Strengthening Fabricated Columns of Premanufactured Steel Buidling 2

[Derek1684]

I am trying to strengthen the building columns of a metal fabrication shop. There is a 10 ton overhead crane, and I am adding two more 15 ton overhead cranes. The runway beams are good if I keep hook centers 15' apart with bumpers. I just need to reinforce the columns.

The building is a prefabricated metal building. The bailing columns are steel plate stitched welded into a W colum. The web is about 36" and the flange is about 10" with a 3/8" thickness. The crane runway rest on beam seats welded to the side of the column. It seems like to much to get into the total engineering of the column because I would have to analyze the whole building loads. The columns were designed for the 10 ton crane, so I really only need 15kips of additional strength. I was thinking I could do it a few different ways:

1) Cap the front of the column with a C-channel.
2) Box the front of the column with a MC-channel. (Weld flanges of MC to front flange of column). I would think this would give a lot more strength since it creates box column
3) Weld flat steel bar to the front of the column - increasing the flange thickness.

I'm a mechanical engineer, but it's been a long time since I have done this detailed of a structural calculation. I figure I can be conservative on the steel and be assured I have a good safety factor.

Does anyone have experience strengthening fabricated metal building columns like this?

Thanks!

 

[haynewp]

With IBC, if you increase the loads above a certain percent you have to look at upgrading the element to current code which includes wind, etc. The options you list could work but there may be more required than that (ex., web reinforcing and bracing).

 

[hokie66]

With three cranes now in the building, I would want to add crane columns under the existing brackets rather than trying to reinforce the building columns.

 

[JAE]

So you are going to weld on some steel to the columns and then just assume they are adequate for the additional loading without any rational/calculated method applied?
That sounds like a hope and a prayer method by a non-structural engineer. This does not sound like good engineering to me.

The interplay between the axial loads and the flexural loads in the columns are not easily understood and just adding steel without, as haynewp suggests, doing the proper building analysis is not defensible.

The required steps, in my view, would involve full measurements of the steel columns, girders, etc., a current-code wind/seismic analysis along with the gravity/crane combinations, possibly sampling the steel to determine the Fy/Fu, etc. If you don't know how to do this I'd recommend you get some help.

 

[hokie66]

I would think my solution of providing separate crane columns would lessen or eliminate the need to evaluate the entire PEMB, which we all know is likely to be a difficult and fruitless exercise. Another advantage is that most of the work can be done on the shop floor, with most of the in place welding avoided.

 

[JAE]

hokie - agree 100%

 

[JAE]

One other add-on thought - additional columns might require foundation work to accommodate the column anchorage. But that would still be easier than analyzing a PEMB bent system.

 

[racookpe1978]

I did not see explicitly that the OP was going to analyze or predict the stop/start loads from the new crane rails either: Worse case condition, his cranes (two at once ?) add combined sideways loads unto the truss and PEMB bents that could warp the whole frame system. Downward (static gravity) loads from the crane rails are easy to anticipate, but acceleration loads also matter.

 

[paddingtongreen]

You make an assumption that only the columns are affected, not so, if it is a portal frame, as it most likely is. Moments carry upward into the frame as well as downward. Much the easier and cleaner fix is hokie's added columns. I always go that route on retrofits if I can.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[Derek1684]

So to sum it up, consensus is strengthening building columns without a full building load analysis is indefensible and equates to doing a rain dance? Yet, adding columns is perfectly acceptable? I find this to be a little contradictory because adding columns just adds marginal strength in the same manner that strengthening the columns would. So what I am hearing is there just is simply not a good understanding to the degree of strength increase these types of modifications would make, so it is easily criticized. That is good to know. These metal building are largely designed by cook book reference charts. A 1/4" increase in the original flange thinness would easily account for this increase in load. So, it wouldn't take much math to extrapolate the additional strength gains to structural stiffening. We know the columns have already been designed to account for all building loads, and we just need to marginally increase the strength of the column for the additional loads. I don't think multiple cranes are really a factor on the loading. I understand these buildings are designed based on the full load of the crane in one bay (between two building columns.) Thus, as long as you don't allow the crane hooks to get closer than column centers, adding additional cranes is no factor to the building loading.

It is good feedback to understand there is not good understanding on column strengthening techniques in the community. I will just add a column under the beam seats that I can select based on strength from my structural reference book. Probably go with a W8x24 with a 12x12x3/4" base plate. At 25' that should be more than enough strength for the additional 12kips or so for the crane. Would it be worth while to stitch weld the sister column to the building column for added strength? Also, I assume having the orientation of the sister column opposite of the building column would add the most strength to the structure?

Am I off base on any of this?

 

[jwilki]

Quick question, is this sister column cantilevered from the ground? If so it will need a fixed base (including footing) as lateral crane loads can be significant. If you weld the new column to the existing column then you are essentially relying on the existing footing and the existing structural support system (portal frame, bracing etc) to take the lateral crane loads.... seems to me this won’t achieve the objective and the questions still remains, can the existing frame take the additional loads?

 

[JAE]

I think you are off base on everything you just posted - especially the first paragraph.

 

[hokie66]

I agree with JAE that the first paragraph makes no sense. In the second paragraph, orienting the crane column orthogonal to the building column is the correct way, but then you mention stitch welding, which is a no-no in crane supports due to fatigue issues. Bracing for the crane column about its weak axis should be bolted and flexible. As this is a fabrication shop, I suggest that you use your contacts with a local structural engineer to obtain assistance with the design. Don't you need certification for your modifications?

 

[SAIL3]

with all due respect and it may be just the wording of the OP's response, but , I do not get the impression that there is a sound understanding of the engineering principles involved and would advise, as others have done, to consult with an experienced engineer in this case...

 

[msquared48]

From my experience with PEMB's, I would not touch the existing structure - the designs were are tight to save materials and $$$, and you probably would not get the building to work under current codes with such a modification.

As previously suggested, use separate crane columns with separate footings. That approach will be the simplest and most cost-effective approach.

If you were going to rry to modify the columns, you would just get a progression of problems to solve - a ripple effect in the structure. Just keep it simple, and above all, hire a local structural engineer to do this.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[Derek1684]

I will get a PE to sign off on the design, but I am trying to do the leg work here.

Right now I am looking at using a W8x24 with a large base plate to spread the load. It will be mounted on the floor with 4 anchor bolts and be welded to the underside of the beam seat.

 

[paddingtongreen]

One thing we all have had in mind, but haven't said it loudly, if you modify the existing structure, you own it, you accept responsibility for it. I speak of strength and or function modification, minor connections are okay.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[TLHS]

There's plenty of knowledge of how to strengthen columns, it's just not the only thing involved. It tends to open a can of worms when you try to strengthen a pre-engineered building significantly. They're very good for what they do, but part of that is designing right down to the letter of the code. That can make it incredibly difficult to figure out exactly what code assumptions they've made and actually get the thing to pass code when you're looking at it from a retrofit perspective, especially taking into account odd tapering sections, custom members, and connections that aren't generally fully detailed on the drawings. Then, if there's been any code change since it was installed it will often fail the new provisions. That's not a slight on the designer, it's a niche of the industry where you get judged pretty much entirely on installed cost. Done right, they're very interesting and challenging engineering problems.

Installing a new load this size would generally require reanalyzing the structure and ensuring that it meets current code. Do you know when it was built? What does this do to the seismic requirements for the lateral force resisting system? Is it a moment frame system? If so, how does this affect the spanning beams, the beam connections, the column anchorage and the foundation? Even if it's not a moment frame, the foundation is a big deal. You might end up with deflection concerns as well. From the most basic standpoint, it's something that deserves a great deal of consideration because you're adding a load that's likely a large percentage of the existing structure capacity. Supporting a small platform or something at the top of a column, yeah maybe just throw reinforcement on your column that equals the applied moment and compression loads and take a quick look at the beams. Add something an order of magnitude bigger and you're starting to be in a situation where you have to look more in depth.

Personally, I'd be terrified of installing crane systems where the operators have the ability to run it in such a way that the structure is overloaded when the cranes aren't. What happens five years down the line when it gets sold to someone else and the only reason the thing works is that you keep the hooks fifteen feet apart? The crane capacities are written in giant letters on the actual equipment and need to be certified regularly, so you can be pretty comfortable that people know to keep the cranes at or below capacity. I wouldn't be comfortable in a situation where you could overstress the beam by a factor of two due to operator error.

Also, just because the load is between two columns doesn't mean it only affects that bay's structure. A crane rail is likely a continuous beam and depending on the stiffness of the system could pass load to a number of bays. Any thrust loads will also get pulled down the structure based on the brace or column stiffnesses.

This doesn't even get into the issues of connections, specific code requirements for crane structures, or all the fun dynamic stuff.

Yeah, reinforcing the column might be the right solution, but there's a lot more involved in deciding that it is. The reason people are suggesting that you install an independent support system that doesn't tie into the building is that it's likely that you'll have problems making a pre-engineered building with this much additional load pass code without significant additions, so it's better to go with a simple system that you can fully control.

 

[SAIL3]

THLS just about summed it up...I will add that overhead cranes in fab shops are one of the few instances in engineering where the structure will more likely see the design loads and repeatly...they really get a workout that can go on for many years and the issue of new ownership, lack of maintenance, inspections, etc are also factors to be considered....I designed a retrofit sheave bm for a 120ton crane in a fab shop a few years ago...a few years later the plant was shut down and the equipment(cranes) sold to some outfit in the far east...God only knows what it is being used for now and under what conditions...the only saving grace in the OP's case is the loads are small for crane loading, but, then again, everything is relative..