Internal pressures do not contribute to foundation reactions???

[KevinChez]

A design example in FEMA 550 says the following:


Determine external pressure coefficients for the various building surfaces. Internal pressures, which act on all internal surfaces, do not contribute to the foundation reactions. For sign conven- tion, positive pressures act inward on a building surface and negative pressures act outward.

I do not follow why they do not contribute to the foundation reactions?

 

[JedClampett]

If you have a building or vessel that is pressurized, the pressure on the roof negates the pressure on the floor. So no net pressure.

 

[KevinChez]

Thank you. I follow what you are saying.

ASCE 7 GCpi is a +/- value. The reactions from a shear wall or moment frame will include the internal pressures?

 

[aeoliantexan]

Be carefull! Non-spherical structures are seldom stiff enough to distribute the loads resulting from interior pressures to the opposite side of the structure.

An example is a cylindrical tank storing liquified ammonia or natural gas. The vapor pressure (often limited to about 1 psi) acts against both the dome roof and the flat steel floor. But the roof pulls up on the walls, and the thin flat floor can't hold them down. It is necessary to tie the walls to the ring foundation and use a wide tee footing or deep foundations to resist the uplift.

Another example would be a hangar. Foundations in tension are required to resist the uplift forces the roof generates in the columns. The downward air pressure on the floor can't be mobilized to resist the uplift.

 

[JedClampett]

No net pressure, but for a building the attachments to the floor must be designed for the internal pressure.

 

[Ron]

Internal pressures may be additive to external pressures that affect the structural in a "global" sense. In that context, they apply if the foundation is used to counter such forces.

 

[KevinChez]

Thanks for the comments. I am going to continue to research. I don't like the FEMA foundation design examples. I've been directed to them because we are raising houses in NY from storm Sandy damage and checking someones calculation spreadsheet. It basically mirrors FEMA examples which are very basic rectangular house.

 

[aeoliantexan]

Back in the mid-70s, a major tornado plowed through Omaha. I lived a few blocks from the storm's path. Many frame houses sat on basements, and the frame was not anchored to the basement walls (the code allowed it at the time). Some houses were picked up off the basements and thrown into the yard. Some were picked up, rotated about their vertical axes, and set back down at a skew. Others, of course, had the roofs torn off. The code now requires reinforcing and anchor bolts in the CMU basement walls and tie-down straps between the roofs and the walls.

I still do not understand how FEMA intended the statement you quoted to be applied.