Sorry if the question is confusing as such, here's an image that makes it hopefully clearer:

enter image description here

So let's say this building consists of two elevated concrete slabs. The first elevated slab is supported by the 9 massive reinforced concrete piers arranged at even intervals of 5m x 5m, and connected with massive reinforced concrete beams. Thus the building is 10m x 10m in dimensions.

The second concrete slab is supported by eight of the 9 massive concrete piers, the ones around the perimeter, but the concrete pier in the middle is only one-storey high. Instead two less massive piers (labelled A & B) start from the height of the first elevated slab and support the second slab.

In this case, my questions are

  1. is this design potentially safe & practical as a way of supporting the top slab?
  2. does this mean that the beam along the junction of A-B requires extra reinforcement compared to a design where the top slab is supported by the same (extended) pier as the bottom slab.
  3. are there any other major ways that the forces differ, e.g, in terms of the forces transmitted by the lower columns, or the loads experienced by the lower slab?
  • $\begingroup$ I am assuming that the beam A-B requires extra reinforcement as it looks like it is carrying the load both from the first slab and for the slab above it. I guess you can add piers outside of the beams also, e,g., you could place a pier bang in the middle of the 5mx5m square. From what I can figure the biggest load is the weight of the pier itself? E.g., let's say the upper slab is supported by piers @ 3.3m and is 0.12m thick. So around 800kg for each of 4 piers (total 3.2tonnes). Then a 30cmx30cmx3.5m pier = 750kg. So the slab at that point supports 1550kg on 0.09 m^2 = 17.2 tonnes/m^2 ??? $\endgroup$
    – thelawnet
    Dec 4 '16 at 10:44

Questions 1 + 3:

Your first question is unanswerable. Any design is theoretically safe. Just create a massively thick slab with high-strength concrete and massive reinforcement and you can do just about anything.

That being said, your two stories are completely different. The first floor can be best described as four different slabs, all of which are fixed-fixed-pinned-pinned. The second floor is a single slab which is pinned around its entire perimeter and at two points near the center. The second floor will certainly suffer larger stresses.

The points where the slab is supported by the columns are especially critical. There will be massive shear stresses in that region, and specialized design will be necessary to handle those.

Question 2:

Yes, the beams will certainly require additional reinforcement. After all, in this situation they will need to absorb the load transferred by the columns. This will lead to a massive shear load in the beam and probably massive bending moment as well. If the support system from the first floor was repeated in the second floor, then the beams in the first floor obviously wouldn't suffer those loads and therefore would require less reinforcement.

  • $\begingroup$ " If the support system from the first floor was repeated in the second floor, " Do you mean the same structure of columns supporting it? Or the beams? $\endgroup$
    – thelawnet
    Dec 5 '16 at 17:47
  • $\begingroup$ @thelawnet: Both. $\endgroup$
    – Wasabi
    Dec 5 '16 at 17:48

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