Cantilevered Pier Footing w/ Axial & Moment

[PBW2]

Designing a second story addition to an existing residence. The structure is a typical wood frame building on CMU piers. I am considering that all of the horizontal loads (wind seismic) will be equally distributed to the piers by the diaphragm action.

This gives me a rather large lateral load/pier and in turn a large overturning momoment. The existing footings are 3x3. I can design the footing modifications to take the axial and moment but I end up doubling the footing sizes. I was thinking about adding steel moment resisting frames to take the moment or adding grade beams between the footings. I am looking for other suggestions/advice.

 

[BAretired]

I would opt for moment frames.

BA

 

[JoshPlumSE]

Adding a grade beam between footings is common. That's just a combined footing or strap footing. It can be more economical to go with one of these combined footings rather that to size each footing to resist the load independently.

 

[MiketheEngineer]

I beleive some codes require that the individual piers be tied together in some fashion - like frames or grade beams.

 

[hokie66]

You could also use cross bracing between some of the supports. May need to use steel columns instead of CMU.

 

[PBW2]

Thanks for the responses.

BAretired - Is there a reason that you would opt for moment frames over grade beam? Simpson makes a prefab moment frame that I might consider.

JoshPlum - For the design of the grade beam, can the grade beam be designed for the moment only and the footings for the axial or am I making it to simple?

Mike - I will be sure to check the code

hokie66 - Walls between columns have to be break away (coastal) bracing or shear walls are not an option.

thanks again

 

[BAretired]

PBW2,

I prefer frames because they tie the piers together thereby reduce bending and deflection in the masonry piers.

Grade beams work too, but the masonry piers are then individual cantilevers which do not appeal to me, particularly if the lateral load has a torsional component about the center of gravity of the piers.

BA

 

[rscassar]

If the CMU piers do not have sufficient capacity to resist the lateral forces then I would look to create L-shape walls at the corners of the building.