Industrial Platform Design 2

[Ipetu]

I am designing an industrial platform to support 1 4000 lb compressor unit in an indoor application (no wind). I am able to brace the frame in the x-y plane with cross bracing. I am not able to brace the frame in the z-y plane as a clear opening is required underneath the platform for forklift access. Please see attached sketch. Three questions:

1) Are the braces to be designed for the resulting forces from the application of a horizontal force equal to 2% of the column load applied to each frame in the x-y plane?

2) In the z-y plane can FR moment connections between beam and column be used in lieu of cross bracing?

3) For (2) above are the design moments for the FR moment connections the resulting moments from the application of a horizontal force equal to 2% of the column load applied to each frame in the z-y plane?

The 2% value referenced above is from Cl. 9.2.5. of Canadian Steel Code which states that "bracing systems shall be proportioned to have a strength perpendicular to the longitudinal axis of the braced member in the plane of buckling equal to atleast 0.02 times the factored compressive force at each brace point in the member or element being braced."

Thanks very much.

 

[hokie66]

The 2% rule is for bracing a member to prevent that member buckling, not for a complete frame.

In an industrial application, the first rule is to make structures robust. Granted, you may not have wind loads. But you have to decide on an appropriate lateral loading condition based on the useage of the platform. Seismic may control, but with forklifts running around, vehicle impact is likely the one to worry about.

 

[JAE]

What about seismic loads?

 

[msquared48]

I agree with Hokie. The force os seismic action is nothing compared with the forced induced by a drunk forklift driver.

No contest here, from experience.

No, I was not the driver...

Mike McCann
MMC Engineering

 

[Ipetu]

Seismic is not an issue in these parts. However, the forklift impact force is indeed a big concern. For a moment if we can leave seismic and forklift impact forces aside, what is a respectable percentage of column axial load to be used as a lateral force to ensure a robust frame? Also can FR moment connections between beam and column be used in lieu of bracing?

Thanks,

 

[apsix]

From BS5950 (UK steel code); 2.4.2.4 Notional horizontal forces
To allow for the effects of practical imperfections such as lack of verticality, all structures should be capable of resisting notional horizontal forces, taken as a minimum of 0.5 % of the factored vertical dead and imposed loads applied at the same level.
NOTE For certain structures, such as internal platform floors or spectator grandstands, larger minimum horizontal forces are given in the relevant design documentation.

From AS/NZS1170.0 (Au/NZ loading code) notional lateral load = 2.5% of (G + fcQ).
G = dead
Q = imposed
fc = combination factor which is typically 0.4 or 0.6.

 

[hokie66]

I would use a notional horizontal load of not less than 1 kPa (20 psf) on the projected area of the structure. Not from any code, but that is based on industrial experience. But again, the mobile equipment impact loading is the big consideration.

 

[Stillerz]

how big is this thing?

 

[Stillerz]

a 4000 lb compressor isnt much.
I'd say your bracing design will wind up being based on KL/r(even though the code may not dictate that).
Depending on the length of the braces, and whether or not you are using tension only type bracing a limiting KL/r to 200 or even 300 will probably yeild a pretty robust design. I cannot see actual loading controlling the design.
I see no reason why you can use moment conn.'s in the open ends.
Obviously you want to orient your columns so strong axis bending is in the plane of the open ends for your moment connections.

 

[Stillerz]

Correction:
...I see no reason why you CAN'T use moment conn.'s in the open ends....

 

[Ipetu]

hokie66,

Do you mean:
1) 1 kPa on the projected vertical area of the x-y plane for design of bracing in the y-z plane, and
2) 1 kPa on the project vertical area of the y-z plane for design of bracing in the x-y plane?

Thanks.

 

[Sawbux]

What kind of compressor is this?

If it is a reciprocating piston type, you have dynamic forces to consider. Maybe need enertia block and stiffness (both vertical and lateral) to consider to keep structure natural frequency away from the operating frequency.

Could get complicated!

 

[hokie66]

Ipetu,

Yes, the notional load in each orthogonal direction is perpendicular to the surface and parallel to the bracing.

 

[ISCK]

Reading this post gave me significant insight. After some online searching, this source,

http://www.safety.rochester.edu/ih/pittrain.pdf,

page 21, says that a 5,000lb forklift with a 4,000 lb load driving at 10mph can deliver 135,000 lb of destructive energy. That would cause tremendous shear at the bottom of a column, which will lead to gravity loads redistribution to the other 3 effective columns. If those columns were slender and not braced, buckling will occur very quickly. I guess, that scenario will look something like this:

http://www.youtube.com/watch?v=5ezI_LLOTVw.

 

[paddingtongreen]

Check your units. Is that a force or energy.

10 mph is about 14.7f/s

9000x14.7x14.7/(32.2x2)=30200ftlbs

It looks as though they are figuring on about 3" of crunch.

What is the access? can the forklift get up to full speed. Can you put bollards in to protect the columns?

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