Differential Settlement of two adjacent structures 1

[strucEng36]

As per the enclosed sketch
I am working on steel moment frame structure resting on clay soil attached attached to concrete tower resting on rock
Both structures will be based on shallow isolated footing with no tie beams.
The soil report provided allowable bearing to be 125 kpa
The report mentioned that the maximum differential settlement under the maximum allowable pressure is expected to be 25mm with is within the limits of angular deflection limit as per the code.
I have also calculated the maximum separation joint based on lateral load analysis for the two structures to be 1" As per NBCC seismic separation requirement.
I am also considering to size the footing to have approximate nearby bearing pressure between interior and perimeter footings.

I am concerned to hear you opinions and thoughts on this case as it is new to me, Also is there any precautions I should be looking into?

Thank you,

 

[oldestguy]

As to the sizing of footings for equal settlement, one may try to use equal bearing pressure, but that is not very precise. Instead, I'd get back to the geotech and provide the column loads for each situation and the footing grade. Ask for a specific determination of footing size for each column load based upon an actual settlement analysis for each situation. Ideally they would provide a chart showing a plot of lines of equal settlement with the ordinate as column load and the abscissa the footing width. While they may want to run consolidation tests, they may be able to estimate these consolidation test results and come closer than you can with equal bearing pressures.

An old paper by BK Hough provided relationships between N value and some of the values used for these settlement analyses.

ASCE SM 4, # "Compressibilit5y as the Basis for Soil Bearing Value". August 1959 His value C=1+e/Cc called Bearing Capacity Index

 

[strucEng36]

Thank you very much for the valuable input
why is this case specific load calculation would be needed if the footings have similar pressure


Generally in clay the increase of least width of footing will decrease the bearing capacity?


As the report mention maximum least width to be 6 m and beyond that the bearing capacity will be reduced and geotech should be consulted

 

[BigH]

strucEng36 - if you look at the bearing capacity (shear) equation, as B increases (the footing width) Qultimate increases - the nuance that you refer to is the allowable bearing pressure - as the footing width (size) increases the zone of influence of the stresses below the footing increases and, in clayey soils, that typically means additional settlement - hence, the allowable bearing pressure will decrease. As oldestguy says, charts can be developed for allowable bearing pressure vs footing width . . . you can do this for a single footing and you can do this for adjacent footings using elastic solutions of stress overlaps, etc.

 

[strucEng36]

Thank You for the valuable input
Is there any other consideration in this case I should account for ?