Minimum Steel in a Arched Concrete Slab

[cpb213]

Hello All, We have a underground structure that has an arched concrete roof. The roof is in complete compression and transfers the loads to the exterior walls. Someone had mentioned to me that we need to provide at least 1% reinforcement ratio because its a compression member. This arched slab spans 70 feet and is 40 feet wide. Does anyone agree that it needs to meet the minimum column reinforcement ratio of 1% per ACI or can I go with a lower ratio? I have always believe that refers to tied columns. Bearing walls are in compression and they don't require reinforcement ratios that high.

 

[WARose]

I'd still go with 1% as per ACI 318. ACI 334 (which covers shells) still kicks you into ACI 318 for minimum steel.

I've done a lot of arches.....and I've always used 1%.

EDIT: I’ve got an old reinforced concrete design text that (IIRC) treats this subject. Will look tonight and see what it says.

I don’t think the wall analogy works because ACI 318 is only allowing that for certain cases (i.e. empirical criteria, etc.). To my way of thinking, treating it like a wall doesn’t work because you are (or should be) checking it via a different buckling criteria than a wall. At that point……you are into Chapter 10 of ACI 318 (where Chapter 14 throws you outside of the special circumstances I mentioned before).

[red]EDIT #2[/red]: Checking the reference I mentioned (i.e. 'Design of Concrete Structures', 8th edition, by Winter & Nilson, (1972)), they are treating the members of the arch like column members. Basically figure the forces and design like any other column. (1% min.)

The folded plate design example in that book.....he doesn't use less than 0.35% anywhere where there are tensile stresses. (ACI 334 says something similar.)

 

[FloydLloyd]

There is a Code Requirements for Concrete Thin Shells document 318.2-14, a free download
It lists min. steel as 0.0018*Ag, typical temperature steel requirement for crack control.

 

[IDS]

For a 70 ft span arch to be in full compression it must have a very high axial load, so the minimum 1% requirement would be appropriate. Most large span buried arches have significant bending moments and the minimum flexural reinforcement requirements apply.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/