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- To: mony(--nospam--at)destination.com.lb, seaoc(--nospam--at)seaoc.org
- Subject: Increase Bearing Capacity - Case study -Reply
- From: Hector Buitrago <HBUITRAG(--nospam--at)BAS.CI.LA.CA.US>
- Date: Mon, 01 Dec 1997 07:16:51 -0800
I agree that to our way of thinking, they probably haven't lessened the bearing pressure agreat deal. The fact that the footings are in good shape (my assumption) indicates to me that there is slab action in the footings. But who am I to argue...the french have been building a lot longer than we have and with great success. >>> Sleiman Serhal <mony(--nospam--at)destination.com.lb> 11/28/97 02:33am >>> I'm involved in a project in Beirut Central District - currently the largest construction site in the world - consisting of the rehabilitation of a 70 year old concrete building. The overall condition of the building is pretty bad due to fires, explosions and deterioration of concrete. A new floor with a heavy concrete dome is to be added to the building. The strengthening of the existing concrete skeleton is done by an experienced structural engineer with a French college education. Footings were revealed and soil borings were done. The allowable bearing capacity given in the soil report was 2 Kg/cm² (4,000psf) and the actual stress on the footings was calculated to be around 4 Kg/cm² (8,000psf.) The allowable bearing capacity was based on Vesic's 1" maximum permissible settlement criterion. I argued that the soil report did not take into consideration the fact that the building had already completed most of its settlement after 70 years and that the relevant criterion was the added settlement from the new floor and dome. Anyways, to cut things short, the Contractor and structural engineer in charge of reinforcing the structure did not accept this argument and decided to enlarge the footings to meet the 2 Kg/cm² bearing capacity. Now the interesting part was how the footings were enlarged: The French design their footings in a way different from ours; they assume that the footing is too rigid to simply design as a slab with uniform load. They compute the required steel according to the "Bielles" method wich is a truss analogy approach. The load is transfered from the column at 45° angles through concrete compression struts and the steel takes the tension, either as a tension chord for rectangular footings or as radial stresses for circular footings. So the structural engineer capped the old footings with a new circular "footing" without even connecting the two footings with dowels or anchor bolts. He argued that the column jacketing would transfer the load to the enlarged footing just as the circular Bielles footing would work. I argued that: 1. The angle between the column base and the side of the enlarged footing, thus the radial steel, was too small (13° instead of 45°) to work as a truss and that the load would be transfered to the old footing. 2. The soil underneath the old footing is more "rigid" than the unconsolidated soild underneath the enlarged part of the footing thus making the old footing more stiff and prone to take up much more of the load. 3. Adding to the above that the old and new footings are not interconnected makes predicting the behavior of this composite footing very hard. Well my arguments were ignored and the footings were enlarged. This was facilitated by the fact that the client did not mind the extra costs.
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