Checking Adequacy of Existing Suspended Floor Slab to Support Replacement Steel Frame Tower
Checking Adequacy of Existing Suspended Floor Slab to Support Replacement Steel Frame Tower
(OP)
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.
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: Checking Adequacy of Existing Suspended Floor Slab to Support Replacement Steel Frame Tower
RE: Checking Adequacy of Existing Suspended Floor Slab to Support Replacement Steel Frame Tower
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 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 seems 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.
RE: Checking Adequacy of Existing Suspended Floor Slab to Support Replacement Steel Frame Tower
A 4.5" suspended slab won't carry a lot of load.
STAAD, or any other computer program, won't give you the answers you need. You need the assistance of an experienced structural design engineer.
RE: Checking Adequacy of Existing Suspended Floor Slab to Support Replacement Steel Frame Tower
This tower is to be a replacement for an almost similar but older tower already on the slab. The slab is not just a regular 4-1/2" slab, but concrete slab on ribbed steel deck. Just pure concrete is 4-1/2", but with the steel deck, the thickness might come to about 6". Good as-built drawings are hard to come by, unfortunately.
Analyzing the plates under the steel tower will provide me the stress contours on the slab. I can tell what stresses are being imposed on the slab. I am trying to make sure that the slab is not overloaded beyond its capacity. Is this the right approach, or is there something else I need to look it.
It is difficult to have to re-design the slab again, because so many other steel frames have been built on this slab, and it is very congested real estate. Not the best of situations but this is what sometimes happens out on the site. It would surely take much effort to find out what those other loads and weights are. I am trying to make sure that the area where I am replacing the tower is structurally adequate. at least.
RE: Checking Adequacy of Existing Suspended Floor Slab to Support Replacement Steel Frame Tower
Your computer isn't going to model deck-slab bond.
If you don't have much as-built information then the proper approach, in my book, is to come in with supplemental framing under these legs to take 100% of the load and leave the slab as just a non-participating element that serves only as "grout" between your tower legs and the supplemental framing.
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