Transferring Earth Loads thru Rigid Diaphragms at Exposed Basement

[enigma2]

I have a building roughly 300’ x 175’. It is built into a hill. There is a concrete foundation wall between the basement slab and the first floor. Between the first and second floor, the north and west walls are concrete and built into the hill. The south and east walls are mostly exposed (at grade). The floor construction at both elevated floors is concrete topped precast plank supported by steel beams and columns. The second floor is at grade at the north and west elevations. It is steel columns and girders and bar joist girders. The roof is metal deck.

They will be backfilling after the floors are in place, because against my better judgment they do not want the expense of counterfort retaining walls. I am incredibly concerned about unbalanced earth pressure particularly the ability to transfer it through the diaphragm at the first floor. I know I’m going to need to dowel the concrete topping to the foundation wall to help transfer shear. Any words of advice as to what else I might need to consider? Should I be considering passive pressure on the single story basement wall?

 

[hokie66]

I would not consider it passive pressure, as the building is not moving into the hill. At rest pressure is more appropriate. I agree that you must make the first floor structure act as a diaphragm, so the topping must be a structural topping, reinforced to distribute the diaphragm forces, and connected to the perimeter walls.

 

[Larryhd2]

You must take the reaction from the restrained retaining wall into the diaphragm. Section 1610 in the CBC... not sure if it's the same in the IBC.

Be sure you also include the seismic loading on the retaining wall and take that wall reaction into the diaphragm.... This load can be huge. Be sure you have a soils report and they tell you what loading to use.

 

[enigma2]

Well my initial thought with regard to considering passive pressure was that the section of wall that is only one story below grade opposite the side of the wall two stories below grade would want to move into the soil. I thought I could possibly use the passive pressure that develops as additional restraint against lateral movement. But, I think I would need to ensure the passive pressure would develop and I’m not sure I know how to determine this.

If I consider at rest pressure everywhere, I have a net force of about 14 klf that needs to go into the diaphragm.

 

[GeoPaveTraffic]

Keep thinking in terms of load paths. All of the horizontal load has to go somewhere. To develop passive pressure, the wall would need to move upto 5% of the wall height. If you don't take the load via at-rest/passive pressure, then you will need to take it in some other fashon and get the load there.

You don't indicate what load you are designing for, but remember that if the wall on the high side doesn't move, then the applied load will be at-rest (not active). Also, make sure to account for any water pressure that may develop on the uphill side.

Mike Lambert

 

[RFreund]

14 klf, wow, Thats a pretty good load. I'd be curious to see the geometry or section... maybe a sketch if you have time.

Also I'm curious what others would say about designing the walls for at-rest pressures but checking stability with active pressures. i.e. if the structure is going to overturn it needs to move.

EIT

http://www.HowToEngineer.com

 

[RFreund]

Sorry forgot to ask what sort of at-rest coefficient and soil density you are using?

EIT

http://www.HowToEngineer.com