Integrated Buildings & Structures

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Column size reduction and soft story query

  • 1.  Column size reduction and soft story query

    Posted 06-25-2020 09:04 AM
    I have some questions about concrete structures, anybody can answer if possible-

    1.When we have to reduce column size in between two floors, how to place upper floor longitudinal reinforcement as per codal provision? I want to be sure of the actual method. If you have please suggest any reference.

    2. Building with soft storey some code approaches to designed for 2.5 times the story shears and moments calculated under seismic loads neglecting effect of infill walls. Hence what it means by "under seismic loads"? Increasing 2.5 times seismic load combination or increasing only induced earthquake force which you can refer to as base shear. It is too confusing, hope you can clear this confusion.

    Thanks in advance

    Md. Rifat Bin Ahmed Majumdar S.M.ASCE
    Structural Engineer

  • 2.  RE: Column size reduction and soft story query

    Posted 07-02-2020 04:23 PM

    You have not mentioned which jurisdiction you fall into, so it may be difficult to suggest. Assuming that you are using ACI code, you may refer clause 10.7.4 of ACI 318 -2019, which gives the required method of how to place the reinforcement when the column faces don't match. For even more detail you can also see section of ACI Detailing Manual SP 66 which directly addresses the issue of smaller upper story column.

    Basically you have two options if the offset is small less than 3" and you have sufficient depth in the beam column joint to slope that offset rebar into the upper column stirrups with a slope of less than 1 in 6 that you can do that.  Else for offset more than 3" you have to curtail the lower bars in the slab or beam and provide dowels for the upper floor column rebar.

    Secondly the 2.5 times requirement is for the soft lower story. Typically what happens is that the lower floors do not have in fill wall which are present in the upper floors and sometimes because of Open Basement and first floor requirements, buildings omit shear walls from the ground floors. This leads to a significant loss of stiffness in the lower floors and can be catastrophic. The upper floors more of less move in sync and the lower soft storey will end up having a higher seismic load and deflection, since it is less stiff and will absorb more of the seismic energy, If the lower storey is not strong and ductile enough to absorb those loads and deformation it may collapse. The shears and moments in the soft storey beams and columns due to seismic loads they all need to be multiplied by a factor of 2.5 for the design purposes.

    Hope this clarifies.

    Mandeep Singh Kohli CP, M.ASCE
    Senior Engineer