deflection of steel truss and failure deflection distance 2

[srr]

Was recently called to look at a building with 43 foot steel trusses.They measure two feet high with 3/4 cold roll as truss material.Top is two 1 inch angle with the 3/4 welded in between the bottom cord is made of two pieces of 3/4 welded cold roll on each side of truss steel,ends have attachment plate and are mounted solidly,bracing is eight foot out on truss.Trusses are on 11 foot centers. After recent snows,with over thirty inches on the metal roof there was 3/4 inch deflection.This was measured after roof was shoveled off.Any one have any idea what the total deflection for safety or roof collapse would be?

 

[Zulak]

A common total deflection limit of l/240 for that span would equate to a little over 2" at mid-span. While large deflections doesn't necessarily mean member failure, other factors could be influenced by the large deflections (such as roof ponding).

 

[srr]

thank you for the info,this is not a totally flat roof so ponding would not occcur. I guess this roof was in no danger of collapse at this time.Always a reassuring thought.

 

[epr]

srr, don't be reassured too easily. It sounds to me (based upon the minimal data provided) that the roof trusses may have been underdesigned from the beginning. The top chord seems too small to be much more than a chord size of 1 or 2, which for a 24" deep joist does not even seem to conform to SJI requirements. And the 11' spacing seems excessive for the size and span. Without accurate data, it seems that the roof trusses may only be capable of supporting a TOTAL load of less than 20 psf, which doesn't seem correct at all.
I suggest you obtain accurate measurements of the various components and attempt to ascertain which joist configuration actually exists at the facility, using the method described in the Steel Joist Institute's 60 year manual.

 

[srr]

Thank you;
As for toal load I guess it has already been tested by fire for more than twenty pounds.According to Cornell's snow load data.Medium density snow of which this was weighs twelve pounds per square foot and with almost thirty six inches on the roof it figures to 36.I do agree that I will have to look a little more closely at it.It is not an engineered building,so anything is possible.It is a rather lightweight roof so that helps and does have fall at a 2/12 pitch.Also with bracing eight feet in from both sides the actual clearspan is 27 feet which should help considerably.

 

[epr]

More data always helps.
When you originally wrote bracing at 8', I thought you meant lateral bracing. If the bracing is providing vertical support, it changes the situation entirely. The configuration of the bracing is critical though. If it is only a compression member buttressing the underside of the joist/truss, with no lateral bracing at the same point, then lateral buckling of this additional support is a major consideration, and he analysis becomes considerably more complex. I suggest you model and run a three-dimensional frame analysis, if that is the case.