Question about concrete encased steel beams

[stevenspm]

Hello All,
I am working on a building built in the early 1930's. We are placing some large mechanical unit on the roof. The roof is composed of a reinforced concrete slab spanning over steel beam. The beams are spanning between girders, in some areas they a steel beam girders, in others they are built up plate girders. In one location I have a plate girder supporting other girders. Please see the attached, the dark red are the girders and the light red are the steel beam purlins. All the steel has some concrete encasing them, generally for fire protection only. The question I have is, does the concrete encasement around the girders provide enough bracing to consider the webs braced for shear stress checks? Based on the steel manual of the era, the shear stress of the un-stiffened web is about 1/3 of the stress if the web is stiffened.
Thanks
Phil S

 

[BAretired]

The devil is in the details. It depends on the thickness of concrete cover and whether the concrete is connected across the plate girder. For just enough cover to act as fireproofing, its structural contribution should probably be ignored.

BA

 

[msquared48]

How thick is the concrete cover and does it contain any confining reinforcing?

If confinement reinforcing is present, I would consider considering the flanges, not the webs, as stiffened. If the reinforcing is not present, then I would agree with BA.

Mike McCann, PE, SE (WA)

 

[stevenspm]

Guys,
Thanks for the responses. As to the thickness the existing drawings do not say how thick but it look like it is only a couple of inches and does not look like there is any confining steel. After posting, I was reading through an old steel manual that I received from the time, and it indicted that standard practice for plate girders with a depth between flanges greater than 60 time the web thickness stiffeners at no more than 6' were required.
Phil S

 

[msquared48]

Web stiffeners ... got it.

Mike McCann, PE, SE (WA)