High uplift/horizontal loads in a Freezer Building 1

[Karlos80]

Hi All,

I am designing a large cold warehouse (100x100m) consisting of a standard steel beam to column frame. The building is operating at -30 deg. The frame is pinned and braced X as a big box. I did a 3D stick model, applied a drop temperature of -50deg to each steel member (assuming ambient temp of 20deg) and I was shocked to find out that I have 2000kN tension force in the gable X bracing. This high tension is transfered to my corner columns. I would have to design a baseplate for a such impossible to resist force (uplift and horizontal shear). I wonder if anybody can help me on this. Maybe I am doing something wrong, as those warehouses/cold stores as built all over the world.

I also noted that I need to isolate the baseplates with a thermal breaker. That means that I need to have two baspelates offset and connected by holding down bolts. How even small horizontal forces can be resisted on 4 small bolts, they will fail in bending. Not sure if I am missing something here too?

I appreciate your help.
Kind Regards
Karol

 

[dhengr]

Karlos80:
Don’t all the steel members expand and contract at the same rate? So, relative proportions don’t really change much. It sounds like you didn’t adjust the temp. on a few member, or some such. You may have to adjust the turnbuckles on the x-bracing as the interior comes down to its -30̊ temp. to tighten the bldg. up laterally. And, you likely will need to do this, because if you are using insulated rigid panels as the exterior skin, they are very good shear panels and will try to take the lateral loads if the x-bracing doesn’t. This racking just blows their gasketed and interlocked joints out. I’ve seen 4' wide x 20' high icicles hanging off the sides of bldgs. at these broken joints. The frame lateral delta must be limited. And, the connections btwn. the frame and the panels must be designed to take this kind of differential movement.

The steel frame x-bracing takes most of the lateral loading, except for some shear reaction on each individual column due to wind or EQ acting perpendicular to the wall plane. There are hard foam or ceramic bearing blocks which go btwn. the column base plate and the concrete column pedestal, and you put a small shear stub on the underside of the base plate and down into the pedestal, and then the regular A.B’s. You also have to heat the underfloor sub-grade to prevent it from freezing over time and heaving. Talk with a couple suppliers of the wall panels which you might use, they will have a wealth of detail info. for you.

 

[JAE]

You should have your X-braces placed in the center bay(s) of the freezer. This allows the framing on either end to move freely in and out from the center X brace.

 

[msquared48]

If you allow lateral movement in the connection, how are you going to effectively drag the forces to the X brace?

Mike McCann
MMC Engineering

 

[JAE]

Mike - the "freely moving" I refer to is simply the frame shrinking "freely" when the freezer temperature drops.

Think of an exterior wall of framing where you have 9 bays (10 columns connected by roof beams).
The center bay has an X-brace with 4 open bays on either side.
When frozen, the outer four bays on either side "freely" shrink inward and are not resisted by any X-bracing.

Thus you don't have temperature induced high forces at the base of your columns.
The worst thing you can do is put X-braces at the corner where thermal shrinkage causes them to fight each other.

 

[msquared48]

Got it. Thanks JAE.

Mike McCann
MMC Engineering

 

[Karlos80]

Thank you both for your prompt response. I have tried all possible bracing patterns and it is still attracting a huge tensil forces. It slightly improves when the bracing is located in the center bay, but doesnt solve the problem, it is still 1500kN. It is just not realistic. I am using a standard angle bracing. When increasing the size of the bracing it is attracting more and more thermal load.

dhengr: In my model I applied uniform drop temperature of -50deg to each member including bracing. The building contracts due to shortenning of the steel members. Could you please advise what do you mean by adjusting the temperature on a few members?

 

[msquared48]

What happens if you put all the columns to either side of the X braced frame on rollers?

Mike McCann
MMC Engineering

 

[Karlos80]

Mike, dont have the model with me here, but it will probably sway due to wind. Not sure if the cold room panels can accomodate such a movement as I need to restrict the frame sway to L/360 which is very strict limit.

 

[hokie66]

I think there must be something wrong with your model if placing the bracing in the middle bay does not solve the problem, or if increasing the size of the braces attracts more load.

A 50 degree C change in temperature equates to a change in length of about 58mm in the 100m length of your building, say 25mm from each end to the middle bracing bay. Allowing for this movement is much preferable to trying to overcome it.

In my only experience with a large freezer building, the building enclosure was separate from the freezer, building within a building.

 

[paddingtongreen]

My suggestion is to put a steel ring beam around the foundation, guided in the middle of each wall allowing movement perpendicular to the walls. Have this beam within the cold space and the box can grow or shrink resisted by the friction in the guided slide bearings between the foundation beam and the foundation itself. I don't promise that your troubles will be over, but they will be mitigated.

I did a similar thing to a long pipe rack that was "Hot-boxed" instead of heat tracing the individual lines.

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

 

[Karlos80]

hokie 66 - The tensile force in the steel member is related to its section size so bigger section bigger resultant force. All steelwork is exposed to -30deg temp. as it is an external cladding system. Th ebuilding is divided on two independed sections each 100x50m, the biggesr problem is along the short section even with the bracing in the middle bay forces are still very high.

Michael - there are multiply doors so wont have the continues ring beam.

Maybe applying -50deg to each member is incorrect approach? I would imagine there is an answer as there are cold stores built everyday.

 

[hokie66]

Karlos80,
As I said, there is something wrong with your model. Just increasing a brace size does not increase the force due to volume change. The type forces you are experiencing suggests that you are modelling the eaves struts with pins rather than rollers, i.e. not letting the columns lean. Do you agree with my second paragraph above?

 

[paddingtongreen]

@Karlos80, what are the loads like if you neglect the thermal effects? since you have to avoid the thermal bridge anyway, see if you could guide each column to allow movement on a line going through it's own polar axis through the center of the room. As the room cools down it gets smaller.

I didn't know enough about the rooms in my previous post, I assumed there would be a framed floor with part of the cooling system beneath it.

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

 

[Karlos80]

hokie66 - I did a 3D model and have pinned supports at the baseplates only. The frame is a standard stick built model. I am not restraining the top of the frame, it contracts due to temperature, but still transferring tension to the X bracing. I am using the Master Series software.

Michael - without thermal effects the loads are max 200-300kN (much smaller comparing to 2000kN). If I put columns on roller they will just sway due to wind action and I need to comply with H/360 for this building. So on one hand I would like very flexible frame, but from another side it needs to be quite rigid to comply with this delta limit.

 

[paddingtongreen]

Karlos, I was thinking of it being fully braced The guided "rollers" would only allow movement toward and away from the center. Instead of rollers i would use guided slide plates. something along the lines of:

http://www.amscotnj.com/pdf/Amscot_ABG_Guided_Catalog_Sheet.pdf

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

 

[haynewp]

What happens if you replace the braces with a moment frame with pinned bases?