seismic tie beams

[structengineer]

I have a project that is requiring seismic tie beams between column pads per 1999 SBC. I have discussed the design of the tie beam with others in the office and some stated that it should be treated as a column design, while one stated that it should be designed as a beam (the senior engineer). Is the senior engineer right? Also, can a tie beam slope from one footing to another such as from footing elevation 99 feet to footing elevation at 96 feet with 20 feet between the columns. The senior engineer stated that this is not a problem. The code is vague in the requirements for tie beams, any help is greatly appreciated!

 

[haynewp]

If the slab on grade is adequately supported by the soil and the only purpose of the tie beams is to keep the footings from spreading during an earthquake, then I would design them as compression members. Maybe the other guy is thinking of grade beams that are used when there is inadequate soil support.

 

[JAE]

haynewp is right. The tie beams are intended as axial members (tension and compression) based on a percentage of the axial force in the columns.

 

[bkgd]

Out west the code requires that grade beams are required to resist 10% of the column axial load, so thus they act as a column, not a beam at grade. Sometimes we will beef up the slab on grade and use it to meet the tie requirement and eliminate the beams and save some dollars, however we add bar centered on the grid lines.

Check ACI, i think it touches on the subject.

regards,