permanent truss bracing

huangyhg

permanent truss bracing
i'm going to be entirely upfront here and point out that i am a relatively new structural intern.  as such i often find myself surprised by accepted practice and the occational rediculous "generally accepted practice" that doesn't really work at all.
my question is this:  how do you handle the refusal of the wood truss manufacturers to design and take responsibility for the permanent truss bracing system?
a little bit on my overall understanding, perhaps for edification, perhaps for rebuttal, but here it is:
  wood trusses are typically spec'd by the building designer as trusses @ x dist. o.c. and in a given direction, with requisite design loads.  from there the truss supplier runs their truss plate manufacturer produced software following (if you're lucky) your truss layout and inputing your required loads.
  the software provided by the plate manufacturer outputs locations of lateral support for given members.  this is of course to reduce the effective unsupported length of members in compression and enable a more efficient design.  also, they are looking to eliminate the need for t-bracing of members, since this makes staking the trusses more complex and difficult, if not (as is sometimes the case) impossible.
  from here, the contractors (nearly always) interpret the bracing requirement to mean simply installing the bracing shown on the drawings (typically 2x3 or 2x4 perpendicular to the truss between similar trusses throughout) between the trusses and leaving it there.  i would question the ability to get just a small number of nails to work, let alone take the loads which would normally be present.  and let's not even get into a discussion of load paths and/or the fact that you can't connect two laterally unstable systems together with a single simple connection and magically call them braced.  two laterally unsupported wall are still laterally unsupported if you connect them together with a simple 2x4 and a few nails, and so are trusses!
  nearly all of the literature out there seems to be very biased in favour of the truss plate manufacturers who seem to just want this problem to go away without their further consideration, expense or effort.  a really good example of this can be found in dr. frank e. woeste's paper "wood truss bracing" which can be found at
  2.  the drywall on the underside of the trusses is effecting the same load path as above, simply on the horizontal.
  now, there are many things in our everyday lives that do simply work and have worked and will probably continue to wok for a long time.  hoewever, that is not what engineers do or what our profession is about.  we are here to ensure the safety of a structure.  to give the client the most economical design possible.  to defend the safety of the layman against unsafe or unscrupulous workmanship.  and even if this system does work, the "shear panel" and the "underside drywall" suggestions are only qualitative models of what makes this work, not quantitative solutions for the given system.  and i certainly have no interest in being the one left holding the short end of the rope when something does "just happen" to go wrong.  on top of it all, without further reasearch or guidance, it would be impossible to predict when the current "it just works" solution could fail.
  hopefully some of you guys can explain all of this to me.  and if you've gotten this far, thank you very much for sticking it out.  sorry for the long read.
permanent lateral bracing (bracing intended to stay in place and transfer lateral loads) is the responsibility of the engineer of record (eor).  the bracing shown on pre-engineered shop drawings is bracing assumed by the truss designer to brace the bottom chord of the truss.  it serves no other purpose.
the eor is responsible for the permanent lateral bracing because that person alone knows the lateral load path and the lateral loads to be resisted.
the truss supplier is providing a vertical load bearing   
i appreciate the rapid responce sperling, however the fundamental problem remains.  i'm not talking about the bracing required against wind loads, where i of course see the good sence in their being the responsibility of the eor.  wind loads are typically quite easy to transfer into the roof diaphram and then down through the rest of the structural system.  rather it is the the premanent bracing required in lieu of t-bracing on compression members within the truss system that are of concern to me.  
i recognize, and fully accept, that the responsibility for tranfering live wind loads to the foundation should remain with the eor.  however i don't see responsibility for the internal bracing of truss system   
youngstructural, not so much cause for concern. i have been designing apartment buildings, tract and custom homes for 26 years, and have never seen a collapse caused by inadequate bracing (that is, after the house was completed).
the only thing that i am responsible for is the gable end roof truss bracing detail, which is a standard detail shown on my drawings (i am the eor)
are we talking about the same thing?
bracing (to prevent buckling, reduce the un-supportted length) that is shown on the truss shop drawings should be designed by the truss designer.  the truss supplier is trying to get you to design this bracing?
young, i have seen truss calculation submittals that have shown strongbacks, bridging, t-bracing, etc....have never been  "asked" by the truss mfr to specify those on my contract drawings.
i have never had to design the bracing but i typically show a generic call-out of bracing and specify to refer to the truss engineer for details.  most of the truss engineering submittals i have reviewed have showed this bracing.  the eor doesn't have access to the software that the truss engineer uses nor typically the loads and design procedures, it is the truss engineer's responsibility to design bracing for lateral stability of the truss   
first off, 'thanks for the comments...  
ucfse, i know exactly what you mean.  more and more some of what we do seems like voodoo, but you know what...  i think most of the time that's okay.  there are some things that have worked and really do work (such as a maximum notch out of a joist...  really you've butchered the joist, but in the field, she keeps working!).  my problem here is that it is just too complex of an issue to be certain of it working all of the time, and especially since the truss manufacturers seem to be making an extra special effort to pass the buck.  these are, after all, the people with the research and who are experts in these systems.  how are we to know where the lateral members requiring bracing are, let alone design them?  yes, we do get shopdrawings for the trusses, but like you said, there isn't even enough information (loads, hello?) to do the design.
my solution, and one that i have gotten support for from even the voo-doo engineers (whom i truly, honestly respect since it's the plethora of experience, not really voodoo in the works here), is to specify the use of t-bracing in lieu of simple bridging on all truss   
being a practitioner of "voodoo" engineering, i would like to object to this derogatory term. i have plenty of experience, which tells me to worry about important things, not silly trivial issues. the truss bottom chords are amply supported by the attachment of the ceiling board. there are plenty of redundant systems of resistance against collapse/instability in most structures. we engineers just get uncomfy because we can't devise a scientific, quantatative (sp?) calculation to everything that worries us.
what typically happens when we worry about small issues, is we overlook the big errors.
since i work in the wood industry, i know you will get a large range of opinions regarding this topic. you can probably find some of those opinions if you search past postings of this forum's data base.
most press plate truss manufacturers consider their product to be a componet.  a component which the eor has specified for use as part of the building system.  most truss plants want to be in the business of producing trusses, they do not want to be in the business of providing structural design  of the system.
as a structural engineer, i am not sure i agree with them although i can understand their point of view.  a large volume of press plate trusses are used for single family housing. however with the growth of the industry more and more trusses are being used in commercial buildings such as banks, shopping centers and large warehouses. these larger projects require more engineering.  engineering which some truss plants are able to provide but a lot can't.
in the past i've handled it by the way i have specified the requirements on the structural drawings.  while doing that i was often told by the truss supplier that they do not provide the services specified.  my response was that it didn't matter to me who supplied the trusses however the requirements of the specifications would have to be met before i would approve the truss shop drawings.
along the same lines however i have a question regarding bar joists which i consider similar to press plate trusses.  when you specify open web joists do you place the same requirements for designing permanent bracing and other items as you do for press plate trusses?