Large wood truss collapse - Lansing, Michigan 2

[plasgears]

I have seen two major wood roof truss collapses in several years in this area, and I wonder if residential wood trusses should not be built in large sizes. This is the type with 2x4 and 2x6 using formed shear plates with projecting spikes. I suggested using steel trusses as a replacement, and the plant mgr opted for wood, even with no price difference. (???)

The latest collapse was at a state government building. The span was about 150 feet. The failure was in June, not during the winter snow load condition. Any comments?

 

[14159]

Are these designed or just picked out? It makes a huge difference in the responses you should get.

DBD

 

[UcfSE]

Maybe the high school data entry person was distracted while designing the trusses.

 

[SlideRuleEra]

It is almost always the connections. See my 5 July comments here
thread507-127734

http://www.SlideRuleEra.net

 

[hamlet104]

The thing most often missed is the lateral bracing of the individual web members - they are long and slender, and truss manufacturers claim that it is the role of the EOR to design them - their design sheets just show where the bracing is needed.

(This came as a surprise, certainly. Why it should be the EOR's responsibility is a mystery to me, since this bracing is entirely within the truss system envelope, and we have no idea when we design a project what will be needed. I think it is just a way for the truss manufacturers to shed some liability and cut their own costs, even if it drives the total project cost up. They can supply members needing bracing every two feet, then palm the design, purchase and installation onto somebody else's budget)

 

[shepherd]

I have heard of a situation where the truss was tied down to a non-bearing wall below at mid span. Due to changes in moisture content and restraint of the bottom chord, the vertical web member snapped. Detail connections to account for changes in moisture content. This would be even more significant in larger trusses.

 

[pba]

hamlet104,

When we do a truss drawing, we indicate a truss bracing system that is our estimate of the truss designer's requirements. We put a big note next to it stating that the bracing system indicated, is to be assumed by the truss manufacturer.

When the truss calcs are supplied for our review, we correct not only their loading but their bracing assumptions. The bigger companies we deal with now tend to get it right but it is still hit and miss...

 

[EdStainless]

Of course I have seen steel joists fail too. Almost always related to improper bracing as well. This is an issue with any long slender truss.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[hamlet104]

pba:
Thanks for your response. Just to clarify your note, are you speaking from the point of view of the Engineer of Record for the project, or the Specialty Engineer that reviews and seals the truss manufacturer's drawings?

 

[pba]

I work in the UK so officially neither! In reality I was talking from the EOR position. The point is that as EOR (or equvalent), we are specifying a product. It is up to us to ensure that we specify what we want. If we don't know what we want, we cannot expect the truss designers to know either and we can't reasonably complain if they assume something which causes us further problems.

I confess - I learnt from experience.

 

[RARWOOD]

My personal opinion, which many of my colleagues in the wood idustry would not agree with, is that press plate wood trusses should not be used on long spans. My opinion is based on my design experience with heavy timber trusses which use larger members such a s 6 1/8 " x 12" through 24" and 1/4" or 3/8" thick steel truss plates with 3/4" to 1" diameter bolts.

As your truss span increase the magnitude of the loads which need to be resisted increase significatly. When you are dealing with heavy timber trusses with spans over 100' you need very knowledgeable and experienced designers; because all the design problems which you have to deal with in normal truss design become more critical with the longer spans.

 

[Chewey68]

Lots of opinions here. Let me try to detail for you. The "truss member" bracing that must be specied by the truss mfg. is only to contribute to excessive bending forces on long members inside the truss. This is in no way intended to be accepted as permanent bracing. Permanent bracing is to be specified by the building designer. IRC-2003 points to ANSI/TPI-2002 which clearly designs who is responsible for what.

 

[MikeHalloran]

Wow, a 150 foot span for a matchstick truss? What cojones!

I claim no special expertise here. My exposure to such trusses is limited to picking through the ruins and making makeshift repairs (including a replacement truss of ~30 foot span) after a couple of hurricanes.

Given how wobbly the individual trusses are without bracing, I'm just amazed that anyone would even try to make them that long. How would you handle them in such a way as to not damage the joints or the members during transportation and erection? If you did accidentally apply enough bending moment to crack a member or partially pull a plate, how would you know? If someone else damaged it and didn't 'fess up, how would you know?



Mike Halloran
Pembroke Pines, FL, USA

 

[plasgears]

Mike,
I think it is evidence of regional complacency in wood construction. Michigan has been a source of lumber in the midwest, and the locals still tend to favor it over steel, even in large projects.

In the aviation accident arena they call it:

CONSPIRACY OF COMPLACENCY.

 

[MichSt]

I will veer off subject a bit here…

This building is located down the street from my parent’s house, and I happened to be visiting the morning it collapsed. Luckily, it was a Saturday and no workers were in the building. I have several photos of the building just after collapse and during demolition of the collapsed portion of the building, which was done that day. If anyone is interested in the photos let me know (I will need a site to post them on).

The first thing I thought when I saw the collapsed building was the design of the trusses looked very unconventional. The trusses were about 25’ tall. The top 10’ or so each truss was a typical prefab truss and below that was a lattice of vertical members that looked liked they were hand framed and not properly braced. In addition, the bottom chord looked like it was only a couple of 2x4’s. I have no expertise in truss design, but what I saw did not look like a sound design.

Plasgears…are you aware of any report on the cause of collapse?? This happened during the summer and I have yet to hear anything in the local news about the cause.

The contractor who built this building is very big in the area…I hope his other buildings with roof trusses are being review by someone.

 

[SlideRuleEra]

MichSt - A good, free image hosting site is

http://www.myonlineimages.com

(there are many others)
Signing up for an account and uploading the images, if the files are not too big, is easy. Then you can post the link here.

http://www.SlideRuleEra.net

 

[MichSt]

For those who are interested I’ve posted images of this building just after collapse and during demolition here

http://www.myonlineimages.com/public.aspx?memberid=15688.

SRE – That for the hosting site…very easy to use.