Horizontal ties at Change in retaining wall thickness 1

[MileyDiley]

http://ladbs.org/LADBSWeb/LADBS_Forms/PlanCheck/PC-GRAD.Corr11-GradingPlanCheck.pdf

The link above is to Los Angeles City's standard grading plan check list.

Item G. 2 (on page 5 of 6) states "Provide retaining wall details on plans, show: surface drains, subsurface drains, slope of backfill, tie at change in wall thickness and reinforcement."

What is meant by "tie at change in wall thickness and reinforcement"?

Please see attached sketch. The ties drawn in that are what the plan checker is requesting - I have never seen these and do not know what purpose they could serve. Anyone have any idea?

 

[EngineeringEric]

It could be some sort of confinement tie for the top of the wall, but I am not sure as i have never seen that before.

 

[dik]

I wouldn't normally do it... but, like chicken soup... can't hurt!

 

[msquared48]

Actually, I have done this as common practice on my own for many years to limit the lateral splitting force of the top rebar to the top of the lower wall below., particularly where PS planks sit on top of a wall with a concrete stem wall above.

I didn't know any of my details got to California!

Mike McCann
MMC Engineering

 

[MileyDiley]

Thanks for the responses.

My initial thought was the plan check list is poorly worded and these ties are not the intent, as so far no one can provide a code reference.

The item starts out very general. "Provide retaining wall details on plans," then mentions a few key things these details should show. I think the "tie" part is a really bad way to say "show the required embeds/laps where wall thickness changes."

The plan checker says it's in the UBC masonry code, but they have an example worked out with 12" o/ 8" block with no mention of these ties (though that example may only use a single layer of reinforcement in the 12" stem).

My only contact with the plan checker is through a third party, as he won't respond to my phone calls or emails directly.

 

[AELLC]

Those responses above make sense, because there may be diminished shear capacity at the joint between the 8" and 12" CMU, plus this is California, seismic.

 

[MileyDiley]

How would those ties add shear capacity between the stems?

 

[AELLC]

The ties are preventing the verts from the 8" CMU form splitting the 12" CMU, as pointed out by Mike.
Therefore in a sense, you get more shear capacity, although it is impossible to calculate the actual shear capacity in the case you don't have those ties.

Likewise, it is impossible as far as I know to calculate the contribution of the ties. All I know is those ties should prevent the 8" CMU from shearing off the 12" CMU - something I never worried about here in AZ, but probably an issue there in CA.

 

[AELLC]

ps - if the grout the 12" CMU all the way to the top of the block (in error) there will be a lot of shear capacity reduction. Make sure they hold the grout down 2"

 

[MileyDiley]

AELLC - we hold down 1.5"

Just got off the phone with the plan checker. The ties aren't in any code because he's simply referring to tying the vertical bars in place.

 

[msquared48]

I do a lot of things that the code does not require. Not being required does not mean that it is not needed.

Mike McCann
MMC Engineering

 

[EngineeringEric]

As AELLC mentioned... , i am guessing this is something that cannot be calculated by a general equation but is rather something developed through testing/experience post failures. And if it is in a standard detail by a requirement it is my opinion that it is overkill but it serves a real purpose. And if it is required for LA County then it is required and the owner has to foot the cost of installing it.

 

[AELLC]

Now if the plans checker is saying they aren't required by Code, the are just holding the bars in place, that is easier done with prefab wire bar holders put in the joint of the bottom of that 12" CMU top course. I would at least put in #3 Z-bars anyways for the reasoning expressed by Mike.

 

[MileyDiley]

That's what the plan checker said. I'm planning on showing the ties and referring to our specs that call for securely tying vert steel in place.

 

[AELLC]

If the plans checker wants the rebar positioners near the top of the 12" wall, why doesn't he need them near the bottom?

 

[MileyDiley]

My guess: there's no "change in wall thickness"

Our note specifies top and bottom of wall and at 8' on center.

 

[ESse]

Long time reader, first time poster.

This retaining wall detail with an "offset splice" between the flexural reinforcing bars is problematic. My company has had a bad experience (partial collapse) in a very similar situation with a cast-in-place concrete retaining wall with stepped changes in thickness on the non-exposed face (the flexural tension face of the wall). If this wall were concrete, ACI 318 requires that non-contact lap splices be not spaced transversely farther apart that the smaller of one-fifth the required lap splice length and 6 inches. The offset is permitted to be transverse only (i.e. in the plane of the wall) since splitting will be resisted by the plane of the wall itself. Offset splices in the "out-of-plane" direction are not specifically addressed by the code. In this detail, there is a compression strut that develops between the bars, unfortunately, there is no "tie" to complete a strut-and-tie system and thus the detail is prone to splitting failure before the bars can be developed. Draw a free-body diagram with the flexural tension on each of the bars and you can see that the eccentricity between the flexural tension bars is unresolved unless confining ties are present.

See the attached article on tests that were performed by the Washington State DOT on offset splices at bridge column shaft to caisson interfaces. Their conclusion is that splitting occurs prior to yielding of the bars unless the splice is confined with ties.

As it is uncommon to have confinement ties in wall construction and I believe that this detail should be avoided.

 

[hokie66]

Good post ESse. Please come back, as we need more contributions like yours. The inherent defect of this detail is probably not apparent to many of us, and in concrete walls, it probably exists infrequently. It could be a lot more prevalent in CMU walls. The lack of anecdotal evidence of failures of this detail may be due to conservative assessment of soil pressures or some other factors.

 

[MileyDiley]

Update and new question:

 

[MileyDiley]

There's no way to edit posts here?

Anyway, the update is that the plan checker was apparently not asking for vertical bar ties. Instead, he wants the top horizontal bars (wherever we use two layers of reinforcement) tied together with seismic hooks each side.