Braced Frame Footing Design - Lateral Seismic Load?? 5

[DavidStructural08]

Hi all,

I came out with this interesting thought. Say I have a braced frame isolated footing which will take 300 kips seismic lateral shear (footing takes all lateral shear, no slab on grade involved). My question is that:

Do we usually consider this 300 kips in the footing design, besides overall frame overtuning?

To be specific, say for example the footing is 42" deep, do we consider the extra 1050 k-ft (300x42/12) moment in the soil bearing pressure design check for this footing? The extra soil pressure will be 1050 k-ft / (bh^2/6) if no uplift, with b the width and h the length.

 

[escrowe]

Why consider shear force to be
a component of axial bearing pressure?

Well why not. If we assume that the structure and the footing
are moving 'out of sync' with the ground as a result
of the stiffness/dampening properties of the building...

Then first determine the axial and lateral components of
the increased and total load, and particularly the resulting maximum pressure
(eccentricity) at the foundation edge due to overturning
during the seismic event.

Accepting that axial bearing capacity depends on soil type/strength,
groundwater depth and foundation size,
also remember that the bearing capacity of the soil
may be impacted by the seismic event,
particularly through liquifaction
(so determine Seismic Shear Stress Ratio of soil).

So it seems that the bearing capacity of the soil may be reduced
even as the axial foundation bearing pressure
increases due to overturning forces.

So in the case of increased axial foundation bearing pressure,
yes foundation bearing capacity
and settlement must be considered.

For the lateral component, typically a pseudostatic analysis
based on ground acceleration is appropriate
for laterally loaded structures (i.e retaining walls).
Of couse in the normal case we assume
that the ground motion is accelerating the structure--
the opposite is true in this case.

The lateral movements of the foundation are resisted by
the footing base friction (shear) and by the weight
of soil adjacent to the foundation face orthoganol
to the direction of force (passive earth pressure).
So it seems prudent to maintain sufficient embedment
and size of the foundation to reduce movement
resulting from the lateral component to an accepatable range.

 

[JAE]

Yes, you consider all the applied loads for each and every load combination required in the applicable building code.

The footing will actually see that moment at its base due to the lateral shear and it will see the soil pressures from the overturning of the overall braced frame.

One thought - in higher seismic areas we usually try to avoid isolated footings as part of the lateral system. Tying the footing to an adjacent footing with an underfloor grade beam would be a way to ensure that your column legs wouldn't do the splits during a seismic event and also be a way to reduce this overturning moment effect.

 

[WillisV]

Agreed with JAE - I typically run a grade beam between the two footings, that way when one of the footings is in uplift due to the global braced frame overturning moment you have the friction generated by the downward force of the opposite column to resist overall sliding.

 

[DavidStructural08]

Thanks, guys.

Another question: is there any reference example I can look at for the connecting grade beam design?

Should it be designed for axial load only (seismic lateral load), or a moment capacity should be checked?