Shoring Walls & Lateral Stability 3

[Trenno]

I'm hoping to create a discussion about best practices in capturing the influence of shoring walls on lateral stability behaviour (backstay effect).

Typically speaking in my region of the world, the shoring system consists of ~600mm reinforced concrete soldier piles at ~1800mm c/c with 200mm thick shotcrete infill panels (1x layer of mesh and dowels to the soldier piles). Generally we connect the ground floor podium slab into the soldier pile capping beam. Even if you don't tie the ground floor and basement slabs into the shoring wall with rebar, the argument could be made that the frictional forces would be able to transfer a degree of slab diaphragm shear into the shoring wall.

I know the answer will lie somewhere between modelling the full stiffness of the shoring wall within the ETABS model and completely ignoring it. Would be good to hear what others opinions are on:

- Modelling individual soldier piles as frame elements including stiffness modifiers.
- Modelling shotcrete panels including stiffness modifiers and boundary conditions.
- Modelling the capping beam as a frame element including stiffness modifiers.
- Use of springs to simulate horizontal and vertical boundary conditions of the soil-structure interaction.
- Enveloping soil stiffness interactions.
- If using non-linear springs, how to deal with the pitfalls associated with Response Spectrum Analysis at the same time.
- The need for semi-rigid and cracked (membrane) stiffness modifiers for slab diaphragms.

Just to make it more complicated, the soldier pile and shotcrete panel is usually Contractor designed. So conveying all of the non-gravity design actions to the shoring designer is a nightmare.

 

[HTURKAK]

In my region, the shoring system is not connected to ground floor podium slab , other basement floors and raft mat. But we provide perimeter wall all around and backfill the gap with selected material..
My opinion is ,
The second picture ( back stay effect ) could be assumed reasonable in case of the podium slab designed as transfer slab and wall shear is transferred to the raft via podium slab and perimeter walls..
In this case total base shear will be resisted by raft - soil friction, passive thrust of the front basement wall and friction btw side walls and soil.. and in order to mobilize passive thrust , you need some substantial deformation and side friction will be limited ( for water proof membrane , protection paint ...) so eventually the base shear will be resisted by raft-soil friction and transferred by perimeter walls and shear walls ..

In your case , the back stay effect will develop partially ..













If you put garbage in a computer nothing comes out but garbage. But this garbage, having passed through a very expensive machine, is somehow ennobled and none dare criticize it. ( ANONYMOUS )

 

[KootK]

1) First, some resources.

a) Structure mag article by Tocci: Link. Probably a little entry level for you.

b) British Columbia now has a practice guideline that touches on this: Link. See section 3.4.5.

2) In my experience, the key to not getting unmanageable shear reversals in your below grade walls is modelling some manner of flexibility in the diaphragms and basement walls. It doesn't take much to accomplish a lot but, at the same time, I know of little the way of published guidance. The BC thing mentions a range of 5% to 100% which is, obviously, a pretty big range.

3) I can't imagine excluding the wall panels from such an investigation unless you're doing an upper bound / lower bound thing where there is some case in which you consider them.

4) Where does your diagram come from?