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I am checking the adequacy of an existing suspended floor slab

I am checking the adequacy of an existing suspended floor slab

I am checking the adequacy of an existing suspended floor slab

I am checking the adequacy of an existing suspended floor slab with STAAD which a like-for-like replacement is being made for an existing 4-column steel frame tower, 40 ft high, 15 feet wide with 3 equally spaced platforms. I am ok with the steel tower, but would like feedback, advice or critique about my approach and accuracy, or not, to analyzing the adequacy of the floor slab, which is reinforced concrete, f’c = 3000 psi, supported on steel ribbed metal decks. Thickness of concrete on steel metal deck is 4-1/2”

The floor slab is modeled in STAAD with 4-noded plates. The steel tower is pin-supported on suspendedthe floor slab, which is about 20 foot above grade elevation. Other loads, such as self-weight, equipment loads, live loads, wind, etc. are placed on the structure to mimic real world conditions to generate base level reactions.

The adequacy of the floor slab is evaluated by studying all the various contours to ensure that none of the stress contours in the Postprocessing mode exceed the allowable compressive stress of the concrete, i.e. 0.4 * 3000 psi = 1200 psi. Some of the stress contours are:

Max Top (Principal Major Stress)
Min Top (Principal Minor Stress)
Tau Max Top
Tau Max Bottom
Max von Mis
Von Mis Top, etc.

Both one-way, or beam, shear, as well as two-way, or punching shear, will also be checked to ensure compliance.

RE: I am checking the adequacy of an existing suspended floor slab

I think it is too simplified to just check stresses, since the amount of reinforcing steel is critical to the deigns/capacity. A few FAQs about concrete design in STAAD.pro including some related to plates can be found here:

RE: I am checking the adequacy of an existing suspended floor slab

Thank you so much for your prompt response, Seth. I gather that you are referring to Question & Answer #10, which is reproduced below. From reading it, it seems that I almost have to re-design the whole floor slab again to find out its capacity.

One of the issues that I have is that there is inadequate information about the old existing floor slab, even thickness and reinforcement is not clear. Also, on this particular floor slab, there are numerous other structures sharing the same real estate, so it become quite a task finding information about those structures in order to almost re-design the floor slab again.

Is there another way to verify that I am able to support my structure on the floor slab without having to go through the whole design exercise again? All I need to know is basically what kinds of loads my steel tower is imposing on the suspended floor, and whether the concrete slab can support those stresses. I do not really have to know what is the capacity of the slab, only that it is able to support the stresses from the tower. Can I make use of the stress contours generated by STAAD, and use these to compare to the allowable compressive stress of the slab? In a way, it does seem to be an over-simplication, like you said, but wouldn’t this be theoretically ok?

10. For an existing concrete member, I need to compute the capacity of the section. How do I do this?

You can do the following to compute the capacity of the concrete section:
Model the strucuture.
Specify the existing profile to the member properties
Specify all the required member specification and Support condition
Specify the load on the strucutre
Specify the Concrete design parameters
Specify the parameter MinMain and Maxmain to the provided bar size
Do the design
Check the results.
Adjust the load and redo the design until the reinforcement matches with the provided steel.

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