Holdfast Training Services Trial Blog

As part of the Liverpool street station the northern wall of the Blomfield box which will create the interface between the Blomfield superstructure and the London Underground (LU) Circle and District line was due to be an RC pour insitu wall with aluminium rain screen cladding. Well not any more. Following extensive campaigning by Laing O’Rourke since 2012 to produce this wall in precast and countless rejections Crossrail finally agreed to completing the wall in precast concrete, however this has left Laing O’Rourke only 4 weeks in which to turn the designs around in time for the next design review which is critical to the 3D modelling.

The Issue.

Build-ability. The original wall and cladding were due to form the interface between the LU and the permanent structure of the Blomfield Vent Relief Shaft. Between the LU and the construction site is a temporary heavy duty hording. The space between the hording and the back of the RC wall was only 350-400mm, which given the size of the form work sections required to pour it would prove difficult to fix in place and even harder to then remove.

H&S. The only means to remove the formwork would be to remove the heavy duty hording. This presented a considerable H&S concern due to working next to live track.

Commercial. Working track side could only be achieved during track closure (engineering hours, 0200-0500). Working during engineering hours would present considerable time constraints and incur additional cost.

The Proposal.

Replace the RC wall with a precast twin wall. Precast twin wall could be constructed off site and the lowered into place reducing construction time and also negating the need to work track side (when constructing the wall). The overall cost for the wall could be reduced with reduced working hours, reduced transport cost of only transporting the twin wall rather than rebar deliveries and concrete pouring deliveries.

The Actual Project managers instruction

The RC wall and cladding are both to be replaced by architectural grade single skin precast wall. Of which LOR are to take design responsibility (actual instruction a little more indetail)

Having been given the responsibility to deliver the precast facade as it is now known as it is doing the job and the cladding but is not structural I have become embroiled in the dirty world of commercial and design responsibility. While i have very little engineering input with respect to design the facade for loading but I have all the responsibility of coordinating the effort of the architects and engineers from Crossrail, LOR and Arup. Lessons learnt to date.

Open to Interpretation. The PMI was written in such a way that Crossrail believe that it allows us the scope to think outside of the box and come up with our own design. As LOR do not have in house architects or detailed design teams we have had to employee Arup to complete architect designs while LOR would use its subsidiary company ‘Expanded’ to complete structural designs of the precast facade panels. The vagueness of the PMI has resulted in each stakeholders understanding of what is required being different and therefore what the facade is suppose to achieve what loads if any it is to carry and how it will look.

Design responsibility. The vagueness of the PMI has also resulted in endless wrangling over who owns what design responsibility. The architects claiming that they cant design the facade overall appearance unless they know how it is to be constructed fixed and loads transferred to the superstructure and the engineers from Explore claim that until they know how the facade is to interface with the superstructure they can’t design the internal rebar and how the loads are to be carried. At present there is no clause in any contract apportioning design responsibility either in the Crossrail contract, the PMI or the LOR to Arup subcontract. Until this is assigned then designs will not progress.

Ground Truth. The discussions were further stalled by the realisation that the as built drawings that were being worked to by the architects from Arup and engineers from Explore were in fact wrong. This was only picked up during a meeting when I noticed that the drawings that were being used and assumed did not match with my knowledge of ground truth. Lesson here always check that as built actually reflect what has been built and not what people want others to believe has been built to avoid penalties.

Commercial Knowledge. As John has often stated do not assume that others know what they are taking about. This goes for contracts as much as engineering principles. Members from the Crossrail team had been stating a claim that I found out later to be false; Cross rail claimed that Laing O’Rourke were responsible for removing the heavy duty hording. This turned out to not be in the scope of works and is important as it removes the problem of removing a concrete plinth that would once expose a section of the superstructure not covered by cladding. Not the end of the world structurally but for Crossrail and the architects a disaster. As we do not need to remove it the problem of extending the facade beyond the concrete plinth goes away.

Relationships RA and foresight. There appears to be an approach in the industry that its not my problem i don’t need to consider it. The original proposal state a twin wall to replace only the RC wall on H&S grounds and build-ability grounds. When the PMI was issued complaints were made that it was not inline with the proposal, however the engineers and construction managers (who are the guardians of RAs) had failed to note that simply replacing the RC wall did not remove the risk posed to a further sub contractor who would then be required to install the aluminium rains screen cladding. This should have been picked up at the design stage but then again by LOR. If the risk is too great for your own work force why pass on the risk to a further organisation, why not eliminate the risk for all. The replacement of the RC wall and aluminium rain screen with the single precast facade does just this.

Conclusion

The whole contractor, designer and client relationship is crucial and I am not convinced that many in the industry understand that. A client and their designers must make use of ground truths and experience from the coal face as much as the coalface must understand the designers and clients intent. When designs change and there is an opportunity to pass the buck to someone else people will take it, and architect and engineers just aren’t capable of playing nice.