shell roof (dome shape) reinforcement 1

[shei]

I am currently working on the design of a shell roof (dome shaped)with beams supporting on each side slicing the dome to 8 pcs curved triangular roof panels. Resulting to an approximately 6m base and 8m x 8m curved sides.

The design requires to limit the roof thickness to 75mm. In the analysis (using STAAD) minimum reinforcement area is required. From ACI, the required protective cover is 40mm for top surface since it is exposed to weather and 15mm for bottom surface. Therefore only 20mm space is left from the 75mm thick panel. If I use 10mm dia rebar bothways with spacing based on the result of the analysis, thus there will only be one layer of reinforcement on both direction.

questions:

a) are there any codes that requires top & bottom reinforcment for shell roofs?

b) would one layer of reinforcement suffice provided that they meet the area requirement for top & bottom bars from the STAAD analysis result?

c) other suggestions?

Thanks and regards....

 

[JAE]

First, are your top bars really "exposed to weather"?? - raw concrete exposed to rain, wind, etc? I just keep imagining some kind of roofing. If so, then you are not exposed.

ACI does have a paragraph (in chapter 14) I believe, requiring walls to have double layers when larger than 10" thick (don't have my calculator handy to convert - I'm stuck in the non SI world...mentally!!).

A single layer should work provided you have designed for the actual depth, d, that will occur and you have considered the lower stiffness from a smaller d that results... for a thin shell with flexure and some axial load, you could have some P delta effects to consider.

 

[ishvaaag]

I also would detail the intersection of cylindrical surfaces likely thicker. By the way, I think a mandatory minimum for any RC shells in Spain stands around 5". This surely address some instances of local buckling seen to have happened in the less thick shells; it also helps to the general integrity of the shell.