Column Cover Plate and removal of X-Bracing
Column Cover Plate and removal of X-Bracing
(OP)
I do a lot of work with existing structures and a number of retro fitting projects. This particular project I need to remove x-bracing and a horizontal beam in a particular bay of a braced frame to make room for a monorail and clearance for its cargo. Essentially I will be increasing the effective length(KL) by x2 in the Y-axis for one column, and x2 in the X-axis for the other.
Being that the structure is existing, loads are not easily determined and would take quite a bit of time to collect necessary info to come up with accuate loads.
My plan is to add cover plates to each column to match their "old" capacity corresponding to their "old" effective length. Without being able to add vertical bracing, how can I make sure the added cover plates will provide enough strength to resist lateral forces? (Turning a braced frame into a moment frame) The frames above and below are both braced, and will somehow need to transfer the lateral forces to one another. I will be able to estimate the wind loads acting on the structure to get a rough idea of how much load will need to be transferred.
EIT
Being that the structure is existing, loads are not easily determined and would take quite a bit of time to collect necessary info to come up with accuate loads.
My plan is to add cover plates to each column to match their "old" capacity corresponding to their "old" effective length. Without being able to add vertical bracing, how can I make sure the added cover plates will provide enough strength to resist lateral forces? (Turning a braced frame into a moment frame) The frames above and below are both braced, and will somehow need to transfer the lateral forces to one another. I will be able to estimate the wind loads acting on the structure to get a rough idea of how much load will need to be transferred.
EIT






RE: Column Cover Plate and removal of X-Bracing
RE: Column Cover Plate and removal of X-Bracing
Since I don't want it to fail in weak direction buckling I would like to stiffen it so my kl/rx closely approximates my kl/ry. That way it doesn't matter what the load is.
That does what you need for gravity, after that you need to look at Ch 35 of the IBC or IEBC (or your local code). You are definitely changing the lateral characteristics of the structure so you will likely have to dip into Ch 16 and design it like it was a new building.
However, since you are only punching a hole in the building, could you move the x-brace down a bay or two? Then you would not fundamentally be changing the lateral characteristics of the orthogonal loads. If you get resistance to moving the brace from an architect or owner, show them what they might have to do if you need to go to a moment frame. That cost could be a deal killer, and could persuade them to come around to just moving things.
RE: Column Cover Plate and removal of X-Bracing
See the link below for a picture. You can see the monorail in the photo and where they have removed half of the X-bracing. The lower portion and horizontal beam will also be removed. I appreciate the help/input!
RE: Column Cover Plate and removal of X-Bracing
dougantholz-
the x-bracing runs up the entire side of the structure (check out the photo link above). I agree with your thought about moment frame ---> cost killer.
RE: Column Cover Plate and removal of X-Bracing
As recommended above use coverplates on the columns to ensure l/r is not reduced.
RE: Column Cover Plate and removal of X-Bracing
Removing the one level of x-bracing seems reasonable since both the bay above and bay below are x-braced. The visible column splice looks good - assuming a continuous column should also be reasonable. With that said, the x-bracing looks a little "flimsy" for a generating station. IMHO, if you "upgrade" the x-bracing (and their connections) to both the bay above & below, the problem is solved (with the cover plates added, too). What I mean by upgrading is to (at a minimum) add another angle to each existing x-brace member. That is, put the new angles back-to-back with the existing. This looks like it would make the bracing symmetric and a little more robust. Of course the connections for these should be checked and re-welded as needed.
As you know, most industrial structures are typically designed more conservatively than buildings since the loads are somewhat unpredictable, and it is almost "a sure thing" that changes will be made over the years. In the photo, this unit looks to fit that description. This may be one those opportunities to take advantage of the "wiggle-room" in the design. Computing the real loads on that portion of the structure is almost a wild guess anyway.
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RE: Column Cover Plate and removal of X-Bracing
RE: Column Cover Plate and removal of X-Bracing
In a Generating Station, such as this one, there isn't a "real" building or even a wall. Just a massive braced steel frame. What looks like a wall is just thin (non-structural) sheet metal covering the frame.
The heavy steel frame is needed since a utility boiler is suspended. The boiler has no "base", as such, it hangs from massive beams at the top of the frame.
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RE: Column Cover Plate and removal of X-Bracing
RE: Column Cover Plate and removal of X-Bracing
No bracing can be placed in that bay, as the riggers want the area totally clear for monorail cargo. I have already pushed for chevron braces or "K" braces, both of which were rejected.
SlideRule-
I like your thoughts, and will potentially be the way I go. I did get my hands on some prehistoric drawings and was able to build a model. I'm going to estimate some lateral loads and see how the model distributes these without that bay being braced.
Now one other question. Above people have mentioned to try to cover plate to maintain the same KL/r as before the braces and horizontal members were taken out. Can I just calculate the old compressive strength for that section (with the old effective length) and then design the coverplated modified section to match this compressive strength? I will have a higher KL/r but with the added steel area the compressive strength is much higher. Does this make sense? I can attach a calculation if needed.
RE: Column Cover Plate and removal of X-Bracing
RE: Column Cover Plate and removal of X-Bracing
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RE: Column Cover Plate and removal of X-Bracing
This could be a difference between 2in cover plates and 1/2in plates.
I have attached a calc for the compressive strength of the existing conditions and for the modified conditions. Maybe it will make more sense as to what i'm doing.
Thanks again!
RE: Column Cover Plate and removal of X-Bracing
Adding the cover plates will be field welding. You don't want to be totally dependent on "shop-quality" welds to make your design work as intended.
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RE: Column Cover Plate and removal of X-Bracing
One thing that I am not sure anyone mentioned is the fact that all columns will now suffer from additional bending from the sway action of the lower frame. The upper bracing only restrains the node points leaving the upper columns free to flex between these.
Because of this, you should check the influence of bending on the columns for the full height of the structure. I would imagine that you will need the cover plates for at least the bottom 2 levels if not more.
RE: Column Cover Plate and removal of X-Bracing
RE: Column Cover Plate and removal of X-Bracing
RE: Column Cover Plate and removal of X-Bracing
One clarification for LRFD design. I know in ASD if you are modifying an existing flexure or compression member, you have to take into account residual stresses and design accordingly. For a compressive member using LRFD does the same apply? In other words, can I just calculate section properties for the built up column section and find the compressive strength without taking existing load (on the column) into account? They will not be unloading the column completely (only the conveyer will be unloaded).