Holdfast Training Services Trial Blog

Hi all,

Merry Christmas, I trust like me everyone had intentions to do loads of thesis but ended up doing none.

Getting a little bored of working for a management contractor I decided to move to phase 3 mid November. I though now would be a good time to give a little update.

I’ve started at a small office which have recently branched out from Australia into the London market called BG&E. I haven’t yet worked out what that stands for, for those interested here is the website.

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There are 9 in the office, 4 structural engineers, 1 BIM technician, 1 CAD technician, 2 RC detailers and me. So far the office has been doing temporary works for some of the big London contractors and some mid rise residential developments in Cyprus.

Since starting, I have been getting bits of work but as can be expected nothing substantial. I’ve outlined what type of stuff I’ve been doing.

Task 1 – Reinforcement rates

Part of a wider package with Getjar, as a second checker I used a mixture of RC intents and details to calculate tonnages for costing as part of a tender return.

Task 2 – Transfer Beams

The Broadway, London is a resi/commercial development with 6 buildings between 14 – 20 stories. The structural engineers, RBG, have detailed a steel section encased in RC as a transfer structure for a line of columns which go the hight of the structure. The step that the beams make is an architectural feature of the building creating large lobbies, see the section through below.

The issue was the contractors crane strategy did not have the lift capacity/radius to lift in the transfer beams. So they engages BG&E to conduct an option study, which was then given to me. There was no information on column loads, just the GAs and floor load diagrams. Unfortunately for me, the column tributary areas are different for each column and also the loads. So I did a column run-down using load reduction permitted in EC1 to get the loads at the base of the supported column. I then used a software called RAPT to ‘play’ with possible options. The most effective solution would have been to increase the depth of the beam and use normal reinforcement, however generally beams cannot be increased in depth due to the other trades. So I increased the width of the beam to 1200mm and it worked with 3 layers of 32s @ 100 (in the bottom), this was proposed to the contractor and we wait for further work. Clearly PT could have been used, but the it is not clear if the floor plates will be PT as they are c300-350mm which is thick for the spans.

Task 3 – Climbing Screens

Wembley Park, is a large resi development to the left of Wembley Stadium for those who have been recently. The contractor has won the job and is trying to work out their climbing screen and back propping strategy, the climbing screens can be seen below. This seems a little late to me, how much floor back propping is required and how quickly it can be struck must surely be known to form a programme. Unless it is a guess….

Getjar ask BG&E to conduct a concept study to see what is required for the climbing screens. Again the RC intent was not available as the design is not that advanced, all that was available was the floor plans and loading diagrams. Looking at the typical floor layout it was clear that there were three different spans. Knowing that the RC slabs would be designed for the loads in the diagrams, i.e. permanent case. I built the spans in RAPT (programme) and calculated the forces and reinforcement required. I then used this to compare to the construction case with the loading from back propping, climbing screens and a construction LL.

This showed that the climbing screens did not need any additional back propping, if the loads where not applied at the same times as the back propping to construct the wet deck. This is unlikely, as the buildings a residential the SDL and LL are low, 1.5Kpa each. The 250mm RC slab exerts a back propping load significantly larger than this and would need back propping over at least two floors. I calculated that for the loads to be combined the reinforcement would need increasing locally by up to 15% above that required in the perm case. It is now up to Getjar to do a cost exercise, material cost v programme to see which is preferred.

As a side note I also check deflections to ensure deflections under construction loading where not excessive. Whilst doing so I noticed that one of the larger spans was deflecting 30mm under permeant loads which is over span/250, which would be bad for internal finishes. I raised this to the contractor and this may come back to BG&E to design a PT slab for this area.

Task 4 – Tower Crane Grillage Cat 3 Check

The foundations for tower cranes are generally considered temporary works and as such fall under the guidance of BS 5975. The level of the required design check is outlined generally on the complexity and consequence of a failure, the category’s can be seen below.

My previous project, One Nine Elms has 5 tower cranes, all supported off grillage which is more complex than normal, I have discussed one at length in a previous post. Clearly the consequence of a failure could be dire, for those with a strong stomach YouTube ‘tower crane collapse Mecca’. It is also bad for business, failures are very public affairs and to make them worst developers/main contractors like to put their company logo on the crane. Anyway, a cat 3 check should be conduct by a person outside of the organisation which did the design. The check should be done without the calculations, just the drawings and any pertinent information, loading etc etc. This task was given to me with the direction to see how far I get…… shit. I was given some information about the crane and Carey’s design drawings.

My first start was to check Carey’s had used the correct loads from the crane supplier, it was slightly concerning to find they had missed the most onerous case a storm hitting the front of the jib. I built the steel grillage in a software called Microstran (similar to STAAD) and applied the loads with a number of different combinations to get forces and deflections. I then realised I have forgotten everything Neil taught us about steel and spent a day going back through section and member checks by hand, the software also does a check which it useful. Clearly somebody sits in the crane cab so a large differential deflections between the legs gets amplified higher up. I also believe the crane suppliers factor up the loads they provide, as a way of reducing deflections to stop Pdelta effects.

By far the most difficult check was the connections, with my main issue being whether to fix or pin the members at the joint. I found the 16mm FW on the fin plate insufficient by some margin in either case and suggested an alternative solution. The solution (see below top) uses and 8mm FW which can also be done with one pass so Saving labour costs. In checking the apex type of connection I found that the plate welded to the plunge column plate resulted in a bearing stress great than the capacity of the section and would also be difficult to weld on site. After some head scratching and advice I came to the solution below.

Quite a bit if stuff in a couple of weeks and I am learning lots quickly. One thing I have noticed is that the engineers do almost everything on some form of analysis software, Excel and Bluebeam with very little being done by hand.

I hope I haven’t bored everybody to much but you already know I’m a geek.

Brad

James, these are the loads supplier by Select for a 70m tower crane. The overturning moment drops to almost nothing when the crane is tied.