied in all cases there. Section 2303.4 of the 2006 IBC does a good job defining the truss design process and truss design drawings. Unfortunately we could not reference BCSI in the IBC but that is what is meant by “standard industry details.”
Thanks for the interest in bracing issues and please call any of our WTCA technical support staff for any questions that this may elicit. Here is link to WTCA staff and contact information http://www.sbcindustry.com/staff.php.
Kirk Grundahl, P.E.
Executive Director of WTCA - Representing the Structural Building Components Industry
http://www.sbcindustry.com
RE: Wood Truss lateral bracing
I appreciate the information you have provided. I am still wrestling with this issue, so bear with me. The documents that you referenced specifically say the EOR is responsible for specifying the size, grade, and connection for CLB of web members where required by truss designer to reduce the unbraced length of the web memebers and that this bracing should be designed for the appropriate "lateral loads". What are these lateral loads that we must design bracing for? Is there a standard that defines these loads?
RE: Wood Truss lateral bracing
For a CLB to be effective in bracing web members, there need to be diagonal braces at some spacing. The CLB acts like a drag strut, and the daigonal brace prevents all of the webs from buckling together.
On our wood truss projects, we show a typical web member stabilization detail. We show the CLB and specify the size and spacing of diagonal braces. We tell the contractor to coordinate with the actual truss configuration by the truss designer for which webs need bracing. We also specify that the truss designer is responsible for the design of T or L bracing when dissimilar braces are next to eachother and a CLB is not possible.
RE: Wood Truss lateral bracing
Thanks for the input. That is the exact document I was looking at when I posted my previous question. I understand the concept of how CLB works. What I'm trying to figure out is how I determine the loads that the CLB needs to be designed for so I can provide a size and detail the connections. Any references would help.
RE: Wood Truss lateral bracing
It may help to read TPI 1 chapter 2 and BCSI B3 (good cure for insomnia) and let this information settle in some as there is a good deal of information here that I think makes good common sense but the volume of words is a bit tough to get one's brain around immediately.
Again our tech staff can help via phone as some of this is hard to explain in a forum like this.
Thanks and hope that this helps some.
RE: Wood Truss lateral bracing
RE: Wood Truss lateral bracing
1) Use 2% of the max. axial compression force and accumulate it to a diagonal brace to the diaphragm or some type of bearing.
2) You can use BCSI B3 standard permanent bracing details.
3) You can use T, L or scab bracing of the web members.
4) You can select your own % of the max axial force you want to use to accumulate the lateral bracing force restraint needed.
a)Based on the testing we are doing we do not believe that 2% is accurate and we are going to fix this in the future.
5) Our new industry testing facility can test full scale structures up to 30' wide by 32' high by 90' long, so we will get a solid understanding of temp. and perm. bracing in the near future. For more information here please see the http://www.sbcri.info/.
Again any questions you have on this that anyone would like more inforamtion with respect to, please call our tech staff.
RE: Wood Truss lateral bracing
I'm confused. The document "Commentary for Permanent Bracing of Metal Plate Connected Wood Trusses" says that where continuos lateral bracing (CLB) is required to reduce the buckling length of a web member, the Truss Designer determines the location but it is the responsibility of the Building Designer to determine "the size, grade, and connection of this bracing". But you're saying that this bracing should be on the Truss Design Drawings. Am I missing something?
RE: Wood Truss lateral bracing
I think what you may be missing is that with CLR's (I believe the industry is changing the terminology from CLB to CLR to help clarify that these alone are not fully bracing the webs) there are two components to the bracing. One is the CLR which is connected from truss web to truss web. This is generally spelled out on the truss enginnered drawing (size/grade and connection to web). The second component, which I believe is the responsibility of the EOR, is to design the diagonal bracing (or the bracing for the CLR's). The connection of this to the webs and to the sheathing/drywall/shearwall etc. is the responsibility of the EOR. I hope this helps clarify things a bit.
RE: Wood Truss lateral bracing
you said,
"4) You can select your own % of the max axial force you want to use to accumulate the lateral bracing force restraint needed.
a)Based on the testing we are doing we do not believe that 2% is accurate and we are going to fix this in the future. "
I'm curious, does testing indicate that 2% is overconservative or not enough?
RE: Wood Truss lateral bracing
We believe that 2% is a pretty conservative number.
Kirk
RE: Wood Truss lateral bracing
Do premanufactured metal building designers / engineers rely on the engineer for the foundation to specify bracing above the column baseplates to keep their building from falling down when subjected to its design loads? Not that I've ever seen.
Do premanufactured metal silo designers / engineers rely on the engineer for the foundation to specify bracing above the base to hold the silo in place under its design loads? This would be unusual, to say the least.
Do manufactured home designers / engineers rely on the engineer for the foundation to specify bracing above the foundation to help their structure resist the loads it was supposed to be designed for? I don't think so.
What we are seeing is no more than a cheap tactic by the premanufactured truss industry to have the building engineer share liability / responsibility if their trusses fail. What they don't realize is that truss companies that persist in requiring the engineer (for the building) to assume any responsibility for helping to brace off their trusses, will gradually see themselves become less relevant as those companies that take care of this obtain a larger and larger market share.
When working with our clients, if we come across a truss company that tries to pull this, we encourage the client to go elsewhere and they listen. We've been with them from the start and the truss company is a late comer in the process. Who are they going to listen to?
RE: Wood Truss lateral bracing
I think a more appropriate comparison is steel joists systems that indeed provide all the lateral bracing required for their system. They don't ask EOR to brace their webs or chords because they are too slender. I do not agree with your comparison with metal buildings in they are a complete (stand alone) building structure above the foundation.
I am somewhat understanding of the Wood Truss industries delima but I do not understand why they can't solve the problem as the steel joist industry has chosen to solve it instead of putting the responsibility on the EOR to adequately brace the wood trusses?
Perhaps, it is the wood truss industry's desire to show how low cost wood trusses are, but then there are the hidden bracing costs which are not known until the engineer and contractor see the wood truss shop drawings.
The GC tries to evaluate the cost of various truss manufacturers that may have different costs of bracing because of the way they choose to configure and the material they use to build their trusses. Generally, the lowest cost trusses may require more bracing. If the trusses and bracing were furnished as one complete package, then the costs could be evaluated on the same basis.
RE: Wood Truss lateral bracing
RE: Wood Truss lateral bracing
WTCA and TPI have worked very hard to define reasonable responsibilities for all parties involved in the construction process. The latest version of ANSI/TPI 1 Chapter “Responsibilities in the Design and Application of Metal Plate Connected Wood Trusses” can be found at this link -- http://www.tpinst.org/my_TPI1PC.htm#Downloads. This was a very challenging development process and from my perspective is the best consensus document that could possibly have been developed. The National Council of Structural Engineers Association was intimately involved in this consensus process as well which was extremely helpful.
Given that engineers typically do all their design prior to having the trusses designed the truss roof system design is left to be done “by others” (very inefficient design process by the way from an engineering economics perspective). Given this, the default roof system bracing is defined in BCSI B3 on permanent bracing. See http://www.sbcindustry.com/bracing.php for more information on industry bracing.
BCSI is referenced in the IRC for residential construction so B3 should be applied in all cases there. Section 2303.4 of the 2006 IBC does a good job defining the truss design process and truss design drawings. Unfortunately we could not reference BCSI in the IBC but that is what is meant by “standard industry details.”
Thanks for the interest in bracing issues and please call any of our WTCA technical support staff for any questions that this may elicit. Here is link to WTCA staff and contact information http://www.sbcindustry.com/staff.php.
Kirk Grundahl, P.E.
Executive Director of WTCA - Representing the Structural Building Components Industry
http://www.sbcindustry.com
RE: Wood Truss lateral bracing
RE: Wood Truss lateral bracing
Just hope that everyone considers the work that has been done in this realm as work that has been the best work we can do at arriving at a consensus with the intention of good being done for the industry overall.
If not that is OK as well.
RE: Wood Truss lateral bracing
I would think one would have a little more respect for someone who has spent years of work attempting to define the responsibilities in the industry clearly to all. To me, the responsibilities are very clear. I have never seen it to be a problem with any of the structural engineers in my area complying with the responsibilities spelled out in WTCA/TPI.
RE: Wood Truss lateral bracing
Is your question relevant to the topic at hand? Or was it sarcasm for the sake of sarcasm?
Kgrundahl's post assumed that I had not read "the material referenced in this post". Reposting a previous post did not help in that regard. I have looked through these documents and have seen what they hope to require of building designers. Needless to say, I disagree with some of what they have w/r/t professional responsibilities surrounding the trusses.
It was mentioned that "National Council of Structural Engineers Association was intimately involved in this consensus process as well which was extremely helpful." This may be the case, but according to the one referenced document, they held only one seat of apparently 47 "project committee members" (see pages 4 and 5 of the document).
Most of the committee members are / were from the truss / "building components" / wood industry. Names such as MiTek (three seats on the committee; the most of any organization; also chaired the committee), ITW (two seats on the committee), Robbins, Eagle, Keymark, and WTCA should be familiar. Others related to the industry on the committee include Cherokee Metal Products Inc. (truss plate mfr), Rigidply Rafters Inc. (roof truss mfr), CompuTrus, Inc. (truss connector plate supplier), Letherer Truss Inc. (truss supplier), Lumber Specialties Ltd. (truss mfr), Stafford Inspection (truss inspection), etc. These clearly represent the interests of the truss suppliers / manufacturers, plate suppliers / manufacturers, etc.
Many others were from aspects of the industry that probably don't really care who ends up with the responsibilities we're discussing in this thread, just as long as someone has the responsibility. Their goal is to promote construction with trusses.
There did not appear to be too many members representing the building engineers. Calling it a "consensus document" or a "consensus process" seems to be a bit misleading. Hint: Not all of the committee members with names that end in "Engineering" represent or speak for individual building engineers.
RE: Wood Truss lateral bracing
RE: Wood Truss lateral bracing
Mr. Michael Kozlowski, P.E. Apex Technology
Mr. Tom Zgraggen, P.E. Aries Engineering, Inc.
Mr. Colin Bailey, P.E. Baily and Son Eng.
Mr. Timothy W. Ott, P.E. Callahan Associates, Inc.
Mr. Kent Reimschussel, P.E. Church of Latter Day Saints
Mr. Samuel A. Greenberg, PE Dansco Engineering
Mr. Aaron M. Reed, PE, MBA Deadlines Engineering, Inc
Mr. Dennis Wish, P.E. Dennis Wish, P.E.
Mr. James R. Brown, P.E. Engineering Consultants, Inc.
Mr. John E. Grenier, P.E. Grenier Structural Engineering, Inc.
Mr. John E. Meeks, P.E. John E. Meeks, P.A. Consulting Engineer
Mr. William Walsh, S.E. P.E. KRW Consulting Group, LLC
Mr. Kenneth A. Watters, II PE KW Engineering
Ms. Stephanie J. Young, P.E. Mattson Macdonald Young
Mr. Scott Douglas, P.E. National Council of Structural Engineer Assoc. -- DCI Engineers
Mr. David Weaver, P.E. Nederveld Associates, Inc.
Mr. Norman Scheel, S.E. Norman Scheel, S.E.
Mr. Shawn Reeder, P.E. Performance Engineers
Mr. Larry Beineke, PhD, P.E. PFS Corporation
Mr. Philip Brazil, PE, SE Reid Middleton, Inc.
Mr. Brad Crane, P.E. SCA Consulting Engineers
Mr. John Gruber, P.E., S.E. Sheppard Engineering, P.C.
Mr. Ed Huston, P.E. Smith & Huston Inc.
Mr. Patrick Edwards, P.E. Timber Products Inspection
Dr. Michael Triche, P.E. University of Alabama - Civil Engineering Dept.
Prof. Steven Cramer, P.E. University of Wisconsin - Madison
Mr. John G. Ernst, P.E. Wade-Trim, Inc.
RE: Wood Truss lateral bracing
Why aren't many of these names listed on pages 4 and 5 of the document under the "Project Committee Members"? In other words, it appears that many of these names weren't part of the committee that voted on the approval on the document.
RE: Wood Truss lateral bracing
1) Not everyone desires to spend the time needed to read, digest, comment, improve and vote on the entire TPI design standard and that is the role of a pure project committee member and it does take time and industry knowledge to do that well.
2) Not everyone has the time to devote to even all the details that were worked on with respect to Chapter 2.
3) Our desire was to ensure that we had a broad base of input from the engineering and building official community that has expressed an interest in our industry. To that end we sent out more than 1000 invitations to everyone we had on our mailing list that was a technical and/or code person to attend a meeting in May of 2006 to discuss these issues.
4) These are the people that provided perspective, challenge and advice and are bright people that were very helpful to us and this document.
As I believe I have said previously; from my perspective the development of this chapter 2 language was one of the most challenging projects that I have ever personally been involved with and the best consensus based document result we could provide at this stage of this work.