Holdfast Training Services Trial Blog

ith the 30% concept design completed and off for approval I thought I had arrived at the start of the detailed design. Armed with my vast amount of knowledge and more importantly a phone line to the rear guard in Chatham I was ready to dive into some detailed engineering calculations…or so I thought.

Turns out that since January the design has been turned inside out and some fairly significant design issues uncovered that should have been mentioned in the 30% design.

Concourse Slab Strength. The concourse slab has not been design to carry any further load other than those loads present I the permanent state. During the temporary state the concourse will be required to hold the form work for the roof slab. As it turns out it has not been design to hold the additional load of the wet concrete roof slab as it cures. As a result the propping for the roof slab will need to extend through the concourse slab with either large props passing through the concourse slab or a forest of props at both levels. As this is a bottom up build the contractors plan and the project programme is based on the M&E fit out occurring concurrently as each subsequent level is completed. With the proposed forest of props at the platform level the contractor will not be able to complete the M&E fit out until the roof slab has cured. This puts the project programme at significant risk.

Current sections of architect model. The odd shape accopunts for a Highway fly over foundation,road junction and buried services. Section shows platform, concourse and roof slabs

Steel or Concrete Roof. With the issue of concrete curing times and excessive propping the contractor proposed a change to the design that saw the concrete roof slab replaced by a steel beam and precast concrete roof. This particular issue was passed to myself to see if it could be done. Given that some of the spans are nearly 30m this would require some fairly large beams…turns out about 2m deep plate girder beams with 80-100mm flanges should just about do it. The issues faced when designing the concept for the beams are outlined below:

Supports as the rest of the structure is concrete the tying in of the steel beams to the concrete supports to make full moment connections would be particularly difficult and is likely to result in excessive cracking at the interface of the steel beam and concrete. Cracking as my previous blog is a particularly sensitive issue on this project. Therefore the beams would need to be simply supported.

Simply supported beam results in a high bending moment at the mid-section of the beam as the beam is free to rotate at the support and no moments are passed to the concrete. As a result the beams would need to be deeper than if the beams supports were encastra.

Deflection. Given that the beams would be in some cases spanning nearly 30m the deflection at the midpoint was in some cases up to 100mm. Not a large deflection I’m told but any deflection of the beams would encroach on the architects and M&E services spaces. To counter this I suggested that the beams could be pre cambered i.e cut to account for the deflection so that as they did deflect they would then settle at a level.

Elongation. With deflection now sorted my attention was turned to the elongation of the beam. As the beam would deflect from a pre-determined camber to a level that beam would also elongate. As the beams were to be simply supported but would also be required to act as props the beams would have to once extend and rest on a bearing to carry the axial load from the ground and support the box. To ensure this actually happened would be very difficult given design and construction tolerances.

Bearings. It occurred to me that as the roof slab would also be forming the foundations for a mina high way, in a way I was designing a very wide bridge. Any steel bridge usually sits on some sort of bearing and bearings need to be replaced usually every 25 years. The design life of the building is 150 years…and the question to the team how would any maintenance of the bearings be completed, how would bearings be replaced.

Pictures of the King Fullah Road that the station will be beneath at the end oc construction.

There were already a number of problems that I felt the team had missed or were choosing to ignore. As the report was being complied to inform the client that the roof could be completed in steel which would aid the construction programme I felt that we were not advising the client on the second and third order affects of the changes from concrete to steel.

With the risk of making myself a very unpopular individual…well more so then currently. I raised the issue outline above along with: transportation of 30m spans of steel, the fact that Riyadh does not have a steel industry and therefore steel beams would need to be fabricated and imported. My aim was not persuade the client either way but simply to ensure they had all the facts and could make an informed decision.

This has made for a few uncomfortable meetings, however I’m sure that we as the designers have the moral obligation to ensure that a client is fully informed and not simply to follow the blind beliefs of the contractor.