Column Cover Plate and removal of X-Bracing

[UtilityStructure]

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

 

[bmccann]

An elevation of your existing lateral load resisting element would be helpfull in this case. A picture can go a long way. What seismic zone are you in. What is the height of the building? Have you identified the system you are dealing with per ASCE 7-05 table 12.2-1. Hopefully you can classify this region of the lateral load resisting element as an ordinary steem moment frame (OSMF) since your design options are not limited. If you get into having to design an intermediate or special moment frame (IMF, SMF) and you have not done a design according to the new AISC 341-05 "Seismic provisions for structural steel buildings" you may need to purchase yourself a night light for some bedtime reading. To date there are only three acceptable "prequalified" moment connections that can be used when designing an intermediate or special moment frame. They are listed in AISC 358-05 "prequalified connections for special and intermediate steel moment frames for seismic applications". They are the RBS, BUEEP, and the BSEEP. The reduced beam section, bolted unstiffened extended end plate, and bolted stiffened extended end plate respectively. Hope this leads you in the right direction.

 

[dougantholz]

When stiffening here is the approach I might take:
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.

 

[UtilityStructure]

Seismic is not an issue at the location as wind will govern. The height of the structure is about 120 ft.
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!

 

[UtilityStructure]

I should mention that this structure is the coal unloading transfer mechanism. There is a conveyer (not shown in the photo) that is connected to the top of the building which runs perpendicular to the bracing. This is where significant lateral loads will come from as the span from the conveyer support is quite a ways away.

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.

 

[apsix]

I would look at introducing a K-brace, with the braces coming to each side of the monorail.
As recommended above use coverplates on the columns to ensure l/r is not reduced.

 

[SlideRuleEra]

UtilityStructure - I looked at the photo yesterday, and have thought about the situation overnight. I believe the problem is manageable. As you stated, the proposed existing column cover plates should take care of the increased unbraced column length caused by removing the horizontal beam.

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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[hokie66]

Are you talking about the horizontal bracing, Slide? If so, I thnk you are on the right track. That will provide for load sharing from the outstanding bent back to the building wall, and the load can then be shared back out to the exterior below the opening.

 

[SlideRuleEra]

hokie66 - Per my understanding of UtilitySturcture's photo, I have marked the horizontal beam and the two remaining braces in red (see attached, marked-up photo below).

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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[hokie66]

Yes, I understand that he wants to take out the members you have marked in red. But when I referred to the wall of the building, I mean the braced frame which I think is behind the sheeting. And there is cross bracing in the horizontal planes of the outstanding skeletal structure which would make the external bent work with the internal braced frame. As these braces may have been designed for nominal robustness, they should be checked to redistribute the forces which result from the alteration.

 

[UtilityStructure]

apsix-

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.

 

[UtilityStructure]

Also, would like to thank you guys for your time/efforts. Forgot to mention this in my post above but its greatly appreciated

 

[SlideRuleEra]

UtilityStructure - IMHO, don't skimp on the cover plates. Most of the cost will be the installation, not the material. Anyway, you don't have enought hard information (on new / existing loads, etc.) to depend too much on the results from calculations.

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[UtilityStructure]

Point taken (and I agree), but i'm not really doing anything with new/existing loads. All i'm doing is matching the new compressive capacity with the old compressive capacity (based on new effective lengths and modified section properties). I have been working with a more experienced engineer (PE) and he also agrees with me, but I wanted another opinion.

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!

 

[SlideRuleEra]

Ok... but I suggest that you still be somewhat conservative for another reason:

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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[csd72]

So basically you are making the lower level into a portal frame rather than a braced frame.

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.

 

[hokie66]

I don't think he is depending on portal frame action. Above and below the open bay, it is still braced. The point I tried to make before is that the bracing in the horizontal planes at the top and bottom of the open bay needs to distribute the forces.

 

[csd72]

Maybe portal frame action is the wrong word, but to my understanding he is relying on bending action between the braces this will give secondary bending moments.

 

[UtilityStructure]

Thanks again for all of your input. After getting some drawings and after a site visit, I realized that the top of the coal transfer tower will act as a rigid diaphram and will transfer most of the lateral forces from the conveyer into the main building frame.

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).