Design of catwalk supports

[oengineer]

I am working on trying to obtain information regarding supporting a catwalk from a reinforced concrete beam.

The concrete beams are 25 ft above the catwalk and the beams are spaced at 8'-6" c-c. In order to have the catwalk supports attached to the beams above them, the catwalk beam frames will span 8'-6" long.

I am interested in obtaining information about the type of members to be used to attached the catwalk to the RC beams above. I have some preliminary UNISTRUT documents/drawings about catwalk systems being supported by various structural elements (i.e. concrete slabs, steel beams, roof joist, etc.) above them.

Is anyone aware of any other methods to achieve this? Any other technical documents/guides?

Comments/suggestions are appreciated.

 

[jayrod12]

Hanging from the concrete beam isn't difficult. You can use any material that suffices for the loads. Keep in mind the connection details. I'm personally partial to fastening to the side of the concrete beams above to prevent pure constant tension on your anchorage. Also, the lateral support of the catwalk will be the hardest thing to come up with. If you just hang the catwalk with no lateral restraint system that thing will bounce and sway like crazy.

 

[dik]

should be easy to hang them by attaching to the sides of the beams... use a concrete cover meter to determine where the rebar is located.

Dik

 

[oengineer]

Hanging from the concrete beam isn't difficult. You can use any material that suffices for the loads. Keep in mind the connection details. I'm personally partial to fastening to the side of the concrete beams above to prevent pure constant tension on your anchorage. Also, the lateral support of the catwalk will be the hardest thing to come up with. If you just hang the catwalk with no lateral restraint system that thing will bounce and sway like crazy.

Is the swaying you speak of on the catwalk primarily because of lateral load due to wind loading?

If so, the catwalk I am designing is inside of a concrete rectangular vault. So there will be no wind loads applied to this catwalk.

 

[jayrod12]

No, from people walking. There's a lateral load generated on a structure just from walking, I would consider designing it for 5%-10% of the gravity load as a lateral load and seeing what that looked like.

The steel code where I practice has a minimum lateral notional load prescribed to ensure stability.

 

[dhengr]

Oengineer:
I’m with Jayrod12 in preferring to attach the hangers to the sides of the conc. beams, so that the A.B’s. are in shear rather than tension. Otherwise, pay attention to all of your connection details, this is not a very redundant structural system, remember the Hyett Regency, Kansas City incident. You have a pendulum hanging there and you do have to do something to brace it, the more so, the longer the hangers are. You can put x-bracing or truss type bracing in the catwalk floor framing system, but you also have to brace it laterally to prevent it from swaying excessively. Since it is connected to sound structure at each end, it might be o.k. in the longitudinal direction. Remember, you have a couple 300lb. workers and anything they might be carrying as a min. gravity loading.

 

[oengineer]

Based on ASCE 7-10 (see image below), the code calls for a 50 lbs point load to be applied to the handrail at any location & direction.

After applying this load laterally to the catwalk/walkway structure, the catwalk framing is have significant issues with resisting the lateral live load.

In order for the catwalk to resist the load, I have had to use pretty large structural suspension members and still provide a brace to a nearby wall.


@jayrod12
Are you aware of any typical recommended structural detail that could be used for suspended catwalks/walkways bracing or connections? Possibly for something from AISC, PIP, ACI, etc.?

 

[jayrod12]

Nothing that I know of in terms of publication. But there are many many options at your disposal.

If connected to the structure at the ends of the catwalk, and the structure itself can provide the lateral resistance, then perhaps all you need is some plan bracing in the floor system of your catwalk.

If there isn't anything to provide lateral resistance from the structure at the ends of the catwalk. Then some form of vertical bracing or moment frames would be required. The roof beams you're hanging this stuff from would need to be designed for the loading from the bracing. It's no different than bracing down to the ground in a typical structure, just upside down.

 

[Eagleee]

@oengineer though i am not familiar with american codes, i believe such a handrail load you mention above should be considered specifically for the design of the guardrail system and to be able to transfer this small load from the handrail level to the platform level. my understanding is that such loads are meant for people pulling on the handrail, meaning that you in theory would have an opposite and equal force at platform level (friction between the feet of the guy doing the pulling and the grating). in other words, handrail loads stay within the system. as jayrod mentions above, 5%-10% (from vertical DL+LL) sounds reasonable but this depending on the live load you have can be more or less than the applied 0.73kN/m.

 

[STrctPono]

No, from people walking. There's a lateral load generated on a structure just from walking, I would consider designing it for 5%-10% of the gravity load as a lateral load and seeing what that looked like.

I agree with jayrod12 but think that this needs to be evaluated further. This is not just a strength concern. This is going to mainly be a serviceability concern. I would never design something like this unless I ran a MODAL analysis on it to check what the lowest natural frequencies were in the vertical and horizontal direction. I am not an expert in vibration analysis but know that walking induced (first harmonic) vibration of these types of structures can be a huge problem. Especially since you are supporting it via cables. I would model all of this, platform, cables, braces in a structural analysis software program and check your natural frequencies. Do some research because modeling cables in a structural analysis or FEA program is not so straightforward. You can use the pseudo static lateral load for a strength criteria check but that will not answer the question on whether this structure is going to vibrate to an uncomfortable level when people are walking on it. ASCE/OSHA must have some criteria for this. I know that AASHTO provides guidance to Engineers for pedestrian bridges to keep your lowest natural frequency in the vertical direction above 3 Hz and your lowest natural frequency in the horizontal direction above 1.3 Hz. It gets way more complicated than this but this is just an initial check to tell you if you might have an issue.

 

[KootK]

25' is a long way to hang. I've suggested something below but I'm sure that it's nowhere near as light a system as you seek. Tough to beat those unistrut catwalks for economy though.

 

[oengineer]

I would consider designing it for 5%-10% of the gravity load as a lateral load and seeing what that looked like.

Would anyone happen to know where in the code (either AISC, IBC, or ASCE 7) that allowances for 5%-10% of the gravity load as a lateral load is mentioned? My goal is to use this for my analysis, but I would like to show where it is mentioned in the code to justify its use.


@KootK
Thank you for your sketch. Would you happen to know where I can get more info for unistrut catwalks ( I was trying to navigate their website to find some more info, but I did see anything for my situation)?

 

[KootK]

Would you happen to know where I can get more info for unistrut catwalks ( I was trying to navigate their website to find some more info, but I did see anything for my situation)?

I'm afraid that I'd never heard of them until you mentioned them in this thread. I'd call one of Unistrut's sales people. I'm sure they'll be happy to inundate you with useful engineering information. Eng-Tips member Teguci always seemed to have a particular skill with Unistrut. Unfortunately for us, he's not been very active lately.

 

[r13]

I would say 5-10% gravity load as lateral load is an estimate based on experience. As the aspect ratio of catwalk is usually 1B:4V, assume an off center load that causes rotation, T=P*(B/2)=H*V, solving H=P*(B/2)/4B=0.125*V (12.5%). You can follow this link to get in touch with Unistrut. Link