Stirrup Spacing for Grade Beams

[GeauxTiger10]

Looking for the maximum spacing requirements for stirrups in 24” deep grade beams. The stirrups would be used to hold the rebar in place, essentially as ties for the reinforcement as they would not be required for shear. I’m thinking 48” O.C. would be sufficient, but are there any other codes pertaining to maximum stirrup spacing?

ACI 318 Section 11.5.4 provides stirrup spacing requirements for shear; however, minimal shear is occuring.

IBC 2006 refers to ACI 318.

WRI: Design of Slab-on-Ground Foundations states “FOR BEAMS 36” TO 54” DEEP USE #3 STIRRUPS AT MAXIMUM 24” O.C.”

 

[msquared48]

I would use 24". Just feels better...

Technically, to get extra shear, 21" would be the max considering cover for the depth.

A little extra shear capacity never hurt anyone.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[JAE]

We typically use 48" o.c. as they are only used to hold the longitudinal reinforcement in place.

With typically low shear, and in our area, fairly deep grade beams (3'-6" below grade) the shear is way below [φ]V_c/2 so stirrups aren't required by code.

 

[csd72]

I would design it similar to JAE and just increase the depth if required to achieve this. A bit extra concrete is cheaper than the labour required to fix stirrups.

 

[hokie66]

JAE's approach is fine if the grade beam is 42" deep for other reasons, I assume for frost heave. But I don't think csd72 is correct that increasing the depth is more economical than adding more stirrups. Thinking back, most of my designs would probably agree with Mike's thinking to place them at 24", be that right or wrong.

 

[TXStructural]

There is no maximum spacing where they are not required. However, stirrups are an inexpensive way to assure ductility. Installed cost of stirrups will be a few dollars each. I would design for minimum stirrups and make all grade beams the same. Using one grade beam section with stirrups at minimum spacing (unless there are odd conditions) will typically be very economical.

For example, #4 closed stirrups for a 100' x 200' building with a uniform 2' x 2' grade beam, spaced at 21" o.c., should cost about $1800. The cost at 48" o.c. will be about $800. (Based on a rough approximation of installed cost at $1/pound. Wholesale cost of rebar was about $750/ton last month.)

 

[Brad805]

You could leave the non-structural decisions up to the contractor.