Lateral loads from large RTUs

[SocklessJ]

I have been tasked with adding a large 2-story RTU platform to the roof of an existing steel building. This roof is made from structural steel shapes, which are sloped for drainage.

The units are quite heavy (60 kip total), so I intend to attach to the existing columns by welding thick cap plates to the column flanges. However, I’m worried about lateral loads. The roof beam connections might not have the axial capacity to drag out the lateral load to the horizontal trusses that comprise the roof diaphragm. I don’t have connection details yet, but I’m hoping that they didn’t use really thin clip angles.

Does anyone have a good suggestion for improving the axial capacity of existing beams? I guess I could increase the size of my cap plate and weld it to the top flanges of the intersecting beams. I could then add flange plates to the bottom flanges. This would create a moment connection, which would need to be analyzed as such. Has anyone run into a situation like this before? This is an industrial structure, but I know heavy RTUs are frequently placed on commercial buildings.

Attached is a conceptual sketch (without reinforcement)

 

[r13]

Provide ties above the roof to your columns.

 

[HTURKAK]

If i were in your shoes, I would try first, to use two separate platforms. This would simplify the problem and will be more effective for the operation RTU's .

If the use of two storey is the only solution, I would prefer simple connection of new HSS columns on to the existing W 14 columns.I would model the existing structure with new two storey RTU platform, and add or increase the stiffness of the existing beams and columns step by step.

The options ;

İ= Add new bracings to the existing structure keeping symmetry,
ii= Change the beam -column connections to moment resisting connections with welded cap plates and flange plates,
iii= If still necessary, increase the stiffness of the columns with adding new plates to the flanges.

If you post the plan of the existing structure with applicable seismiic and wind loading , you may get better responds.

 

[SocklessJ]

Thanks for the response HTURKAK. I'd prefer two separate platforms as well, but there's other equipment on the roof so space is a concern. Also, the equipment is not actually a condensing unit, so the stacked configuration won't cause problems w/ operation.

I'd also prefer a simple connection to the existing columns to avoid introducing moment, but I don't think that's possible with my cap plate detail. Maybe I could bolt a thin, flexible baseplate to thicker cap plate?

 

[HTURKAK]

Yes this is possible solution. Try two connection bolts . I will not suggest to use thin cap plate , you may need to extend the cap plates to the flanges of existing beams to provide moment connection .

 

[JLNJ]

If you are going to "push" on one existing clip angle connection and "pull" on the other, maybe you can drive all the lateral load through the "push" connection and not have to worry about flexing the angles.

 

[JLNJ]

If you are worried about buckling whole beam as it distributes the axial load? This seems unlikely if it's decently connected to the roof deck or other cross members.

Unless your force is crazy big.

 

[dik]

Any issues with snow accumulation... not so much for heavy units, but due to size?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?
-Dik

 

[r13]

Maybe you have already looked into matters such as the capacity of the existing foundation, and strength of the columns to support the gravity load from this addition. IMO, unless the original design has anticipated this addition, you are facing a huge challenge to make it work as the configuration propose now. Find a way to spread the load out, and lower the platforms will yield better chance to success.

 

[jayrod12]

Sometimes lowering the platform causes the snow-build-up issues dik was eluding to. Lifting them off the existing roof allows the snow to evenly distribute below the units avoiding reinforcing of the existing joists. We do this often since, especially in manufacturing plants, since shutting down of the processes below to reinforce all of the roof joists touched by additional snow buildup is significantly more costly to the owner than the additional costs of the platform.

 

[r13]

I wonder how much additional snow will accumulate under an equipment occupied platform, that is placed close to the roof. Don't forget wind load in your calculation (I believe you won't, just speaking), and moment arm of the lateral load effects.

 

[dik]

r13... if I have 4' or 5' clear... I'm usually pretty happy unless boxed in by a penthouse or something of that ilk. Usually the loads from the equipment isn't large... but snow accumulation around can be a real problem due to the extent of it.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?
-Dik

 

[r13]

dik,

In this case equipment weights 60 kips.... all seasons.