Thickness of Transfer Slabs 1

[oxbridge]

To limit deflections in uniformly loaded slabs, I'm aware of the rules for span/effective depth ratios i.e. 20 for simply supported slabs and 26 where continuous(UK Code). Can span/effective depth ratios be applied for transfer slabs given the non-uniform loading. The code says that deflections do not need to be checked where S/ED ratios are used but i have some reservations about applying this to transfer slabs. Any thoughts

 

[kslee1000]

Your terminology is not familar to many of use here - what is a transfer slab? Also, do you want an opinion strickly stick to UK code, or in general sense?

 

[OCI]

If you are refering to a "Transfer Slab" like a "Transfer Beam" or truss I would treat this as a special situation and check the deflection. The span/depth ratios are for uniformly loaded floor slabs.

 

[BAretired]

If you are transferring column loads from above the slab to non aligning columns below the slab, the slab thickness may be governed by factors other than deflection. Peripheral shear around columns is one. Ease of placing reinforcement is another.

If you can design the slab to meet all of the strength requirements of the code, I do not think deflection will govern simply because loads are not uniform.

Having said that, I prefer a rigid transfer structure and might be inclined to beef it up a bit because of its importance. You also may require more cover over the reinforcement for fire protection.

BA

 

[hokie66]

BA's advice is on the money. The first thing to check in a transfer slab is punching shear, both under the columns over, and over the columns under. Conservatism is the rule with a transfer slab. L/20 is probably a good starting point. Transfer slabs are best designed with a program like RAPT, in which deflection checks are included.

Progressive loading of transfer slabs has to be considered. We usually use post-tensioning, so the loads can be balanced incrementally.

 

[rapt]

oxbridge,

What you have to remember is that deflection of the transfer member is support settlement for the floors above. So you need to calculate deflections and limit them much more severly than for a general slab or beam. Plus you need to detail the reinforcement for the floors above allowing for support settlement. Support settlement can also contribute to ponding on rooves.

The b=next Australian code limits deflection for transfer members to L/500 if support settlement is being allowed for in the design of the floors above and L/1000 if it is being ignored. Therse figures cannot be determined from L/D ratios. Deflections must be calculated.

 

[csd72]

Limit your punching shear to about 0.6 N/mm^2 the depth required to do this will most likely dictate the depth.