Bond beam?

[Redacted]

Hi there,

I am currently designing a reinforced concrete bond beam to rest on top of a masonry retaining wall to help to restrain it from overturning. I designed it like a normal beam spanning between two supports, however I conservatively didn't really take into consideration the fact that it is also somewhat restrained by the vertical rebars that enter it from the masonry block wall.

It is a small bond beam, only 8"x8" and due to this the maximum spacing of the required shear links is 0.7d which ends up needing T6's @ 4" centres. This would require around 30 links for a 10' beam. I understand that the spacing between shear links is directly correlated to the spacing of the shear cracks but for the application of a bond beam that is resting on a masonry wall in your opinion is it necessary?

 

[KootK]

Is the bond beam providing lateral restraint to the top of the wall somehow? What does it span between lateral? Usually these bond beams are just for load distribution and railing attachment and the demand on them is minimal.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Redacted]

Is the bond beam providing lateral restraint to the top of the wall somehow? What does it span between lateral? Usually these bond beams are just for load distribution and railing attachment and the demand on them is minimal.

Hi KootK, thanks for the response so far. Yes the beam is providing lateral restraint to the top of the wall. It will be tied into both the existing and new surrounding walls. See the updated attached sketch that shows the bond beam in plan. The lateral load at the top will be taking about 10kN unfactored, say 15 kN factored.

Apologies for the OP image, it appears that it was flipped the wrong way when I uploaded it.

 

[KootK]

Got it. I don't see how the shear would be helped by the fact that bond beam is vertically supported by the wall since the bond beam is laterally loaded. Perhaps you could use a multi course bond beam to improve capacity. Frankly, I'm not even sure how you'd get stirrups so small and tightly spaced into a bond beam. I'd seek some alternate system here.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Redacted]

Got it. I don't see how the shear would be helped by the fact that bond beam is vertically supported by the wall since the bond beam is laterally loaded. Perhaps you could use a multi course bond beam to improve capacity. Frankly, I'm not even sure how you'd get stirrups so small and tightly spaced into a bond beam. I'd seek some alternate system here.

Yes, that's true, however the shear capacity of the concrete was enough to sustain the shear loads on the beam. The provided shear links were just the minimum required at the max allowable spacing when designing the beam as simply spanning between two supports.

 

[KootK]

Can you not simply go with a design that is not reinforced for shear then? That's most bond beams.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Redacted]

Can you not simply go with a design that is not reinforced for shear then? That's most bond beams.

This is where I am confused. On all of the bond beams that I have seen images of online they do not have stirrups. I was designing this like a normal beam with a lateral load. Perhaps a bond beam design is completely different?

I was designing the beam to Eurocode. In that code even if the concrete has the shear capacity to take the shear force you still need to provide the minimum area of stirrups. In this case the minimum area would be satisfied by T6 spaced at 300mm. However when calculating the maximum spacing based on the size of the beam the max spacing of the stirrups can only be 100mm.

I assume my next option would be to just increase the size of the beam and have it overhang over the wall. This would make the max spacing allowance for the stirrups to be 6”+ if the beam changes from 8x8” to like 10x10”.

This design does not have to be done to Eurocode, it is just the only code that I know.

Is the shear reinforcement approach different in the ACI or Canadian codes?

What other solutions would you recommend to restrain the top of the wall from overturning, other than increasing the size of the footing? As this has already been poured.

 

[KootK]

Any chance the wall has lateral support on three sides (bottom/left/right). Were that the case, you could design the wall as a three sided plate which ought to be quite favorable.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Redacted]

Any chance the wall has lateral support on three sides (bottom/left/right). Were that the case, you could design the wall as a three sided plate which ought to be quite favorable.

The wall has lateral support on the left and right side from two connecting transverse walls with a spacing of 10’ between them. It does have lateral support on the bottom from the footing (not sure if this is what you meant?)

I guess the main issue with that solution is that the footing and wall had already been constructed before I got involved in the project. It is a simple 8” masonry block wall with vertical T12s spaced at 16”. The top of the wall has 2 courses free which gives me about 10-16” of space to come up with a solution to restrain the wall at the top.

 

[JAE]

Not sure about Eurocode - in the US if your beam has small shear - less than half the concrete-only shear capacity - you can ignore minimum stirrup/link spacing requirements.
That may be why you are seeing no stirrups/links in some designs.

I'd agree with KootK that getting these into an 8x8 bond beam would be difficult - not typical of this size of "bond beam".

You could consider a larger cast bond beam overhanging the wall in either direction.

Getting the vertical bars developed into the bond beam to transfer the lateral force from the wall below up into the beam would be tough too.

You could also consider multiple bond beams (2 or three).


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[Redacted]

You could consider a larger cast bond beam overhanging the wall in either direction.

It can only overhang in one direction due to the retained earth on the other side.

Getting the vertical bars developed into the bond beam to transfer the lateral force from the wall below up into the beam would be tough too.

Yes I think this will be tricky.

You could also consider multiple bond beams (2 or three).

What do you mean by this?

 

[JAE]

Stack them - one on top of the other - they would all work together and deflect similarly - sharing the load.

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[KootK]

You could consider a larger cast bond beam overhanging the wall in either direction.

That sounds pretty great to me.

Stack them - one on top of the other - they would all work together and deflect similarly - sharing the load.

Taken to extremes, this bears similarity to my three sided plate concept. You're making a "strong band" at the top. If you can get this done in a few courses, this is probably your lowest cost option. If you need many courses, back to the cast beam on top.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Redacted]

Stack them - one on top of the other - they would all work together and deflect similarly - sharing the load.

Would doing so remove the need for the stirrups? This would work but I it would require double the amount of reinforcement. However, with this said I would probably be able to reduce the size of the rebar since there would pretty much be 4 bottom reinforcing bars instead of the original 2.

That sounds pretty great to me.

I agree this is definitely possible. However, if it can only overhang in one direction (opposite the retained earth). Would a 4" overhang be acceptable? I assume that the upper floor slab would prevent it from toppling over.