Truss Profile 3

[JStructsteel]

Architect has asked me to show a truss profile for the truss manufacturer. I am not sure why I would do this, unless they want a specific look? Isnt the truss manufacturer best at picking the profile to optimize the economy of a truss?

What do you guys show for a profile? Span, slope, and overhang length, and some diagonals would be called out as 'for show only' truss manufacturer to provide layout?

 

[phamENG]

When I say 200ft I mean more like jayrod's example - truss span was about 80ft, and there were 200ft of trusses (100 trusses spaced 2' o/c). They didn't even know it was a problem. They were working on a design to modify the ceiling shape (was a cathedral ceiling, wanted to hang a drop ceiling to flatten it out for an event space) and I discovered the problem while taking measurements for the analysis. They shifted gears real quick from modifying the space to emergency repairs. Building had been up for a little over 10 years at the time.

 

[BAretired]

So you're trying to place blame at the wrong place in the chain.

Perhaps I am, but affixing blame is not really my goal. I used to wonder how I could avoid having to inspect the same roof trusses multiple times until the general contractor superintendent got it right. I was unaware of all the rules cited above by KootK, but they were not followed on some projects which I am remembering.

I think the fabricator should be fairly compensated for the work he is asked to do. I guess my problem always was, "what should I ask him to do". At the time, asking the fabricator to supply all permanent bracing was not normal practice. Perhaps this has changed since then. There should have been an agreed upon method of resolving this problem at the time, but there was not.

BA

 

[phamENG]

BA - no, still done that way. Every truss package I've seen in the last two years specifically calls out bracing by others. It says where it needs to be, but what it is and how it's anchored to the structure are "by others." I get that, and it makes sense. The truss manufacturer isn't responsible for making sure my wall can resist his brace force. The trick is conveying it to the GC, architect, and owner that there may be changes to the structure to accommodate bracing. I've taken to adding this note to roof plans with trusses:

"TRUSS MANUFACTURER TO IDENTIFY ALL REQUIRED PERMANENT BRACING FOR TRUSS WEB MEMBERS AND TRUSS SYSTEM STABILITY ON SHOP DRAWINGS, INCLUDING MINIMUM BRACE LOADS. EOR WILL PROVIDE ADDITIONAL BRACING AND CONNECTION DETAILS, IF REQUIRED, AT SHOP DRAWING REVIEW."

 

[BAretired]

When I see comments like the following, it convinces me that this topic needs to receive more attention than it has been getting:

I saw a large church sanctuary almost come down.

I have seen whole buildings come down.

Every instance I know of it was long webs that required 2 rows of CLBs (Continuous lateral braces) on one or more webs. In some cases the CLBs were not installed. In others there were 2 rows of CLBs, but they were not anchored to anything. So the webs all bowed the same direction.

It it thankfully not common. But it does happen.

I'll chime in on deficient truss bracing examples. There's been at least two I've seen personally in the last few years. One was a giant garage for farm equipment, 60 foot truss spans, 200 foot long building. Put all the trusses up one day with minimal bracing, came back the next day to a pile of matchsticks on the ground.

Second one was a 4 unit townhouse, this time instead of being on the ground, they were all laying over on top of the walls.

I never saw anything like any of the above, but I do remember seeing the absence of bracing where such a thing could have taken place with a heavy snow load or high wind acting on a structure. Clearly, a better practice is needed when specifying prefabricated wood trusses to prevent such disasters from occurring in the future.

BA

 

[KootK]

Every instance I know of it was long webs that required 2 rows of CLBs (Continuous lateral braces) on one or more webs. In some cases the CLBs were not installed.

Makes sense given that:

1) It takes a lot to fail most incorrectly constructed things in practice.

2) A double braced web can be expected to have 1/9 th of it's design capacity when unbraced.

I guess 1/4 th just isn't enough to cause problems in most cases.

 

[RontheRedneck]

It's important to keep in mind that there are 3 different kinds of bracing.

Erection bracing is what it takes to hold the trusses in place until the plywood goes on the roof.

CLBs keep webs from buckling when they're under compression.

Building bracing is for keeping the entire building straight and square.


They often get confused. The subject might be worth a thread of its own.