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# Retaining Wall - block wall with reinforced concrete core4

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## Retaining Wall - block wall with reinforced concrete core

(OP)
Hi all,

I need some guidance on how to calculate for the reinforcement steel in retaining wall.

I have 2 block skins, infilled with a concrete core. each of 100mm thick. I was being conservative and ignoring the blocks in the design, just evaluating the concrete core. however this gives me 175mm thick C35 concrete core with T16 bars at 100centres - no compression steel needed (for a 2.6m high retaining wall based on designing as a cantilever beam per metre run).

I think that if I back to basics and try evaluating the capacity of the entire wall, composite "beam" including the 100mm thick block skins I can make design more efficient. I'm sure that 100mm thick concrete core is sufficient. This 100mm thick core seems to be a standard detail. However I want to make sure that I can justify this reduced size....

any tips / guidance on how composite design is done.... then how to calculate area of tensile steel in the concrete core part of the "Beam".

### RE: Retaining Wall - block wall with reinforced concrete core

Well, sounds like you are using 4" thick block, so installing rebar in that size block is not doable.

So, you will need to analyze the tension steel in the concrrete core, and use the f'm of the block for the concrete strength instead of the f'c of the concrete, and try to generate interstitlal shear at the interface for composite action.

That scenario could work to increase your effective "d", but could also be counter-productive with the lower f'm.

Mike McCann, PE, SE (WA)

### RE: Retaining Wall - block wall with reinforced concrete core

Is the wall constructed of two 4" wythes separated with a space that is filled with concrete? What is the overall thickness of the wall? A 2.6m cantilevered wall constructed using masonry may not be a good idea.

As the Oldestguy noted... segmental precast with geogrid is a good alternative.

Dik

### RE: Retaining Wall - block wall with reinforced concrete core

(OP)
Hi all,
thanks for the feedback. Apologies for not responding sooner. I am grateful.

the wall was going to be two skin 100mm block with a central 100mm cavity. As dik mentioned, using just masonry wasn't an option, which is why I am introducing the concrete core. This led to the question on whether I can somehow introduce the block strength into the design to increase "d" instead of using an ultra conservative approach. At moment I have simply ignored any contribution from the blocks, essentially considering the blocks as formwork only (to help cast the concrete core). This approach led me to have a 175mm thick concrete "cavity" with T12 reinforcement at 100centres.

I have seen a similar design for a basement (of a friends house) where only 100mm cavity infill was designed by engineer for similar load cases. I can't figure out how this was done as if I do similar design with 100mm concrete cavity it fails in every case?! which got me thinking I am either being too cautious, my friend's design was a little bit risky or actually the way I have approached it is standard and I should stop trying to engineer it to be smaller.

Due to the relatively low mass of wall ontop of increasing cavity to 175mm, I have had to include a deep heel beam to the wall to counter sliding. And a long toe to counter overturning.

thanks

### RE: Retaining Wall - block wall with reinforced concrete core

You are getting a variety of advice here, so I will give you another option.

I have no idea why you would use 2 wythes of block when one will do the job. The only thing I can imagine is if you need split face block for aesthetic reasons, but you didn't say so.

Instead of 2-100 skins and a 100 concrete filled cavity, why not just use 300 block? Less work, less likelihood of one of the skinny 100 skins failing when you fill the cavity, and greater bending strength. Just remember to put the bars on the right face of the wall. And if you don't want staining on the exposed face, remember the waterproofing.

### RE: Retaining Wall - block wall with reinforced concrete core

Wilko78 -

Depending on the method of placement of the "concrete"/grout in the approximate 100 mm cavity,you may actually be dealing with a thick (about 300mm) solid grouted composite masonry wall. I assume your concrete masonry units are actually 90 mm thick that are each over 75% solid and are classified as solid masonry units.

I wythes of the wall were built and the grouting of the cavity between them may have been adequate to develop enough shear strength at the masonry/grout interface. - Not much shear strength is required. When a proper grout (8" to 11" slump)is placed, the bond is developed by the mechanical friction and the absorption of the moisture in the grout is great enough the justify a composite wall performance. Walls with filled 3/8" collar joints between masonry units are analyzed as a composite wall sections.

The compressive strength of the is not a factor in the wall strength, just as the mortar is not a limiting factor.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

### RE: Retaining Wall - block wall with reinforced concrete core

the block is where the strength is in masonry design. ignoring the block and designing around the grouted core seems akin to designing steel around the web and ignoring the flange.... maybe i'm misunderstanding what is going on here.

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