Driled Pier Horizontal Deflection Limit 1

[haynewp]

I have drilled piers with sizes ranging from 3ft to 5ft. I have looked into this and I have found from 1/4" to 1/2" deflection, or 1% of the diameter of the pier may be common limits used as the limit for horizontal deflection at the head.

The building is concrete with brick facade. 1/2" seems OK for seismic, but there are large basement walls that are putting force into the floor diaphragm, which loads the columns, which loads the piers. What limit would be common for horizontal deflection under soil pressure alone, not considering combined seismic?

 

[JAE]

haynewp - not dealt with this on buildings before but for monopoles (traffic mast arm poles) we try to limit lateral deflection to 1/2". It is difficult to tie in calculations, models, etc. to perfectly match the reality of the pier deflections since lateral soil stiffness is harder to model/calculate than vertical soil stiffness.

 

[leo5cn]

I deal with monopoles (utility steel poles). we limit lateral deflection at ground level from 20mm to 25mm. We model the soil layers as linear springs, the drilled shaft is rigid and the rotation point is somewhere along the shaft.

 

[molerat2210]

IBC limits the service lateral load to 1/2 the load that produces 1 inch of deflection. I presume the seismic lateral earth pressure is a service load.

 

[FixedEarth]

"What limit would be common for horizontal deflection under soil pressure alone, not considering combined seismic?" The induced deflection at the pier head alone is meaningless measure. You look at the the induced lateral pressure in comparison to the allowable passive resistance. So for example, if your pier head deflection is 0.2 inch, but you exceed your allowable passive resistance, then either increase pier diameter, increase f'c or improve/remove the upper 3B - 5B soils - where B is the pier diameter.

I am currently incorporating this into our LATERAL FOUNDATION program. For a free headed pier/pile with a given shear load, it works out to that you get higher top of pier head deflection with an increasing moment load but you get lower induced passive pressure with that same increasing moment.