ACI max bar spacing - grade beams 2

[abusementpark]

I notice a fair amount of engineers violate ACI's maximum bar spacing for flexural reinforcement in grade beams (i.e. a 18" or 24" wide grade beam with 2 bars top and bottom). Is there anywhere in the code that allows you to do this for foundation elements? Or is there a particular reason why engineers would deem this requirement not critical for a grade beam?

 

[ToadJones]

could it be designed as a plain concrete footing/?

 

[sandman21]

Thats not a grade beam, it a simple cont. footing. with cont. support. A grade beam transfer or takes bending then it is a beam.

 

[PUEngineer]

I guess I would question its function, and understand if it's a flexural member. I just worked on a project that required struts between pile caps. That would look like a grade beam, but in reality it's a tension/compression member. They were 24" square with 2 bars top and bottom.

 

[BAretired]

I wasn't aware there was a requirement for maximum spacing of bars in a grade beam. Six inch spacing seems reasonable, but can't really see the harm in exceeding that.

BA

 

[steellion]

Agreed with PUEngineer. I've seen tie beams detailed in that manner, but grade beams should follow the same detailing rules as above-grade concrete beams.

 

[a2mfk]

What about Sandman's question- are you talking about grade beams or continuous wall footings that are assumed to be fully supported by the soil?

 

[steellion]

The max spacing of flexural reinf in a concrete beam is defined in ACI 10.6.4 and is approx. 12". The max spacing of reinforcement in footings is 18" by ACI 10.5.4.

 

[abusementpark]

What about Sandman's question- are you talking about grade beams or continuous wall footings that are assumed to be fully supported by the soil?

I've seen it on both, but it is probably less prevalent on grade beams that are pile/shaft supported.

Even continuous soil supported grade beams have to behave as flexural members at door openings in the wall.

 

[sandman21]

Spacing has no effect on the strength of the member. 2-#9 next to each other have the same strength spaced at 18". Spacing requirements are more of a serviceability issue, for example deep beam requirements for skin reinforcement these requirements help prevent cracks. I don't care about cracks on grade beams they should not be seen.

Once you start getting into large flexural moments or extending footing to help with O.T. should the footing have the spacing and ties requirements of beams

 

[abusementpark]

I don't care about cracks on grade beams they should not be seen.

Would it be realistic to be concerned about excessive cracking allowing moisture to corrode the rebar easier?

 

[sandman21]

No the cracks they are controlling are appearance related, it takes a large crack to cause corrosion on grade beam reinforcement.

 

[abusementpark]

No the cracks they are controlling are appearance related, it takes a large crack to cause corrosion on grade beam reinforcement.

Ah, I see. Then, it would make sense that some people might not consider that requirement for a grade beam.