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Shear key subject to long-term soil loads

Shear key subject to long-term soil loads

Shear key subject to long-term soil loads

(OP)
A while back at my old job I did some consulting for a precast company and we designed a standard tank structure composed of precast concrete box sections. Guess they liked my work because I work for them now! :)

Anyway, I've been tasked with tweaking the rebar and structure to get as much efficiency as possible out of them as they're getting large orders for these tanks. All is well except for the end-walls which are cast by taking the box structure, flipping it on end, and then casting it as a flat slab with a shear key. See the attached PDF drawing here: http://files.engineering.com/getfile.aspx?folder=e...

I've worked out the strength and load on the shear key and found it to be controlled by low soil cover and a HS-20 vehicle load directly adjacent to the structure (though deep buried soil loads cause about 50% of this stress). This vehicle load causes a factored shear of 8,448 lb/ft at the joint based on tributary area of the end wall. Based on the 1993 articles by Kaneko et al on fracture mechanics of shear keys I calculated the allowable strength of the key as 11,422 lb/ft. Thus, I should have acceptable strength for an un-reinforced key. However, I dislike this so requested they drill and epoxy some dowels with about 2,500 lb/anchor allowable shear strength per ACI 318 app. D.

It's also worth noting that they're sealing the joint with a butyl rubber sealant and surface sealant.

Now, they REALLY want to get rid of these dowels. I can understand this, drilling those holes on each end, epoxying the dowels, then waiting for them to cure adds a lot of time to the production. However, I've seen shear keys fail under long-duration loads in large above-ground concrete tanks and don't want to leave this joint unreinforced. But, I still have nothing I can find in any literature against this practice and the strength calculation shows that it should work.

Anyone have any advise or should I relent and let them have this joint un-reinforced?

Maine Professional and Structural Engineer
American Concrete Industries
www.americanconcrete.com

RE: Shear key subject to long-term soil loads

It appears that the hatch opening is very close to the wall with the key - even near the mid-point of the wall - seems like the little outer key portion at the hatch would be blown out anyway.

On idea might be to use a lap joint but I think your worries over long term cracking would still apply to that.


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RE: Shear key subject to long-term soil loads

(OP)
JAE: Yeah, that is a good point. At least with the dowels I could get the shear load away from the manhole opening. I'll see about moving that opening inward a bit or perhaps request dowels along the top at a minimum.

An option I just thought of would be structural fibers in the end wall in addition to the WWR. Should increase the durability and shear strength. Thoughts?

Maine Professional and Structural Engineer
American Concrete Industries
www.americanconcrete.com

RE: Shear key subject to long-term soil loads

I guess I don't have any strong opinion on steel fibers....seems like they would add to the concrete toughness.
For many steel fibers, their length might not be consistent with the small size of the keyway and the keyway side tongues.

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RE: Shear key subject to long-term soil loads

What about casting some larger Dia ducts into the box which the bars are poked into and they just fill with concrete when pouring the base. As long as they are diligent with their vibration of the concrete it might be an option.

RE: Shear key subject to long-term soil loads

(OP)
Not a bad idea agent as I don't need any tensile capacity of the dowels once they're set in the walls, and the concrete guys here are really good with filling the forms.

Maine Professional and Structural Engineer
American Concrete Industries
www.americanconcrete.com

RE: Shear key subject to long-term soil loads

As another option you might be able to look into the type of proprietary dowels that are used in slab on grade construction joints where you cast in the first part attached to the inside of the form and poke a dowel in after.

RE: Shear key subject to long-term soil loads

Is there no potential for meaningful outward load on the shear key joint? The joint, as a shear key only, would have almost no capacity to speak of in the outwards direction. Frankly, I'd be scared that it would break off during shipping or installation.

Regarding Kaneko's work, isn't substantial joint compression required for any serious shear capacity to be developed? For a keyway into bedrock at the side of an arch dam, I'm in. The degree of compression in your scenario could pretty nominal however.

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.

RE: Shear key subject to long-term soil loads

(OP)
KootK: Little to none. The loads during loading, shipping, and placement are almost always going to be inward with the exception of flipping the piece after pouring the end wall, when it will only have to carry self-weight. The only outward load will be during leak testing or service, both which are only allowed while backfilled with compacted soil to the finished grade. Thus, the backfill will be present to resist outward water pressures and that outward pressure is limited to the capacity of the tank. Inward pressure greatly controls the shear key design.

From this pdf, see page 5 and page 45-46 figure 5 and figure 6: http://www.usbr.gov/ssle/damsafety/TechDev/DSOTech...

I interpret this to mean that the minimum shear stress is at least equal to the cohesion strength of the concrete and calculated the shear capacity as such with a phi = 0.75 for shear. I consider this conservative given that, as the shear key tries to move laterally it will cause compressive forces to build in the wall due to being restrained on all sides. This should cause an increase in the shear strength of the key and a friction load to develop resisting further movement. Please tell me if you have a different interpretation.

Maine Professional and Structural Engineer
American Concrete Industries
www.americanconcrete.com

RE: Shear key subject to long-term soil loads

Quote (TME)

Please tell me if you have a different interpretation.

Nope, my interpretation agrees with yours. Basically, your key strength becomes 0.1 x f'c x 0.75 x key base area, yes? This is really quite wonderful. I've wanted to know how to calc a shear key for upwards of a decade now and I wasn't aware that a method was available. Interestingly, the resulting values are nowhere near the values that I've seen from folks doing first principles analyses.

I've been using what I consider a better shear key lately. In your case, I would make the left interior side of the key vertical and shift the key to the interior as far as tolerances allow. It's less key strip prep and conserves more of your member shear depth.

Congratulations on the new job! I've always thought that precast would be interesting work. Folks in that world tend to be a lot more bold in their connection design than EOR types.

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.

RE: Shear key subject to long-term soil loads

(OP)
I too had been looking for a shear key design strength calculation method and stumbled up this paper about 2 years ago. Very useful and vastly different from the strengths I'd seen other engineers use (both higher and lower than). And, yes, I calculated my strength precisely based on that equation. Interestingly enough this ends up being a higher shear strength than the 6" wall thickness and the shear through the wall controlled the shear strength of the wall. I assumed full depth for shear though as it didn't seem correct to assume the 45 degree shear crack starts 1.25" in at the corner of the shear key. Otherwise the Kaneko equation for shear key strength seems pointless.

Unless of course the shear key strength equation assumes that simple beam shear will not control... Now I've got myself wondering if I need to always include a check for simple beam shear starting from the corner of the key.

Hmmm, I see what you mean about a "better" key. Your profile means they only have to rip one side of a piece of dimension lumber to make the key and gives more strength to that side of the female joint. I'll make that modification.

As for the new job; I've been loving it. At my old job I consulted with them on and off for about 5 years now so we're quite familiar with each other. I'm glad they're willing to move engineering in-house and they're a great company to work for. Very, very good precasters, too. I hope to be even more useful to them as well as they're moving to providing full engineering consulting in the future in addition to precast manufacturing. Basically I'll find my own jobs in the non-construction season and offer people discounts on my engineering fees if the project can include some precast. Plus, as you say, being a manufacturing facility they do like to push the design as far as possible. The boss was also trained as an engineer, but never licensed, so he knows when he can push my design and when not. In the end it's going to be exciting and a great learning experience.

Maine Professional and Structural Engineer
American Concrete Industries
www.americanconcrete.com

RE: Shear key subject to long-term soil loads

Quote (TME)

Now I've got myself wondering if I need to always include a check for simple beam shear starting from the corner of the key.

I agree with this approach. Additionally, I think that the shear capacity should be based on the plain concrete provisions.

I imagine that precast work has some of the features that I miss from the wood truss design world. Engineering problems are much richer when you add mass production, quality assurance, and logistic into the mix as extra dimensions.

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.

RE: Shear key subject to long-term soil loads

(OP)
Yeah, plain concrete makes sense as I'm not carrying the reinforcement into the support like I would on a normal concrete slab/beam.

Maine Professional and Structural Engineer
American Concrete Industries
www.americanconcrete.com

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