What's the reason to connect ramp to walls on both sides?

[UpsideDown]

Instead of simply having a ramp on ground without connecting to walls on both sides, I have seen many engineers do it this way: using dowel bars drill and epoxy into walls and connect to the ramp. May I ask what is the concern? Can't the loading simply be transferred to the soil under the ramp without connecting to the walls? Thank you.

 

[hokie66]

Maybe. But sometimes the soil under a ramp is not well compacted. It is difficult to compact soil on a slope. The engineer in this case may consider that the slab has to span side to side.

Having said that, I don't support the detail of using drilled in dowels to support a ramp. The more reliable way is to use a thicker wall below the ramp in order to give direct bearing for the slab. Or maybe span the ramp the other way, with intermediate walls under the ramp.

 

[UpsideDown]

Thanks hokie66. The reason they don't want the wall to be under the slab is the construction sequence - builders want to build the ramp after the wall. (walls have to be continuous above the ramp too).

 

[hokie66]

I understand that. But they could have cross walls, or walls which step from say 12" to 8" in the vicinity of the ramp soffit.

 

[MotorCity]

Perhaps they considered the walls to be supported at the top (instead of cantilevered retaining walls), resulting in a smaller wall design.

I agree that its not a constructability friendly arrangement.

 

[EngineeringEric]

The walls are loaded laterally by the soil and surcharge. the load doesn't just go down.

So those side walls are retaining walls, if they are cantilevered (not connected to the slab) they have one stress state; if simple span (connected to slab) then they have nearly an opposite stress and far less demand so smaller walls and less rebar.

 

[Teguci]

I connect the walls to the slab on grade in order to provide a basement type retaining wall. Reducing lateral loads on my foundations this way is usually a free return on investment. When I have a concern with differential settlement between the wall and the slab on grade, I detail compressible fill (ex. pipe insulation) around the small diameter rebar to allow for some settlement.

For parking structures, at the ramp to supported deck transition wall, this connection is rigid since differential settlement is the enemy at that location. A recommended sequence of construction should be shown on the drawings.

 

[KootK]

The more reliable way is to use a thicker wall below the ramp in order to give direct bearing for the slab.

Love this. The structural engineering universe would be so much more pleasant if the various hokie-isms could be rolled into a prescriptive, global code.

As a compromise and nod to contractor preferences, you could go with a single thickness wall with a dowel + low key-way joint between the walls and the slab on grade. That way the walls can go up first with a single pour in simple formwork.

In my market, I see a lot of ramps go slab on grade for a short stretch and then suspended slab after that. I believe that, beyond a certain elevation, the suspended slab is actually cheaper than the compacted fill below.

 

[r13]

We have a few elements here - two free standing walls, a ramp, and soil below, each tends to move in different ways if not tied together. If the contractor insists, and the owner agrees, I'll have him chose 1) a thicker wall below (as hokie66 suggested) to provide a ledge for the ramp to bear/sit on; 2) dowel out as usual, and 3) drill and grout.

 

[hokie66]

KootK,
Thanks for that. But the problem with that, for some people, would be that my "hokie-ism" prescriptive code would prohibit the use of some popular stuff, e.g. shear friction theory.

 

[msquared48]

Aah! The shear-friction debate...

I took PT Concrete from Professor Birkeland, one of the proponents/originator of the theory and principal at Concrete Tech in Tacoma, WA back in ‘69 I believe.

I never was any good at PT, either before or after that class.

A man has to know his limitations.

Mike McCann, PE, SE (WA, HI)

 

[KootK]

But the problem with that, for some people, would be that my "hokie-ism" prescriptive code would prohibit the use of some popular stuff, e.g. shear friction theory.

It wouldn't be a problem for me. I'd be all for not chasing this one down the rabbit hole but I still view your perspective as, effectively, shear friction with super kick ass roughening (the kind more commonly known as bearing).

A man has to know his limitations.

That is rare wisdom that I wish I possessed myself. It's especially poignant, I think, in modern times where no self-respecting human is supposed to believe anything other than that they could do anything if only they applied themselves. I see this as the quite irrational conclusion of liberalism. When the machines take over, they'll put a swift end to it.

 

[UpsideDown]

Thanks guys. So if the connection is for retaining walls. What happens if I connect the slab to one wall only? (as the wall on the other side is shotcrete wall that has been design to span on cantilever piers anyway). Won't it be 'weird' for the slab to connect to one wall and the rest sit on the ground and still design it as slab-on-ground?

 

[r13]

Not if you accept the potential to have large, uneven separation gaps along the free edge, also the potential of tilting in the direction transverse to traffic. If connecting the ramp and the walls are highly undesirable, maybe you can consider utilizing edge beams (or thickened edges) to support, and control the movement of the ramp, then provide expansion joint at the edges; provides that both wall were adequately designed to deflect within tolerable limit.