Cantilever vs restrained basement walls

[wikidcool]

2016 CBC 1610.1 has an exception that says:

Foundation walls extending not more than 8 feet (2438 mm) below grade and laterally supported at the top by flexible diaphragms shall be permitted to be designed for active pressure.

ASCE 7-10 Table 3.2-1 has a similar scenario in the footnotes -

For relatively rigid walls, as when braced by floors, the design lateral load shall be increased for silt and clay type soils to 100 psf (15.71 kN/m2) per foot(meter)of depth. Basement walls extending not more than 8 ft (2.44 m) below grade and supporting light floor systems are not considered as being relatively rigid walls.

Does this mean the wall can still be designed as a restrained wall but with the lower active pressure, or does it mean it has to be designed as a cantilevered wall? I realize it's arguable how much restraint flexible diaphragms provide at the top, and the connections can be difficult to work out, and there are constructability concerns, shoring, equipment surcharges, yada yada. But to dumb it down to this one concept, what exactly is the intention of the code with this provision? I encounter this situation on small residential work occasionally. There is rarely a geotech involved and we are using default values.

 

[GeoPaveTraffic]

I can't speak to the code requirements, but soil doesn't read the code.

If the wall doesn't move enough to allow the soil to move from its current pressure state, generally at-rest; then the pressures will be greater than active, period. Also remember that if you compact the backfill the pressure can actually be greater than at-rest.

For plastic silts and clays, it takes about 1" of horizontal movement at the top for a 10' tall wall to go from at-rest to active pressures.

Mike Lambert

 

[wikidcool]

Thank you for your thoughts. Seems like the code is trying to address that at 8 ft tall or less, light-framed flexible diaphragms don't always provide enough restraint at the top to prevent the soil slip and engagement of active pressure, and therefore these walls should/can be designed as cantilevered? I'm not sure why they don't use the terms cantilevered/restrained, and limit the statements to just the pressures.

Interestingly, this language has been removed from the upcoming 2018 IBC. Anyone know the reason why they took it out?

 

[DaveAtkins]

I believe empirical evidence has shown that walls less than 8' tall do not need to be designed for at rest pressure. It has nothing to do with flexible diaphragms, movement of walls, cantilever versus restrained, etc. For these short walls, active pressure results in a sufficiently conservative design.

DaveAtkins

 

[wikidcool]

Would you design this as a restrained wall using active pressure, or as a cantilevered wall? There IS a diaphragm at the top providing at least some restraint.

 

[DaveAtkins]

I would design it as a restrained wall, using active earth pressure.

I know, this flies in the face of conventional design, but I think it works for short walls.

DaveAtkins

 

[BridgeSmith]

If it's restrained at the top, by a diaphragm capable of resisting the applied load, then it can be designed as such.

The major consideration for what soil pressure to use is the level of compaction of the retained backfill. If it's backfilled with minimal compaction, then active soil pressure is likely adequate. If it's fully compacted, at-rest pressure would appropriate. Assuming the restraint conditions at the top and bottom of the wall are modeled accurately, those should apply regardless of the height. I would not rely on the assumption of conservatism in typical design to make an unconservative assumption with regard to the applied soil pressure.