Any ideas for removing props when stressed?
I am currently trying to build my way out of the 22 m hole I have dug for myself in South Brisbane. Progress has recently accellerated now that the excavation is over. In the last 3 weeks the structure has completed Basement levels 6, 5 and 4. we are currently at Basement level 3 and in the next week we will be at the same level as the props (Basement level 2), which presents some challenges. The props are significant pieces of engineering and consist of 900, 752 and 508 mm Circular Hollow Sections in each corner.
In order to keep the job moving we will have to remove two of the props that clash with the permanent structure (highlighted) and build over the remainder. This flies in the face of conventional wisdom but, lets focus on the problem at hand. The removal of these props is potentially very hazardous, since they were installed the wall has deflected inwards by up to 26 mm and as consequence the props could potentially be under 2-3 MN of compression.
Aerial photo taken of site (this was taken 3 weeks ago when we were on Basement 6)
Having just received my sub-contractors method statement (similar to how a lumberjack would cut down a tree). I concluded that they will kill someone so I have consulted the ‘hole diggers bible’ (CIRIA 517). Unfortunately, the struts are welded in to the walers so it removes a lot of potential options. I believe the only method left to follow is to cut progressively larger holes in the CHS while supporting them from either the deck or the tower crane.
Given that the deck will just have been poured and would require propping all of the way down to Basement level 6, I am reluctant to recommend this option. The sole use of the tower crane is equally unpalletable but, if it was conducted at a time when the majority of the site was empty it could work. I am open to any better ideas though.
Access. The props will block access around the site, so I have had to ask our scaffolders to build an access way on top of the walers. Unfortunately ladder access is frowned upon, so I have found some safety stairs that will be able get from the working deck to the temporary scaffold access. The safety stairs will be used in areas marked in yellow (above) that cannot be accessed by existing the existing stairs. We tested this on site today by placing them against the existing scaffold stairs.
Things are about to get very messy – I think its time to go on leave.
They are getting closer! -Photo taken from Basement level 3 of safety stairs against , props and walers in background (note we have built over the ground anchors now).
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I think Damo mentioned props of this nature expand and contract due to temperature variations. As you are baking away in the heat of the Austarlian sunshine by day do they contract at night as the temperature cools? I assume this is minimal but if it the variation were significant would the stress in the props reduce during cooler periods ie at night? I note they dont appear to have been painted to protect against thermal expansion?
Yep a bit tricky; a 10 degree downward shift in temperature would result in a shortening of less than 1mm; This is where equipment props are useful because they have hydraulic jaclks that can be retracted
I’ve seen props erected with greased shims placed in the end plate joints so that they can be driven out- but it looks like these are flush? I’d go for a careful symmetrical ring of drilled hopes
Is there a specified compressive strength for the floors at the time when the props can be removed?
I saw massive variation in prop load with temperature so removing at a lower temperature would help.
I would cut holes in a symmetric fashion to avoid any buckling potential. Keen to know the outcome…
John, Would it be a good idea to attach strain gauges to the props and walers? As long as I keep them away from the hot works, it may tell me, where the load is going and when the stress is released in the prop.
Doug, what risk are you trying to mitigate with the gauges? Health and safety or engineering risks in terms of load transfer?
Going back to your question on removing the props, even though my site had retractable hydraulic props they used valuable crane time to fiddle about removing. So the idea of using a crane to hold a prop while holes are cut might not suit the PM’s programme. Have you confirmed the floor slabs can’t take the prop dead load?
When you come to removing props you have built over I suspect you are going to need some heavy duty kit for that. What about getting some of that in to help at this stage?
Damian, actually trying to mitigate both. Managing H&S risk but also making informed engineering decisions on next stage. Crane use has already been agreed. It will happen at the end of the day when site is clear. Floor slabs are capable of 2.5 KPa. CHS 900 x 19 weighs about 1.2 T per m and 10 m of it.
You forget about the 4 layers of propping needed and the formworkers props. Work will not start on that phase until Jan. So that is a lot of cost.
Doug, thanks. Yes lots of back propping but if you have a couple of floors back propped already be sure you have just done a pour I thought it might not be too ridiculously costly to prop 2 further floors under area needed versus hook time costs. Either way if you can let us know how the removal goes that would be great. Thanks Damo.
Damo, in a sensible world you would be right but with the contractors I have it would cost a fortune to prop and the issues with differences temp work designers would be horrendous. TMR 4 perhaps.
Yes Damo, the floors have to achieve 25 MPa. We have just entering Spring so it it is only going to get hotter I’m afraid.
Guess you’ve done this now? Otherwise you’re at 7 day strength which is about 75% of 28 day strength and this whole worry about slab capacity is receding fast (time is a great healer…).
Actually, I’ve taken John’s advice and gone on holiday. I will give a sit rep when I get back on Tuesday.