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Flat Bottom Gable End Frames adjacent to Scissor Trusses

Flat Bottom Gable End Frames adjacent to Scissor Trusses

Flat Bottom Gable End Frames adjacent to Scissor Trusses

(OP)
I know this not the norm everywhere, but in the mid-atlantic region it is common for truss manufacturers to manufacture pitched bottom chord trusses (scissors, studio vaults, etc.) and provide a flat bottom gable end truss.  It is clear that there is a hinge at the interface of the top of wall and bottom of truss as the ceiling diaphragm is above that interface.  Fortunately, the 2006 IRC does refer to the BCSI for bracing and this condition is mentioned.  The BCSI states that bracing of this condition is difficult if not impossible.  The BCSI suggests building a gable end frame to match the pitch of the adjacent trusses and balloon frame the end wall to the bottom of this frame.  Other options are available such as framing a full height gable end wall to the roof plane.  Most builders balk at this.  They prefer the flat bottom gable to they can build their walls to a constant plate height.

At this time, most one and two family dwellings in this area do not have an EOR.  The contractors dictate to the truss manufacturers that they will give their business to other manufacturers that will provide these flat bottom gable end frame trusses.  Here is the problem:  The manufacturers that provide these trusses can easily get them signed by a PE from most plate manufacturers because these engineers are signing off on the component with loading provided to them.  They are not looking at the truss as it will be used in a system.  They have technically done their part per ANSI TPI chapter 2 as the truss designer.

Well who is responsible here to protect the public.  There is not design professional of record and most plans reviewers and inspectors are not aware that there is even an issue.

Sorry about the rant...I needed to get this off my chest.  I do my part to make sure this is not overlooked on my projects.  As engineer of record on my projects, that condition is addressed on my plans.  I have had the builders reframe walls and truss manfacturers send out the proper gable trusses.  I always request a review of the trusses prior to manufacture, but rarely get them.  Clearly I need them to design the permanent bracing.

woodengineer

RE: Flat Bottom Gable End Frames adjacent to Scissor Trusses

I was always under the impression that unless there's blocking in the flat end truss for the ceiling diaphram, how else can it be done without raking the end wall and having the gable truss bottom chord match?

Builders here prefer one level for all walls too... but after throwing in the argument about the ceiling diaphram needing to be nailed off at the end wall, not the truss... they sometimes can see the whole picture.

RE: Flat Bottom Gable End Frames adjacent to Scissor Trusses

Per ANSI/TPI, the responsibility lies on the "building designer".  Unfortunately, they rarely take responsibility for this.  I do truss design, and will design the end gable as a scissor, and have them "rake" the wall.  Contractors usually don't have a problem with doing that.  In my opinion, much of the problem lies with the building officials.  They are not enforcing their codes in requiring that the "building designer" be responsibile for the lateral bracing requirements of the entire structure.  As an aside, it burns me up that any one can design houses... there is no requirement to be a design professional to design residential structures.  If these "building designers" were taking responsibility for what the code is requiring them to do, I would have no problem with it.  However, I rarely see "building designers" address those issues.                 

RE: Flat Bottom Gable End Frames adjacent to Scissor Trusses

Woodengineer:
I appreciate your "rant!"  I also agree with CJS that in many locales the building officials need to step up and enforce proper construction methods.  I have had good success over the years in bringing these types of issues to the attention of local inspectors.  As I have made an effort to educate them (in a friendly way) on structural concerns, they have repsonded by doing their own research on the subject and then tightening up their enforcement.  Over time, I have seen many "traditional" framing methods changed for the better, including the very one you mentioned.

RE: Flat Bottom Gable End Frames adjacent to Scissor Trusses

(OP)
I have most recently closed my consulting practice after 5 years and joined a truss manufacturer.  I have affected change in the company as it pertains to this particular issue.  Fortunately my company understands why this issue must be addressed.  In the past, I have been invited to speak to building officials about the hinge and the proper way to build this.  It is surprising how few of these officials are aware that there is a problem.  I posted this issue on this forum in the hopes that my colleagues will help spread the word.

WTCA is aware of the issue and have been urged to put out a technical bulletin so that all truss manufacturers are addressing this. We should not let the builders dictate to us to compromise safety because it is earier for them to build.

It is most important to put the public's safety before anything else.  I am hoping to "level the playing field" by having other component manufacturers follow suit.

I thank you all for your comments...at least I am not alone.

Respectully,

woodengineer

RE: Flat Bottom Gable End Frames adjacent to Scissor Trusses

woodengineer,
I'm an engineer and a plans examiner, I'm not a structural, but I try to educate myself.  I hadn't thought about this problem.  Most plans examiners don't have the background to catch this on their own.  If it is sealed, then it meets code, and they may be told to not question it.  Wind load engineers are for the most part not structurals either, and do the least possible to limit their exposure.  Please, do us a favor and educate the inspectors and plans examiners.  Find a code forum  online and post there.  We all want a good, and safe building, but need to be educated.  That's why I'm on here.

RE: Flat Bottom Gable End Frames adjacent to Scissor Trusses

DRsquare,
You mention, "if it's sealed, then it meets code".  Remember that the sealed truss drawings are components of the whole system.  Just because the truss works for the given loading doesn't necessarily mean the structure is designed correctly.  That was my point earlier, in that the responsibilty to design the entire structure needs to fall back on the "building designer".  Unless code officials enforce that, houses will continue to have this problem.
  

RE: Flat Bottom Gable End Frames adjacent to Scissor Trusses

It is possible to brace the top plate, but it would require adding 2x's perp. to the gable truss and count on them cantilevering off the ceiling.  This certainly wouldn't work all the time.

This is a problem that is not only limited to roof trusses.  At stair wells and 2 story open spaces I see framers splice the studs at mid-height 90% of the time.  I make sure to make it clear on all my drawings to baloon frame these areas, but that only seems to work about 50% of the time.

RE: Flat Bottom Gable End Frames adjacent to Scissor Trusses

CJSchwartz,
I couldn't agree more.  Unfortunately, most building officials aren't trained engineers.  In fact, some places avoid hiring engineers to limit liability to the builder and truss designer.  As an engineer, I have had trouble getting certified because I don't have years of experience "in the trade". I see many builders who are great business people, but know little about buildings.  I see truss loads on interior walls with no foundation, and heavy girder loads with inadequate foundation.  Truss engineers design the truss system, sometimes with just a sketch, and can't check to see if it fits the building.  The builder is ultimately responsible, but I agree that the problem will continue unless code officials know how to challenge the problems.

So, volunteer to educate your local building dept. Post comments on building code discussion forums.

RE: Flat Bottom Gable End Frames adjacent to Scissor Trusses

(OP)
DRsquare,

Truss engineers do not design the truss system, but rather the truss component only.  I am pointing out a condition that gets overlooked often.  The EOR is responsible for the system.  If there is not a design professional of record, it is the responsibility of the person(s) who applied for the building permit.  

Side note:  the component engineer is not paid to analyze the roof as a system.  Eventually, this may change.

woodengineer

RE: Flat Bottom Gable End Frames adjacent to Scissor Trusses

Woodengineer, glad to see you taking the correct approach.  In my opinion, there are still way too many "seals for hire" in the truss industry who hide behind the "truss component only" stance.

As the EOR, I am constantly educating truss designers who have almost no contact with their "seals department" or wherever else they send off their trusses to be signed/sealed.

RE: Flat Bottom Gable End Frames adjacent to Scissor Trusses

FSS,
I don't really think the "seals for hire" are "hiding" behind the "truss component only" stance.  Woodengineer said it best "the component engineer is not paid to analyze the roof as a system".  I think most truss engineers would be glad to design the system if they were paid for it.  
As a truss manufacturer, if we were to try to sell a truss package in which we designed the entire roof as a "system", we would go out of business in a hurry.    

RE: Flat Bottom Gable End Frames adjacent to Scissor Trusses

Florida Building Code address this specifically, and wood frame or masonary walls must be built to the ceiling or roof diaphragm. I would think IRC does the same?

RE: Flat Bottom Gable End Frames adjacent to Scissor Trusses

(OP)
Florida is certainly one of the exceptions.  Our Florida plants NEVER do this.  Clearly, areas with active hurricane and/or seismic activity are aware of the gable hinge as they have seen failure.

I just had another salesmen approach me regarding a 20 some odd feet 6:12 over 3:12 scissor truss with a flat bottom gable.  This builder would not even discuss the issue; they are going elsewhere.  I have offered to speak directly to any customer who wants to understand the issue.  Many of our customers have accepted the change.

woodengineer

RE: Flat Bottom Gable End Frames adjacent to Scissor Trusses

This "hinge" condition is not allowed in the locale where I live (Northern CA).  The plan examiners REQUIRE that the end wall be balloon framed...no exceptions...if the the end truss is a scissor truss.

I just attended a BCSI seminar where they discussed the subject of responsibility for the roof "system".  The seminar was put together by a local truss co. and the audience was a room full of engineers and builders.  The engineers in the room were not happy about the liablility that the truss manufacturer's are trying to shed.  This discussion was about truss bracing though.  I believe the end wall condition is a building designer's responsibility (EOR or drafter).  I would suggest that this seminar be searched for and attended by every engineer.  It was very interesting.

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