Distribution of Lateral Loads Along Wall/Column Line

[Trillers]

Greetings All:

A client has requested that we analyze a building design which is topped off with a cast-in-place RC roof. All the loads (vertical and lateral) are born along the exterior walls. The building is 48' x 66'. The face in question (grid line if you will) is 66' long and supported by 5 columns spaced at 16' OC face to face.

What we are to look at is replacing the CIP roof structure with RC precast double tees in the hopes of shaving 75 days of construction time for this structure.

The client would also like one of the corner columns (end of this grid line) replaced with an 8' long wall. This means that along this grid line we would have an 8' long RC wall with a column 8' from its end and the other three (3) columns spaced as described in Para 1 above.

We would like to manually check the lateral loading on each column but are disagreeing on how to distribute the lateral loading among the wall and columns. Once colleague insists on beginning the frame analysis at the interior face of the wall and ignoring the wall which would mean that the remaining four (4) columns would each be resisting greater loads.

Another colleague says we should treat the reinforced wall as a column carrying the equivalent lateral load as the other end column.

While this is a true issue facing us, we have also upped the ante and turned this into a "train the brain, ice cream to the winner" question which we agreed would be decided by our friends at eng-tips.

So please folks - help us decide which approach performed manually would comply with the IBC and ASCE 7702?

 

[Shu Jiang PEng SE]

The roof should be rigid diaphragm in your case, and the horizontal displacement at the top of shear wall and columns should be same. You can find the horizontal stiffness of the column and wall top and then distribute the horizontal force per the stiffness. Generally, the stiffness of shear wall is much larger than that of the columns. you can conservatively assume the shear wall take the horizontal force along the line

 

[JAE]

Per jiang46602, the forces in columns/walls depends entirely on their relative stiffness.

When you say "columns" I'm assuming you have some type of beam on top of the columns and this is a moment frame correct? Please clarify.

If this structure is designed for seismic, then some other rules might apply such as first distributing loads based on stiffness, but then using 25% of the load in the moment frame as a minimum since this sounds like a dual system.

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[Shu Jiang PEng SE]

I agree that if the structure is designed for seismic, the moment frame should take at least 25% of horizontal forces for a dual system.

A new structure designed as dual system, properly configuration of the stiffness of the shear wall and moment frame should be done, so that the moment frame can take at least 25% of horizontal force. The actual force should be distributed per the actual relative stiffness between the columns and the shear wall.

 

[Deker]

Agree with JAE. If this is a dual system, you need to run two different models and design the moment frames for the worst case. See this thread: Link.