Foundation Design 1

[teyobs]

Soil study shows that an area for a future 4 storey structure have a soil bearing capacity of 334Mpa at 2 meters deep and projected 25mm settlement for the proposed load. The soil study also suggest that rigid tie beam should be installed to contain uneven settlement and isolated footings can be used.

Previous design prior to soil investigation use isolated footing at a minimum depth of 1.2 meter design at 200 Mpa soil bearing capacity. The new design is now only 3 storey, the 4th floor is deleted.

Should the rigid tie beam still be necessary considering that the structure weight now is lighter?

 

[apsix]

The same potential for differential settlement will still be there with similar footing bearing stresses. Presumably you will still design for 334 KPa (?) allowable at 2 metres down. The footings will just be a bit smaller for the 3 storey, assuming construction type is similar.

 

[henri2]

Using a conversion factor of 1 MPa = approx 2090 psf, it seems the values you have for bearing capacity are several magnitudes of order too high for soil. I could be wrong though, since I have not used metric units in decades. Is it possible your units were meant to be KPa instead of MPa?

 

[teyobs]

Thank you apsix and henri2. By the way It is 334 Kpa not 334 Mpa. The design still use 200 Kpa at a depth of 2.0 meters. At this value which will and a lighter 3-storey structure, will I still need tie beams?

 

[mitchelon]

A decision cannot be made due to the deletion of a story. The result of a study determined that rigid tie beams were required for the 4 story building. Without the 4th floor, are the results telling you the same thing?

 

[teyobs]

No study were made if 3-storey structure will be constructed on site.

 

[mitchelon]

What I'm saying is that a study was performed for the 4 story bldg. The study determined that differential settlement is sufficient to cause problems and that tie beams are required. The questions should be, will the three stories still experience a significant differential settlement? Unless there is zero settlement on all footings, differential settlement will occur and tie beams may be required.

 

[DRC1]

Generally structues are designed to accomidate an 1" (25mm)of differential settlement, so the rigid beam may have been belt and suspender approach. If they bare to control settlemnt, I am not sure how they would limit settlement unless they are grade beams that are intended to absorb bearing loads. If they are tie beams for foundation rigidity, I suspect they have been included for seismic or other lateral load concerns. Best bet is to talk to the engineer who generated the report.