Question about slabs cast integrally with beams and sits on bearing walls

[HassanZebari]

Hello fellow engineers,

I have a 4m*15m slab with beams cast together on four sides and supported by a bearing wall..Can we consider the slab as one way slab?

Is the slab simply supported or cont.from two edges due to stiffness between slab and beam so we can find its thickness using equations according to deflection requirements?

Also, How can we classify the beam support on bearing walls (length of span and ends supports) to find bending moment and torsion on beam?

 

[r13]

1) Yes. The aspect ratio 15/4 = 3.75 > 2, slab is dominant by one way action.
2) Yes. You can workout slab thickness by limiting deflection to code specified value.
3) If the beams are connected to columns, ignore the bearing wall, and design the suspended beam-column structure. If no columns, the beam will not have much structural reactions. But you shall provide minimum eccentricity, per code, to account for incidental torsion.

 

[HassanZebari]

Hello retired13,

For limiting deflection in code, Should i use L/20 for simply supported slab or L/28 for two end continuous since the the slab is connected directly to beams and they are cast integrally which could give high rigidity connection?

The beams are not connected to columns. There are no columns where it is only a bearing wall structure so should i ignore bending in beams and design for torsion only?

 

[r13]

Use L/20, the beam will rotate (due to torsion) a little to release the energy required for full fixity. It is conservative, but is a good practice. When design, make sure the positive moment region has adequate reinforcement (I like to use wl[sup]2[/sup]/12 for this situation).

 

[HassanZebari]

retired13,
You mean wl[sup]2[/sup]/12 for torsion or bending in beam?

 

[r13]

It is the conservatively assumed positive moment, otherwise, the positive moment is only wl[sup]2[/sup]/24 for a fixed end beam, which is not conservative under this situation, as the beams are not truly fixed, unless you can provide required reinforcement in the wall, such as concrete wall, to restrain the anticipated (small) rotation of the beams. Under this scenario, the negative moment regions are reinforced above the true requirement too.

 

[r13]

Another better way to minimize the rotation of the beams is to divide the slab into 3 regions - 2 end regions and one middle region. The end region has a width equal to a quarter of the slab transverse length, and the width of the middle region equal to one half of the transverse length. Then,

- Design the end regions as slab fixed on the beams, and reinforce accordingly.
- Design the middle region as slab simply supported on the beams. In this case, since the negative moment is zero, you shall provide minimum steel on top face of the slab to prevent crack.

Hope I didn't confuse you.

 

[r13]

FYI.

 

[HassanZebari]

you are good retired13..Thank you

 

[r13]

You are welcome. Glad to be helpful.