Last minute design change
This weeks activity has been focussed on preparations to pour 3 suspended slabs by the end of the month to reach a payment milestone. This has been delayed by high level changes to task priorities that has affected progress. On setting out the first slab it was found that a 300×300 column that was set out to the architectural measurements was positioned on the edge of a slab edge-beam also set out to architectural measurements. The issue was that the structural design of the column assumed that it sat centrally in the beam and was therefore not sufficient for its current position.
This was raised as an RFI to both the architect and structural engineer to resolve the discrepancy in drawings.
Engineering Issue and resolution
The engineering issue here is that because the column was designed to be loaded equally the reinforcement is now not sufficient to withstand the additional bending. To resolve this there were 2 options.
- Extend the slab outwards to move the beam centrally on the column
- Insert a transfer beam to distribute the load between the 1000mm diameter column and the 300x300mm column.
Both of these options created issues. The slab reinforcement was designed to the structural dimensions, therefore was sufficient to accommodate the move out of the beam to sit centrally on the column, but the architect could not accept this option as it would affect the internal dimensions of the control room below making it too small for the intended contents. The second option required a redesign of the structure, rework to the formwork already placed, and the order of additional steel for the beam.
The decision had to be the transfer beam, which as it was designed could be designed to 450mm deep and therefore not affect the formwork already placed significantly. The steel was ordered immediately on a priority to arrive the following day. At the time of leaving site on Friday, all was in order and good to go – I will find out Monday I guess.
The managerial issue is that there was a discrepancy in the 2 designs (architectural and structural) that was not picked up sooner. The time it took to respond to the RFI took approximately 4 days after many to and fro emails between the architect and the structural engineer. A face-to-face meeting that may have taken 30minutes could have rectified this immediately. The final issue is that of costing. The cost of redesign of the beam should be borne by the structural engineer as it is the architectural design that measurements are taken from. Additionally the cost of the additional steel should also be covered by the structural engineer however the relatively small amount of steel required for the beam (3.5m long) will probably just be covered by JHG to maintain relationships.
In other news, the reorganisation is apparently underway and should be in effect on Mon. The notice period of the weekend is probably not enough time for a full handover so I am expecting a fair degree of turmoil this week. It has been good to catch up with the Knowles and Dollimores this week at their leaving parties, and we are having a RE BBQ at ours this week as a final send off.
Holdfast Training Services Trial Blog 
Hi Nik,
Like the issue – a useful discussion to have at CPR in terms of being able to produce a simple sketch and demonstrate understanding of first principles. You would need to show the span to the first innner column of course which would allow comparison of the issue of off centre loading and the lever arm for rectification. From your sketch I guess we have an action that is about 450mm off centre arising on a span of 6m(?). Assuming 5kN/m2 UDL and 4m panel width I’d guess at a midspan moment of less than 1kNm i.e. I don’t think the midspan increase in moment would be significant enought to warrant additional steel in the slab let alone increasing depth to form a beam, but that is based on an excess of assumption on my part! Have you run any broad numbers around this challenge?
Cock-ups always welcome here.
I’d suppose the off centre beam now implies a torsion. I suppose that the torsion throws an additional moment into the top of the column?
Top edge columns are frequently ‘difficult’ …top becuse they’ve little precompression…edge becuase there are always out of balance moments.
I’m not sure what the solution does …but I think that if you draw a free-body diagram of the edge beam ….well there might be a clue…….toodle-pip!
Nik
I agree with Richard – think about sketches such as these for your CPR documentation. You should think about in your reports, your presentation or even during your interview. Simple clear sketches are ideal.
Hope all is well
Neil