Apr 10, 2014
Cast Connex has recently developed a guide for writing performance based specification for AESS Castings, and we are happy to share.
So you’ve chosen to use castings in your project – great! You’re probably using castings to fulfill a specific structural requirement or to satisfy an architectural need – or perhaps both. You appreciate that standard steel fabrication may not easily – or simply cannot – achieve what casting can. You understand that castings generally require a longer lead-time than conventional fabrication, so you’ve allotted enough time post-tender for the castings to be designed, detailed, poured, tested and delivered. You know all of this, but you need everyone working on the project to know it too.
As a designer, your principal tool for communicating with the contractors who build the structures you design are the contract documents. And so you’ve shown the castings on your drawings with some general, overall dimensions and maybe some 3D images to convey intent. You expect that with this information, bidders will understand that castings are required. But is that enough?
Unfortunately, the answer is probably not. It has been our experience working on projects of all types and sizes that that omitting the right kinds of information on castings in the tender documents can lead to undesirable outcomes. For example, bidders may omit aspects of or over-estimate the cost of castings, which often results in the components being eliminated from the project. Discrepancies or misunderstandings with respect to responsibilities and deliverables may arise, adding additional complexity or costs. Even the difference in a) what you thought you would be getting, b) what is feasible with casting manufacturing, and c) what is contractually required, can lead to conflicts.
Asking the right questions early on and clearly documenting the requirements in the contract documents will help steer you away from these undesirable situations. We know that not everyone out there is a castings expert, which is why prescribing specific material grades or excessive non-destructive examination (NDE) requirements, for example, is not the best approach. That doesn’t mean, though, that you cannot establish requirements for structural cast steel components through a performance-based specification to ensure you and your client gets what they want and need at a reasonable price.
Some preliminary questions to ask relate to scope and responsibility. Who is responsible for the engineering of the castings? Who is responsible for preparing the 3-dimensional models and shop drawings the foundries need to produce the components? Who is responsible for setting non-destructive examination requirements? Who is responsible for procuring the castings and coordinating tolerances between the cast steel components and the fabricated steel? Who is responsible for quality control and quality assurance related to the castings? What experience and background is necessary to be responsible for these aspects of design and implementation, etc…
While you may be focused on aesthetics and structural adequacy, making a component “castable” is just as important. Casting considerations like molding, feeding, directional solidification, and hot-spot tearing, for example, have to be considered when designing castings. Successfully designing castings without this knowledge and the requisite experience can be difficult. Assuming that your office doesn’t have that experience, it’s important that the specification delegate these responsibilities to a suitably experienced party. It is also important to state key architectural features and governing load-cases – the performance standards – which must be met such that the casting designer can satisfy architectural, structural, AND castability requirements, simultaneously. A typical casting designer in the structural engineering field will also be able to engineer the castings to meet these performance requirements as well as to prepare the required shop drawings, set appropriate NDE requirements, establish QA/QC procedures, and even procure the castings. An important side note is that while foundries obviously have casting expertise, they typically will not assume design responsibility or assess whether a component they produce is fit for any specific purpose or use. Most foundries won’t even assist in the production of 3-dimensional models or shop drawings. And they certainly will not seal drawings.
Some secondary questions to ask relate to the information you wish to see to satisfy yourself that the resulting components are adequate. Do you want 3D models submitted for approval? Do you want to see and approve the shop drawings? Do you want to review engineering calculations? Do you want the first casting produced from the new set of tooling (the first article casting) approved before the remaining production components are made? Do you want to review physical and chemical test reports? Do you want to review NDE results?
Including these deliverables within the specification can help ensure your client gets what they want. It will allow the casting designer the flexibility (even creativity) in satisfying your architectural and structural requirements, but promote productive and stage-wise communication between all parties. The goal of having these deliverables is to establish onus on the casting designer, but still allow you to retain control over design intent.
There are more questions to be asked, and there are more answers to be determined. But, I hope that I’ve at least made you start thinking about what you should or should not include in your contract documents. Our team has given specification comments and feedback on countless projects, and can help you write your performance-based casting specification for your next iconic project.