wind column footing seems large 4

[EngineerofSteel]

Designing a footing for a wind column

The wind column is 15" c/c from the column of a center frame for a 50 x 50 PEMB.

The building company gives P = 42.8K for DL + RLL for the frame column

For the wind column, P = 43.0K and the moment generated is 66.8K

After working on this, I came up with a very large footing, 7'6" x 6'0" x 36" thick.

This seems really large to me. What is the typical procedure to design a wind column footing?

What is the best design to account for the eccentricity of the frame column & wind column being centered just 13" from the outside of the slab.

Base plates are both 10" square.

 

[Lion06]

I don't think that footing sounds terribly big. Very thick, but the plan dimensions are not very big at all. I've had 12' square x 2'-6" thick columns for a canopy structure, just because it has no DL.

 

[EngineerofSteel]

Looking at it fresh this morning, I think I see my error:

A wind column takes no vertical loads from the building.

The building company's analysis report gives the following reactions for seismic (which controls as being twice as large as the wind reactions):
Horz: 4.8K
Vert: 43.0K
Moment: 66.8K

I am summing the vertical 43.0K with the effect of the moment. I think the report is giving the Horz + Vert forces as the resolved moment, and so I am basically doubling the force.

I am reading this report correctly?

 

[steellion]

It would be impossible for any of us to tell without seeing the report. Run a quick sanity-check calculation to figure out approximately what forces you're looking at, and then call the PEMB designer and ask what the numbers mean. Designing foundations for PEMB's require a lot of coordination with the PEMB manufacturer. The footing will likely be controlled by uplift/lateral forces.

 

[EngineerofSteel]

A wind column, by definition, takes only lateral loads (wind & seismic).

So, part of my question pertains to the nature of a wind column.

I will call the building manufacturer.

 

[EngineerofSteel]

I phoned them & left a message, as requested. I'll follow up with a post about the structural calculations report.

 

[EngineerofSteel]

The engineer from the NCI confirmed that the moment is calculated from the horizontal shear force. The horizontal and vertical forces in the report do not need to be added.

 

[jimstructures]

DDesigner;

There is some confusion in names. Some PEMB companies refer to endwall interior cols/posts as wind posts and these posts do not have a moment at the base which needs a moment resisting foundation under it. Other PEMB companies have bracing components called windposts which serve to transfer bracing loads into the foundation and have moments at the base which require a moment resisting foundation.

Your reactions which you give for the moment at the base of the column are expressed only in kips. This is not a bracing column and does not have a moment at the base. If the base of column load is expressed in kips for shear and inch-kips (or foot-kips) for moment then you do have a moment resisting column. These are very different columns and require very different foundations under them.

As somebody else has said without seeing the report it is very dificult to tell exactly what is going on at the base of your column.

Is your column or post located along the plane of the endwall (between the sidewall/corner columns) and supporting girts and panel or is next to (or part of) a sidewall column and/or a corner column (may or may not be supporting girts)?

Your comment (from the NCI engineer) about the moment being calculated from the horizontal shear force tells me that this column is an endwall post and not a moment resisting windpost that has a moment resisting foundation. The moment in your foundation is an internal moment ( the shear force times the distance to the bottom of the foundation) to the foundation.

There are some questions that need to cleared up before much useful help can be given.

Please provide a sketch of the location of the post and the direction of the forces.

Your statement that "A Wind column, by definition, takes only lateral loads (wind & seismic)." varies between manufactures of Metal Buildings. It is also important how it transfers those forces into the foundation. Whether it has a pinned base of column or whether it has a fixed base of column.

Hope this helps and is not to confused.

 

[msquared48]

Rather than a separate footing, why not anchor the wind column to a concrete grade beam that wpans between two mainframe footings?

You will pick up dome dead load resistance and have a longer moment resisting arm in the length of the grade beam. The volume of extra concrete could be less too.

Mike McCann
MMC Engineering

 

[EngineerofSteel]

Jim,

Thanks for the excellent response.

The Reaction number listed under the section "wind columns" lists only this one column, which is nested into a column on one side of the central frame in a three-frame steel building.

This wind column is a T-section, a half W beam. The flange is situated nearest to the frame column.

I've designed somewhere around 10 or 12 of these during the last 6 years. I've seen "wind frames", light sections designed for gravity loads only, and to transmit wind and seismic to other frames. I've only seen the a "wind column" twice: in this 50x50 PEMB and in a much larger project in Nevada 5 years ago.

I also google-imaged "wind column". The results give several examples of what I have here: a T nested very close to a frame column. The separation here, c/c on the Anchor Bolt groups, is 16"

MEA CULPA
The report does not give the kips, inch-kips. In the heading, it says "--Reaction (k, f-k).
When I put 66.8K above, I only meant to use the K as "thousand". I was leaving out the ft-lb units. Sloppy on my part.

Thanks again.

 

[jimstructures]

Without seeing an anchor bolt detail sheet and layout sheet as well as the reaction sheet it is very difficult to understand what is going on.

The previous manufacturer I worked for used a separate column that carried very little axial load, to provide bracing in the the longitudinal direction for wind load on the endwall. This solution was not as stiff as rod or cable bracing and also less stiff than a longitudinal portal frame. This column had a fixed base plate with grout under the wind post but not under the mainframe column. This column requires a large concrete block under it to provide some base fixity to the foundation. It was often designed in combination with the column footing. The Butler Foundation Manual from 1985 gives a method for solving for the foundation size and reinforcement quantity.

Another manufacturer uses a detail similar to what you described, with a T-shaped section welded to both sides of the web on the main frame column. I do not know if this additional column had grout under it's base plate (one base plate on each side of the main columns base plate). I do not know if they had a similar detail with a T-shape only on one side of the column. I would think if they had the T-shape on only one side of the column they would have to another main column some where nearby with the T-shape on the other side for symmetry or windload in the opposite direction.

Until I see more information on your situation I am not going to be able to add much useful comment.

Hope some of this helps.