Design of circular steel beam to support scaffolding around dome on historic building

[Temporaryworks]

Hi,

See attached pdfs for visual aids to my question.

I would appreciate some advice on the design of a circular steel beam around the top of a large dome on a large and important historical building.

I am designing scaffolding around a 20m diamater dome and its cupola to provide access for rendering works. I do not expect to be able to load the dome or its cupola (which is what I am calling the spire at the top). Therefore I could truss over the top of the dome but this would result in a large amount of scaffolding and will be onerous to construct.

I want to investigate an alternative where none of the vertical legs (standards) or horizontal members (ledgers) touch any part of the curved surface of the dome. Instead, my client, the scaffolders will fabricate a circular beam (possible SHS or PFC) upon which the horizontal ledgers will rest. The beam will support the horizontal loads required to provide equilibrium to the system.

For the installation, the legs can load the roof temporarily but before it is loaded for works, it will be loaded up against the ring beam only.

The total weight of the scaffold + working load is approx 100kN/10t spread over the entire dome.

I have never designed such a beam and would like some advise on whether this can be done? Would you have any concerns over the rigidty of the system as everything will be resting on one circular beam.

I will be modelling the scaffold and the beam in Space Gass software.

Thanks in advance.

 

[OldBldgGuy]

I've never designed anything quite like that, but my immediate response is that you need a tension ring beam at the lower levels where you show it bearing on the masonry structure. That should be relatively easy to accomplish by adding tension members into the scaffolding system that would be continuous around the full circumference. Have you talked to system scaffold engineers? In my experience, they've done some pretty amazing things & I've been happy to leave that part to them while I concentrate on the building itself.

 

[Temporaryworks]

OldBldgGuy, thanks for your reply.

If I understand you correctly, the tension ring could be replaced by bolting the standards where they bear onto the masonry structure?

Yes, we have discussed it with the system scaffold Manufacturers, their advice was to truss over the top which is our plan B as this will be a nightmare to install and also will involve alot of additional equipment. We may have to fall back on this anyway.

As another option: the dome has an Octagonal base, I was thinking perhaps to fabricate an Octagonal ring- beam and also arrange the scaffolding in an octagonal fashion so that the ledgers can approach the beam at 90degrees making the connection easier.

 

[CANPRO]

Hi TW, not sure I'm picturing this correctly - any chance of getting a section through your proposed setup?

In the past I have used an exterior tension tie (smaller scale, but similar idea) to keep the scaffold from spreading. We wrapped a continuous cable around the outside of the scaffold and brought it snug tight. Cable is cheap and fairly easy to work with...so you can throw in enough of them until you feel like you can sleep at night.

 

[Temporaryworks]

CANPRO, I provided a section in the original attachment in my first post, did you see this? If not clear I will do another one?

 

[CANPRO]

My mistake, I only saw the 1st page...I understand now.

I’d explore the exterior cable option for containing the horizontal force. Much cheaper and easier to install. Not even sure how you’d install the circular beam...I’m sure it can be done, but my gut says there’s an easier way.

Another thing that jumps out at me is that you’re using your bay braces to transfer multiple lifts of dead/live load...which can be done but you’re not going to get the same capacity as you would with standards. I’m sure you’re aware of that, but thought I’d mention it anyway. If you haven’t checked it yet, you might want to do a ballpark calc on what kind of loads you expect in your braces and check how much they can handle.

Cool project...I miss my scaffold days sometimes.

 

[Temporaryworks]

CANPRO,

Good point on the bracing, each one takes 1t and you can twin them to make it 2t. I will check as part of design.

Yes cable option may be better, in conjunction with the 'trussing action' of the bracing, vertical and horizontal members of the scaffolding.

Though I can model the cable in my structural software, I would worry that the cable would go slack when the scaffolding deflects and then the scaffolding becomes overloaded. Would you 'pre-load' the cable by tightening?