Minimum Bearing length for insitu slab

[sybie99]

Hi,

If you cast an insitu slab onto masonry cavity walls, and you only cast the slab to bear onto the inside skin of the wall, you end up with a bearing lenth of about 100mm (in South Africa for standard brick sizes). My question is, are there limits to the minimum bearing length allowed for concrete slabs and beams? I know the British Standards give minimum anchorage lengths of bars at supports which must be adhered to, in which case 100mm bearing would not be enough. But why then is it common practice to have bearing lengths of about 70mm for precast slabs?

Am I missing something, is it okay to support a house floor slab only on the inside skin of a cavity wall?

Thanks

 

[KootK]

I don't know about code limits but I would think that your concerns would be:

1) Bearing stress on the brick
2) Bearing stress on the concrete
3) Anchorage of reinforcement (as you mentioned)

I've dealt with the precast slab issue as well in a retrofit application. In my case, we justified it by demonstrating that the precast planks could resist resist bending as unreinforced sections at a section inboard of the bearing location equal to the development length of the required bars. One thing does concern me about this approach however. If the ends of the precast planks are restrained against lateral movement, you could crack the section in pure tension due to T&S forces before mobilizing it for bending resistance.