Bar Supports for Mat Foundation >48 inches Thick

[Boiler106]

I've been asked by a Contractor to review/design bar supports for a 5' thick foundation. According to CRSI Manual of Standard Practice:

"Bar supports indicated are generally estimated and furnished for slabs up to 4 feet thick unless otherwise specified on the project drawings or in the project specifications"

I've noted in the Standard that there is a reference to CRSI's "RB4-1 Standard for Support for Reinforcement Used In Concrete"

Does anyone know if the RB4-1 is adequate to design such supports before i purchase it and does anyone have any other references for the design?

What are some methods that are typically used? Heavy bar standees? Structural steel? Posts embedded in concrete?

 

[structSU10]

From what I have seen about it, it should cover what you need. I would think something like a dowel block, or something similar would be used. Even a U bar ties to the top and bottom mats at some frequency, with the bottom mat supported by blocks or chairs may be enough.

 

[oldestguy]

You don't want metal that will not be protected against corrosion. Use bricks or similar.

 

[KootK]

What are some methods that are typically used?

I like a mud slab if you can talk your team into it.

 

[SlideRuleEra]

The Contractor is wise to have (pay for) an engineering review of the rebar support system. A 5' thick concrete placement in going to have heavy rebar mats. Contractor must want (or be required) to do the job right.

Agree with KootK to use a mud slab. Preparation for concrete placement will take time and once rebar is in place access to the subgrade is virtually impossible - don't want to have bad weather to make a "mess" of the subgrade.

Use concrete blocks manufactured for the purpose to support the bottom mat. How many and what spacing depends on rebar details and allowable bearing on the mud mat.

Structural steel (secured to the bottom mat) supports the top mat. Again, the size and amount of structural steel depends on rebar details.

When checking/designing rebar supports (concrete blocks and structural steel) allow for significant construction loading in addition to rebar mat weight. Note that the subgrade/mud mat/concrete blocks support everything - bottom mat, top mat, construction loading.

For a major concrete placement, rebar support design is just as important as design of the overall project. "Cutting corners" to "save money" on rebar support (which is sometimes though of as part of a Contractor's "means & methods") is false economy.

http://www.SlideRuleEra.net

 

[Agent666]

A few years ago I was involved in reviewing some very heavy cages on site which were to be supported and cast directly on exposed basalt rock. The contractor had initially tried using masonry blocks to support the cages from the rock which sort of worked but are a bit of a no-no because of the lower strength and they had to use quite a lot of them to prevent them crushing or splitting.

We made him swap them out for purpose build concrete spacers, being the proper way to do it we thought as the spacers have to be of the same strength as the concrete pour. These simply lasted about 2 seconds, cruching/splitting under the uneven weight when you moved around on the top if the cage. In the end they cast some unreinforced blocks about 100x100x300mm out of the leftovers of a batch of high strength concrete. This seemed to work just fine. In later pours when they were more organised they cast plinths of the exact height required to support the cage above the rock surface along the length of the footings, which also worked pretty good. Engineer wouldn't allow any site concrete (guessing this is the same as the mud mat being discussed, a thin layer to act as a solid working/support surface) due to the need to continously key into the rock on 3 sides to transfer the large base shears from multiple high rise cores.

I'm a great fan of the contractor providing some stirrups at regular centers in larger thicker mats of reinforcement even if the engineer doesn't detail anything. It helps a great deal, some contractors think they know better and you can't convince them that the little but of additional work can make their job a whole lot easier. For critical support steel like this consider also wiring them on with much larger diameter wire than your typical tie wire as they can easily break with the weight of the cage on them. (edit - also seen ubolt shackles used as well for a bulletproof connection that's unlikely to break during cage assembly and concrete placement).

Another important aspect I see contractors continually underestimate in thicker foundations is the need to have access to clean out the base. Best case scenario is to have a dedicated area off to the side of the cage which is purposely lower with a sump to enable all the gremlins that fall through the cage to the site concrete to be washed to and as a point to pump out the water. Ideally some slight fall on the site concrete makes everyone's life easier when it comes to cleaning out all the mud and crap.

 

[JLNJ]

We had some cages collapse recently which caused the bottom bars to lay on the bottom of the forms. The concrete couldn't flow and we ended up with large honeycombed areas.