Murali27,
There is the option of reducing the live and wind loads to 75% per 1609.4.3, OR increasing the allowable stresses (normally by 33%) as specified on the material section. To do both, reduce the loads and increase the allowable stresses, is NOT permitted. I do not think these paragraphs apply to foundations on high velocity hurricane zones.
My understanding about the foundation is as follows:
Paragraphs 1624.1.1 through 1624.1.3 are trying to keep the long term settlement of the foundations as equal as possible. Using that method, the soil bearing under the column with the largest percentage of live load to total load will be the allowable soil pressure when fully loaded. While the actual soil bearing under the other columns will be less than the allowable.
Paragraph 1624.1.4, in my opinion, is modified by paragraps 1624.2 and 1624.2.1. It is a remainder that the foundation has to be checked for dead plus live load, and also for dead plus live plus wind loads, since the allowable soil bearing is increased when wind is included.
To design the foundations I would use the dead load, plus the total reduced live load (per 1604.2) at the member immediately above the foundation for the preliminary sizing of the footings. Then I would check the footings for dead, live and wind loads, without reducing the live and wind loads to 75% as permitted on 1609.4.3.
If the wind load is less than 25% of the dead plus live loads, would neglect the wind load.
If the wind is more than 25%, would increase the allowable soil bearing pressure by 25% when using the total combined loads of dead, live and wind.
In other words, the allowable soil bearing pressure or the allowable pile capacity are increased by 25% for load combinations that include wind (instead of 33% as permitted for other materials.) Differently than clay, sand (this is Florida!) could consolidate quicker under transient loads.
Regards
AEF
