Site Inspection – in at the 1.4m deep end…
Anybody interested in what a site inspection of 1400mm deep, 5.5m cantilevering transfer beam designed to support 18 storeys looks like…
The PT is made up of 15.2mm diameter strands @ 300 centres. The cantilever supports 18 storeys with only 20mm deflection!
The construction of this was not simple. PT design is not in the structural engineer’s package, it is in the PT sub-contractor’s. Their original design saw almost half the amount of PT shown in the photo but and deflection of 38mm. By the time the design consultancy had highlighted the need to restrict deflection to 20mm, the PT had been assembled as per the original design. You can imagine the look on the contractor’s face when they were told more PT was needed. Due to congestion, the additional PT could not be added in a side-by-side arrangement, but by way of a second layer in the vertical plane. The contractor was not happy.
Holdfast Training Services Trial Blog 
This is fascinating. It’s about stiffness not strength. Remember/review the relationships between deflection, moment and EI. Think cumulative sum of curvature with respect to length. Options for reducing deflection then become clearer and, presuming there are no ULS issues, using more prestress to reduce double integral M(dx)^2 might well be much more complex/costly than increasing E with a little bit of compression steel. The former faces all sorts of force limit zone challenges and might significantly influence losses, the latter means trying to support steel in areas that are presently devoid of anything other than concrete…. Not an easy one!
The current in house course start to look at Prestress next week so a very timely post!
Andy,
As I can’t sleep until my co-codamol kicks in from my latest old man injury i thought i’d give you the benefit of my 3am musings.
I assume the deflection of 20mm (1 in 270?) is the total long term deflection? Is the PT contractor given the creep coefficient or do they make their own allowance? How close is this predicted deflection to the Clients acceptable limit and is it a brittle facade that limits this or aesthetics?
Also why the change from 38mm to 20mm allowable deflection?
Tony