Unusual case of shear friction

[ntn94]

I am working with a precast block wall that is going to have vertical cells cast in to allow rebar. The rebar is going to be cast into a foundation below and the blocks will be slipped over the bars and stacked to form a wall. The cells are going to be filled with non-structural fill and the rebar will have a cap plate and nut at the top of the wall. My question is, if I design the cap plate and nut for the tensile capacity of the bar, can I say that shear friction is used to resist the in-plane shear loads of the wall? I essentially plan to "develop" the bar using this cap plate & nut instead of it being traditionally developed in concrete or grout, is this valid?

Our client tends to prefer options that make it more difficult on the structural side of things which lead to the unusual conditions described above, any input would be appreciated!

 

[BridgeSmith]

Looks like a post-tensioned segmental block retaining wall, assuming the bar will be tensioned at the end of the process. I don't see any reason why you wouldn't be able to count on the shear friction, at least the part of the equation that accounts for the resistance at the concrete interfaces. I don't think you can account for the shear capacity of the bars themselves, as the movement necessary to engage them would likely be excessive. I would use the method in your governing design code to calculate the prestess losses in the bars, to get the remaining compression at the block interfaces for use in the calculations for the interface shear resistance.

 

[ntn94]

Thank you for the input @HotRod10, the only difference in my situation is that these bars are not scheduled to be post-tensioned. Assuming they are adequately tightened at the top such that there will be minimal elongation during a lateral event, can I still make these assumptions? I agree that the shaer capacity of the bars should be neglected since they are not in a proper structural grouted cell.

 

[BridgeSmith]

If there won't be any permanent tension in the bars, then there's no permanent compression applied to the block interfaces, other than the weight of the blocks themselves. If you're considering seismic loading, I don't believe you can even count on the full weight of the blocks, as there may be dynamic vertical ground movement during an earthquake. What's left? Interlock of the block infill? I wouldn't count on that for the same reason as discounting the bars - too much displacement before engagement.

Why would you not post-tension the bars? All it takes is tightening the nut so that the bar remains permanently in tension. It's a fairly common procedure, usually using high-strength threadbar, but not much different with lower strength bars.

 

[KootK]

Assuming they are adequately tightened at the top such that there will be minimal elongation during a lateral event, can I still make these assumptions?

I would say no if the rods will not be prestressed. While the code provisions don't mention it explicitly, I think that bar strain matters in addition to bar strength. Shear friction mobilizes a small amount of separation movement and, when it occurs, that needs to stress the reinforcement in a meaningful way in order to be effective. With, potentially, a much longer strain length for your clamping reinforcement, I question it's efficacy.

 

[canwesteng]

The commentary for A23.3 does mention that the crack widening due to the rough surface is what causes tension in the bar. Without quantifiable tension in the bars, you've basically got nothing in the case of a seismic event as hotrod said.

 

[ntn94]

Thank you all for the responses. After digesting everything, I agree that post-tensioning the bars will be required to justify this approach.

 

[jayrod12]

Can we ask why would you fill the cores with non-structural fill, when you could just fill it with structural fill and not be worried about anything? I get the whole crazy client stuff, but seriously....

 

[ntn94]

@jayrod12 -The sleeves are created using PVC pipes that remain in the blocks. The bond between the grout and PVC pipe is not strong enough to develop the bar leading to the cap plate&nut approach.

@canwesteng -Are there any suggestions for the amount of post-tensioning that should be done to ensure proper functionality? I was going to use 10% of the bar capacity.

 

[jayrod12]

What about changing to steel sleeves then? perhaps too costly? Other options to cure your issues might be equally expensive.

 

[SwinnyGG]

Arbitrary check-in from the non-SE dude who reads all these threads..

If the bars are not substantially attached to the block wall (i.e. due to lack of bonding strength w/ the PVC sleeve, no meaningful long-axis attachment) AND they don't have significant tension applied by the nut at the top... what purpose do the bars actually serve? Seems like a PT strand that never gets PT'd isn't doing much of anything, other than absorbing dollars.

 

[BridgeSmith]

I think jayrod's question was suggesting something more along the lines of grouting the cores to provide monolithic concrete across the joints between blocks.

 

[dhengr]

Ntn94:
The bars better be well embedded/secured a the found. if you are going to tension them. Then, I wonder what the condition of the threads at the top will be when you are done placing the blocks over them. And, that the contractor doesn’t bitch about threading these blocks over a 10’ high rod, swingin in the breeze is hard to imagine. I’ve seen this done with actual post tensioned rods, a hydraulic pulling jack at the top and run the nut down once the proper extension is achieved. They often leave a clean-out at the bottom, with a coupler nut on the bar sticking up out of the found, and then install the long bar after the blocks are in place. You apparently have an unusual condition on this project, that this is the first time some core shear, bond or horiz. joint friction is needed and becomes an issue. I wonder that the manufacturing process would change greatly if you got some 2-3” dia. electrical conduit cut to replace the PVC pipe they normally use. Special design conditions sometimes require some special thought or components.

 

[hokie66]

Sounds to me like a search for an answer to a problem that doesn't exist. Conventionally laid block, reinforced and grouted, is a much better solution.