Bent Truss Web (Tension Member)

[bookowski]

See attached photos.

Four full story steel attic trusses, around 65ft. span. Trusses are pratt style with double angle for all members. Built in early 1900's. Upper chord of truss supports concrete slab and steel roof beams, lower chord supports steel framing (no slab) from which is hung a plaster ceiling.

On the two middle trusses the first diagonal web member off the center is bent outward significantly. Same member on each truss and same direction of bend. On one of the trusses the center vertical is also bent although to a lesser degree, on the other it appears straight.

This supports roof and attic loading, shouldn't see much unbalanced load - so the diagonals should be tension members and the middle vertical a zero force (or very small force). This is over a theater and my first guess is that at some time someone was using the truss as an attachment point to hoist from. There is no other indication of a problem - no cracking at plaster in this area, all connections look fine, other members are all straight. I noticed this as part of an overall conditions assessment - it had not been noticed prior or been causing any issues.

Thoughts on how/if this should be addressed?

 

[bookowski]

One more pic (this is the more severely bent of the two)

 

[KootK]

I recently evaluated a steel joist with a bent tension member. I ultimately ended up doing nothing about it. For a tension member that never sees a stress reversal, I couldn't think of a failure mode to be concerned about. I suppose that it might ever so slightly exacerbate tension chord buckling tendencies.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[bookowski]

I agree that it's hard to imagine a failure mode that is caused by it. The deformation is significant and it's carrying a large area which makes me a bit hesitant to say that it's fine... but I'm not sure of a reasonable fix that is actually necessary.

 

[KootK]

As a fix, maybe you could weld a channel over top of the double angle web that could replace it entirely from a capacity standpoint. This still falls into the "unnecessary repairs" category though.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[EngineeringEric]

You could brace it laterally to adjacent straight trusses, and possibly reinforce as Kootk mentioned. The bracing would stop future buckeling and the reinforce would repair plastic damage. Again it all seems like extra but if you were hired to verify it you might as well 'repair' it as long as you don't cause more damage.

I would suspect that was caused from installation or something similar..?..

 

[bookowski]

I wouldn't want to brace it to an adjacent truss since it's a tension diagonal. Under load it will want to straighten, not buckle, and the bracing would then be pulling on that adjacent truss.

The concern that I have is that the diagonal has significantly reduced stiffness because of the bend. It would take quite a bit of displacement before it straightens out and regains the originally intended stiffness. At the same time I don't want to go welding reinforcement on there just for show.

 

[ChipB]

Maybe I am losing my engineering knowledge....

A 65 foot span is relatively long
Which means, the tributary area would large.

Attic loading appears minimal.

The picture showing how much out of plane the diagonal is, is definitely a buckle caused by compression. If it was from hoisting equipment, the two members would have gradually come together, to the point of the bend. They would not be separated as they are now, with one of them having an obvious twist, at the bend point.

Flip the truss upside down, load it vertically, and those become compression members.

With what I see here, I would suspect an extreme wind loading condition with uplift as the culprit, personally.
Even with the concrete slab.

Is it located in a high wind region?

Just my observations.

 

[bookowski]

Roof is draped mesh concrete slab with cinder fill, easily 75psf+ and then there is attic and plaster suspended ceiling, definitely 100+ psf for total system. It would have taken a very large wind event to overcome that and cause compression buckling in the middle of the span. Not a high wind area, design speed 98mph.

By hoisting I do not mean from directly below - it would have been lateral and over a pulley down. This is a theater and they typically have numerous holes through the ceiling for lifting, often a cable comes up and turns laterally to a hoist tied off somewhere. Not sure if that was it or not, but there is a lot of that type of lateral rigging in these spaces.