Drilled Pier Ties

[mlevario99]

I'm working in a 4 story concrete building. The foundation is drilled piers, the seismic category is B, I check the ASCE 7-05 and IBC 06 to check if the piers needed to be connected by tie beams, IBC section 1808.2.5 Stability. Piers shall be braced to provide lateral stability in all directions. IBC section 1808.2.23.1 Seismic Design Category C, which says that you need to interconnect piers by ties, but this is for seismic category C, they do not have anything for seismic category B.

We will have a slab on grade at first level, which I tought the slab on grade will brace the pier.

Does anyone has dealt with this before.

 

[dougantholz]

We have done this, although the slab still needs to be able to take some axial load to stabilize the pier. I would still use the percent of axial to design horizontal "wall" elements .

 

[lrhg]

I typically provide grade beams to tie all drilled piers that transfer base shear (wind, seismic) to the soil, analyzing piers + GB's as moment frames including the lateral soil springs along the shaft of the piers. For drilled piers that only support gravity loads, I usually don't provide grade beams, though I check the shaft (modelled with the lateral soil springs) and provide sufficient rebar to take the moment due to 3" of eccentricty to account for a pier located off center line. The geotech engineer provides the soil stifness values based on the borings.