www.sefindia.org

[P.K.Mallick]

A structure supported on isolated footings will have less mass participating in the dynamic response, while a structure supported on a single raft foundation will have more mass participating in the dynamic response. This is because when the building is supported on isolated footings, the load is distributed over more points, which results in less mass participating in the dynamic response. On the other hand, when the building is supported on a single raft foundation, the load is concentrated in one area, which results in more mass participating in the dynamic response.
PLEASE LET ME KNOW WHETHER THE ABOVE STATEMENT IS CORRECT!

[vikram.jeet]

Respected PK Mullick sir,,
It is after a long gap,, yr posting on seen in forum  ,, pl keep guiding  as forum members need yr  views on Structural & Civil engg matters.

Not an expert of str dynamics,, may not able to say much,, but i think foundation size seems immaterial as ground movement is en-mass with structure foundations and ground moving as one entity,, same could be said about pile  foundations..

Hope some expert on str dynamics may throw real light,

My best regards to you

[pvgraju]

Dear Sir,

1. Structure with fixed base.
2. Structure supported on raft with soil springs.

Columns supported on raft can be considered as fixed for analysis. The resulting reactions are to be used for raft design.

Soil structure interaction will modify time period, increases displacements. This may effect mass participation.

But if the base is considered as fixed and resulting reactions are used for isolated footing, raft design then mass participation will be same.

Regards,

P.V.Gavarraju.

[rahul.leslie]

I haven't fully deciphered the gist of the claim... but to my knowledge,  the mass of the foundation itself is not usually considered as dynamic mass -- see, the displacement value at the foundation level is zero in vibration mode shapes.  From this, you may arrive at your own conclusion.

Secondly,  if it is about mass participation of the superstructure, it may depend on whether the support points are fixed or pinned. Mass participation is influenced by whether the mode shape of the building is "shear" or "flexure" type. When the type of support* affects this aspect, it is an influencing factor. A correct inference can be made only by numerical analysis of a few typical models.

*ie., depending on how much the building supports provide some flexibility at the base (if it is in partial fixity)

[vikram.jeet]