Temporary Platform Connected to a Blockwork

[Aytacoglu]

Hi All,

I am currently working on a steelwork repairs project for a cable shaft. The idea is to cut the ends of the beams where they have significant section losses and splice a new section in where it is going to be cut. I have attached a drawing and a photograph below just to give some idea on the works that will be undertaken and the condition of the beams.

This cable shaft consists of 7no. levels and these works are required to be carried out for each beam. Now, because height between each level is above 2.5m, there needs to be a temporary platform built in the shaft so that the workers can reach the beams and work with them.

Our idea was to install eyebolt/ringbolt anchors to the masonry wall in between which is 140mm thick blockwork, and clamp the scaffold tubes to it so that the scaffold boards can be laid over. The tubes will hence be connected on one side to the wall via anchors, and it will be buttressed tight to the tunnel lining which is 100mm wide. I have attached the proposal for the scaffold arrangement below.

I now want to calculate which anchors I need and at what spacing does the scaffold tubes need to be for it to work. My question here is whether these bolts will experience bending moment due to the way the arrangement is for the platform. The load path will start from the timber scaffold bolts and then will be transferred to the longitudinal scaffold tubes. This load then will be transferred to the scaffold tube that will be installed parallel to the masonry wall. Due to this and the arrangement of the anchors, this will act like a continuous beam with the supports being the anchors and the point loads coming from the scaffold tubes connected via a coupler.

This will create a hogging moment at the locations of the anchors. Due to anchors being very weak in bending, although the loads here are not significant, the anchors are struggling to pass the design checks.

Could anyone please confirm/agree with the method of analysis I am using here? That the load path will be as described and the bending moment created due to the arrangement?

I would really appreciate if anyone can assist.

Thanks!

 

[GeorgeTheCivilEngineer]

Those anchors are ties, to keep the scaffold upright. I would not use them to support vertical loads - there should be scaffolding down to the bottom of the shaft (or onto something competent).

It could work but I'd proceed with extreme caution and with advice from the Hilti

 

[jjl317]

As a former scaffold engineer, I can't see a scenario where I would be ok with attaching a horizontal tube to a blockwork wall with screws (or "ring-bolts"), then using to this tube to support vertical live loads.

Are you going to need shoring to support the steel beam?

Similar to previous, one potential option I would be looking at would be vertical framing that could support both the beam to be repairing and the scaffold deck. And depending on the layout of the steel, perhaps starting at the bottom, and repairing the lowest beam, then utilizing the repaired beam, to help support the vertical loads. Then repeat.

Or potentially hanging a swing stage (or sim. work deck) from above, to support the workers.

FWIW

 

[Enable]

If I'm understanding the setup correctly I do not believe your anchors will experience a bending moment in the manner you are suggesting. The diagram you've drawn is for the continuous member, which shows the internal bending moment. Since the fasteners will act like pins that moment will not be transferred to them save for the extent that the fasteners are in the compression zone and if the slop in the holes is small relative to the strain induced by compression from the moment.

But in that case it's more of an induced compression force, which should be trivial for the bolt to handle given the small nature of the tube wall and larger diameter of the anchor. Though the fasteners will experience bending out of plane from the wall, which would be a tad uncertain since it depends on how much load is delivered at the side closest to the wall vs far from the wall. Stiffness probably suggests most would go to the wall side (negligible bending moment) but should the fastener not be engaged on that side for whatever reason (e.g., hole larger than far side) it would be prudent to design for a moment of V*e.

There's also zero chance I would ever rely on non-through bolts into masonry of questionable integrity as the primary (and only) method of providing worker safety. I understand that you probably don't want through-bolts since that means having to go to the other side (presumably elevated), but then at the very least I would require workers to use a life line and harness with your arrangement. They are not likely to enjoy that when the feel the relative comfort of a full platform under their feet.

For me a better approach would, as the others have suggested, involve getting our loads to grade and working up. If you don't want to scaffold the entire area to save on costs you could continue some scaffold at the masonry side to support your stringers. Build one, do the repair, build on top, do the repair, etc. I might also suggest aluma beams and timber instead of scaffold tubing, which will be prone the roll and all sorts of odd stuff. EDIT - Or as jjl317 puts it we could repair the bottom beams, shore underneath, then joist on top to build out platforms and work upwards. This is also a great option.

 

[GeorgeTheCivilEngineer]

It's not even a massive cost saving. It's only a tower scaffold.

Arguably it'd cost more to not scaff the whole thing compared to multiple revisits and adjustments.