Non Composite concrete floor on Steel Beams

[rfd23]

Hi All,
I would like to have your feedback/thoughts on my current issue.
I am working on old industrial(process) structural steel building 1960 era. it has 6 floors and a roof. All floors have 5'' concrete slab on steel beam. There is no metal deck, no shear studs and no horizontal bracing. Simply concrete slab on steel beam with some large opening in slab for a typical industrial buildings. Roof has metal deck with horizontal bracing. Building has vertical bracing in perimeter frames. Each floor has equipment, conveyors/fans/screens with horizontal loads. Building is in operation last 60 years with no stability or other issues.

Now a days for this type of design, we put studs on top of beams or horizontal bracing to transfer floor horizontal load to vertical bracing (diaphragm action).

My question is how is this working. Definitely I am missing something?? My concern is how lateral load of each floor is transferred to vertical bracing at perimeter.

We need to add floors on top of roof. Can we consider some diaphragm action with such type of concrete slab on steel beams. Can we introduce some dummy horizontal bracing in staad to achieve horizontal stability of building.

Thanks in advance for your help.

 

[hokie66]

Friction is often your friend. But industrial structural engineers in the 1960's were not silly, so how do you know there are no studs or similar connectors? One thing I am sure of, they didn't use staad to design it, which gives me some comfort.

I don't like the sound of "dummy" bracing. You either need it or not.

 

[rfd23]

Thanks hokie66 for your comments.
We have engineering drawings and there is no such type of connectors. I agree with you that engineers in 60 have done very smart design. They do have choice of using studs or other connectors but they did not use studs. There must be some mechanics to transfer the forces to bracing bays. I want to understand the concept and put engineering logic and method to stabilize the building.
I do not want to propose studs or bracing to existing building while doing some modifications.

you are right, friction is the only one but how to quantify this in design to stabilize the building so it can help and transfer forces to collector and to bracing bays.

 

[jike]

There are a two other mechanisms that might be working to provide stability.

1) Commonly concrete slabs were formed with wood running to the bottom of the top flange of beams. This would leave a lip of concrete that can bear against the top flange of beams.

2) Also concrete was usually poured tight to columns and therefore can also provide an additional load path in combination with #1.

 

[rfd23]

Hi Jike,
I appreciate your input. We looked at these scenario and both were not in place.
slabs were flush with top surface of top flange and concrete has two inch gap from each column all around.
I am sure it is only friction, but want to discuss and see how other engineers thought about it.

I would like to have opinion of some senior engineer who have worked in 1960 on such type
of construction, but could not have any one around in my organisation.
Thanks

 

[SlideRuleEra]

I would like to have opinion of some senior engineer who have worked in 1960 on such type of construction...

Well, I was in grade school in 1960, but the majority of my structural education is from sources that predate 1960. Hokie an Jike have provided a lot of insight... I'll just try to expand on their comments:

For this thread I reviewed "Stresses in Framed Structures" Second Edition (1942), by Hool & Kinne. This is my take on it:

The only horizontal force taken seriously was wind pressure, often assumed to be 20 to 30 PSF... and this was typically for "high" buildings. A 6 story building may not have been considered "high". Of course, this wind loading was applied to the exterior of the building. The external framing was designed to resist this force, as you have found. Without computers / software it was not practical / cost effective to attempt to do anything more sophisticated. The combination of friction and conservative design were expected (successfully) to take care of anything else.

Here is an excerpt from Hool & Kinne:

At the risk of telling you something you already know, allowable yield stress for most structural steel in 1960, was 33 KSI. Tests of samples from the building will probably justify using a higher value.

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