Wood truss web member bracing
Wood truss web member bracing
(OP)
I had occasion to look through the access opening into the truss space of an apartment building recently. All web members were 2x4 No.1/2 SPF and were attached to the top and bottom chord using truss plates.
Some of the compression web members were tied together using a continuous lateral brace (CLB) but were otherwise unbraced. Thus there were six trusses spaced at 2' centers with a 10' long CLB terminated at each outside truss.
Clearly, there needs to be some diagonal bracing in the plane of the web members to prevent translation of the CLB, but I am not current on what the TPI or TPIC calls for in this regard. A bracing force of 0.02C where C is the compression in the web member seems reasonable.
In a group of six trusses, the maximum force in the CLB would need to be 0.12C. Does anyone have anything current on what these requirements should be?
Some of the compression web members were tied together using a continuous lateral brace (CLB) but were otherwise unbraced. Thus there were six trusses spaced at 2' centers with a 10' long CLB terminated at each outside truss.
Clearly, there needs to be some diagonal bracing in the plane of the web members to prevent translation of the CLB, but I am not current on what the TPI or TPIC calls for in this regard. A bracing force of 0.02C where C is the compression in the web member seems reasonable.
In a group of six trusses, the maximum force in the CLB would need to be 0.12C. Does anyone have anything current on what these requirements should be?
BA





RE: Wood truss web member bracing
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Wood truss web member bracing
Actually, I am amazed ANY CLB's are there - they are often omitted!!
RE: Wood truss web member bracing
RE: Wood truss web member bracing
MiketheEngineer, if the CLB's had been omitted, each web member could buckle in either direction about the minor axis. The addition of CLB's ties the web members together forcing them to buckle as a group. Without diagonal bracing, it seems to me that their buckling length and hence their axial capacity is unchanged.
hokie, the CLB is a continuous 1x4 nailed to the edge of the 2x4 web members. The face of the CLB and edge of web members are in the same plane. Sometimes diagonal bracing is placed on the same edge, sometimes on the opposite edge but, apart from the offset due to member thickness, they are all deemed to be in the same plane.
BA
RE: Wood truss web member bracing
RE: Wood truss web member bracing
I use the 2% rule. My search on the internet "2-percent rule wood truss" brings up a lot of available information on it.
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Wood truss web member bracing
BA
RE: Wood truss web member bracing
I had interpreted "in the plane of the web members" to mean "in the plane of the truss". Agree that the bracing is transverse to the trusses.
RE: Wood truss web member bracing
BA..., wasn't it you about a year or two ago, on a circular building with many light columns at the exterior, and only a line of horiz. braceing at mid height, who suggested this was like a bunch of drunken sailors leaning against each other for support. And you wondered which one would move first and bring the house of cards down.
RE: Wood truss web member bracing
BA
RE: Wood truss web member bracing
BA
RE: Wood truss web member bracing
thread507-304158: Structural equivalent of two drunks leaning on each other for support
BA
RE: Wood truss web member bracing
RE: Wood truss web member bracing
I do remember the photo and the thread that you linked, and the drunk sailors leaning against each other does seem to be JAE's apt comparison. The thread I was actually thinking of was #507-268847, Bracing ability for a tension bracing member, by leeStruct, 1APR10. And, this post shows my computer acumen (lack there of), in that I don't know how to turn the above into a direct link.
RE: Wood truss web member bracing
In your case, add the word "thread", remove the # sign and don't leave a space between, i.e.
thread507-268847: Bracing ability for a tension bracing member?.
The name of the thread will fill in automatically.
BA
RE: Wood truss web member bracing
Thanks a million BA, even an old dog like me can learn new tricks with a good teacher.
RE: Wood truss web member bracing
BA
RE: Wood truss web member bracing
The CBL's will need to be braced with the diagonal bracing to transfer forces back to the roof and ceiling diaphragm. 2-2.5% of the web force would be good for sizing the CBL, then translate that to the diagonal bracing.
RE: Wood truss web member bracing
RE: Wood truss web member bracing
I agree that 2-2.5% of the web force is a reasonable provision for bracing the CLB but that would require the truss drawings to stipulate the web force. Some do and some don't. When they don't, it is not clear how the EOR is going to design the bracing.
Also, it seems to be standard practice in my area for the truss manufacturer to specify that the Erector shall supply all bracing materials. That brings up the question...how does the Erector bid the bracing at time of tender when the EOR has not yet specified what bracing he requires?
bylar,
I agree that the truss drawings show which members require bracing, usually by a small rectangle shown at the mid-length of the web member representing the CLB. I have not seen the standard brochure you mention which specifies bracing requirements in addition to the CLB. Could you provide a link to that document?
I have come across several instances, mostly on single family dwellings or small residential buildings where the only bracing provided by the Erector is the CLB. In these cases, there usually is no EOR.
BA
RE: Wood truss web member bracing
There have been some amendments to the IBC buildings codes and think they are now requiring all max member forces to be shown on the truss design drawing. This is nice as the EOR will no longer have to dig through the truss calcs to find the required member forces.
Generally when the truss manufacturer is supplying only the trusses, the bracing will be supplied by the erector/contractor. If the truss manufacturer supplies the entire structural building package such as with post-frame then the truss manufacturer would supply the bracing as well.
The whole bidding scenario brings up another issue as you point out. The erector/contractor would get the truss design/layout from the truss manufacturer, then would have to have the EOR review this information and incorporate any additional bracing required by the truss design. This should be done before the bid is put in. There are usually tight time constraints, but shouldn't take overly long for the EOR to review the truss layout and add in any additional bracing required.
Structural Building Components Association publishes a document called BSCI which gives all the recommendations for temporary and permenent bracing for the roof structure. The EOR should have a copy of this and incorporate the bracing recomendations into the overall building design.
Here is a link, http://www.sbcindustry.com/bcsi.php
RE: Wood truss web member bracing
I am actually surprised by how many EOR's do not require the truss designs to be self braced, i.e. designed with L or T-bracing for the webs unless a permanent bracing plan is supplied (and sealed) by the truss engineer.
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Wood truss web member bracing
I like your idea of L or T-bracing for the web members unless a permanent bracing plan is supplied (and sealed) by the truss engineer. That does not seem to be the usual practice in my area but I believe it should be.
BA
RE: Wood truss web member bracing
I would actually prefer the truss web member to be self braced with a T or L brace as opposed to the CLR bracing. The T or L can be installed on the individual trusses before they are set and will help to reduce field labor for applying the bracing. And then you don't have to worry about the contractor not installing the CLR bracing after the trusses are set.
It's probably more economical to use the CLR and diagonal bracing on trusses spaced 2' O.C., but would still need to be sized by the EOR. The T or L bracing is sized by the truss manufacturer and appears on the individual truss drawings.
RE: Wood truss web member bracing
Accessing the web members to attach the braces will not be easy, particularly as the units are occupied (and have been for several years).
It is not clear why the truss designer elected to use compression members as end diagonals. It would have been a simple matter to reverse the slope and make them tension diagonals. Hopefully, we won't get a large dump of snowfall before the work is done. According to my calculations, the unbraced end diagonals are good for little more than the dead load of the structure.
BA
RE: Wood truss web member bracing
I'd brace those diags. as individual members, as T's or L's. Take a bunch of (12) 5' long 1x4 or 1x6's up there and screw them to the underside of the diags. And, that'll be a bitch too because they'll be working (swimming) in 24" of insul. out near the bearing (energy heal). These lengths will be fairly easy to get up through a clg. hatch, and all they will need in the way of equipment will be screws and a screw gun.
RE: Wood truss web member bracing
Depending upon how adventurous you are feeling...: maybe at the lower 18" of the 1x4 or 1x6's which are going to make the T's, you could glue and screw two 18" long blocks, 1.5"+ apart on the same side/face of the T board which will mate with the existing diags. These only have to take 25 or 30 lbs. laterally, in either direction. Then just slide the bracing T's down on the underside of the diags. and reach as close as you can get to the heal to screw up into the diags., but you don't have to get right out to the heal to make a connection. Granted, this is kinda Rube Goldbergish, but a lot better than what you have right now, and a lot easier for the poor dog who has to get up there and do this work.
Alternatively, you could take pieces of 2x12 (2x10, deep enough?) cut to 22.5" +/- long, shove them up against the underside of the roof deck, and end nail (screw) them through the sides of the top chord and the diags. This would transfer the 50lb. CLB bracing load into the nails btwn. the roof deck and the top chord. Maybe cut them 3/16-1/4" long, so none of them or few of them are short. They would fit 2° off of perpendicular to the truss, but so what. Stagger them a few inches either side of the center of the diag. length, so you can always end nail them through the sides of the chords.
RE: Wood truss web member bracing
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Wood truss web member bracing
BA
RE: Wood truss web member bracing
BA
RE: Wood truss web member bracing
RE: Wood truss web member bracing
I considered blocking #2 and #4 spaces. This would transfer 150# to each block/chord junction. Not bad, but I prefer blocking #1, #3 and #5 so that I am transferring 100# factored load to each junction.
Nails in withdrawal are not reliable and I would not count on them to carry any load. The existing CLB is nailed to each web member with 2-2.5" nails and should be adequate to brace the three webs which are not otherwise adequately tied to the blocking.
You are quite right about potential splitting of the blocking. It may be more prudent to substitute 1.25 x 11 7/8 Timberstrand Rimboard for the 2x12 SPF. I will discuss that with the EOR.
BA
RE: Wood truss web member bracing
You're absolutely right..., you are being frugal and smart..., I didn't realize that the CLB 1x4 was in place. I thought that lack of the CLB was why you were doing the rework, and that blocking would be needed in every truss space. In your existing case my L or T brace would not have worked. And, in effect, with the 1x4 CLB in place, my concern about nail tension goes away too, since the 1x4 will act as a tension tie too.
My next question was going to be, are the CLB braces on the two long diags. in place, nearer the ridge? I have seen these conditions many times where the builder put in the CLB but then didn't realize he had to take that load someplace substantial, or the need for bracing has literally been ignored.
Most of the truss drawings around here do include the member forces. The materials from the SBCA link are good stuff, we'll understand most of it pretty readily, but a good refresher nonetheless. The reason you've never seen this literature is because what the truss manuf'er. does send out, around here, mostly to cover his own butt on these issues, goes to the builder with the truss drawings and erection drawing package. It is usually given to the builder when they deliver the trusses, and we engineers never see it. The builder throws these papers in the back/box of his P/U to soak up the beer in the bottom of the box. The builder doesn't know how to read much of this, except the erection drawing and he has trouble with those regarding spacing, bearings, headers and hardware, bracing, blocking, etc. Unless it's a commercial job and the spec. and structural notes call for it, it's like pulling teeth to get shop drawings sent to the EOR, if there is one.
RE: Wood truss web member bracing
That's a good one..."to soak up the beer in the bottom of the box".
Thanks for your help dh. That blocking idea for the end diagonals was a winner.
As Woodman88 suggested, it would be prudent for the Building Designer to specify bracing drawings sealed and signed by the truss engineer. That could save a lot of headaches.
BA
RE: Wood truss web member bracing
see attached for info on anchor brace design.
Regards,
Mark
RE: Wood truss web member bracing
Please note that as the article states “Permanent bracing considerations and design needed for truss chords are not covered in this article.” and that the diagonal web bracing in the article requires this permanent bracing to work.
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Wood truss web member bracing
Garth, wouldn't that be a difficult installation? If the face of the 2x block is in the plane of the diagonal bracing, its edge would not necessarily align with the sheathing. Would it be enough to end nail through the chords at the appropriate angle? That means it would not be attached to the sheathing.
BA
RE: Wood truss web member bracing
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Wood truss web member bracing