Logistical Aspects of Prefabrication

When and how to prefabricate are subtle questions that can’t be answeredby novel hardware, more capital, or blind faith in the factory. And evensolutions that have proven successful in their own time (e.g. precut housesin the interwar years, preassembled wood-frame wall panels in the seventies)– may lose their viability to broader developments in the technologicallandscape. This paper unravels some of the logistical factors that determinewhen prefabrication makes sense and when it doesn’t, and which approachesto prefabrication can succeed in what kinds of contexts.The central concept is specific value, or value density. Primary manufacturersadd value by reducing the net volume of material. A sawmill doubles thevalue of wood while reducing it by a factor of 3, for an increase of volumespecificvalue by a factor of 6. House-framing doubles the value of materialagain, but increases the volume something like 20 times, so specific valueis reduced by a factor in the order of 10. Sawmilling and other primarymanufacturing processes naturally take place close to the resource, to minimizetransport costs. Construction tends to take place closer to the site,again, to reduce transport costs. Building components and sub-assembliescan also be understood in terms of specific value. Vinyl windows, for exampleare of a middling value density and are assembled locally out ofhigher specific value extrusions and hardware that can be shipped from afar.A survey of freight calculations shows that volume is the dominant factor intransport but that it’s far from exclusive. Weight plays a role, but smallerthan may be anticipated; various kinds of fragility and climate sensitivitiesare actually more important. Prepping and handling are also significant.Understanding these factors helps explains why prefabrication is more successfulin some aspects of building than others, and why success dependson geographic factors. The differing importance of capital in securing marginaladvantage in a given process is also important. Accordingly, someindustrial processes are vulnerable to lower-capital competition, others aremore secure. Finally, we need to look at some hidden costs of the factory,and some unexpected economic advantages to the decentralized system ofmobile piecework-subcontractors that have proven so remarkably resilientover the past century and more.Overall we discover that each conceivable building component, preformed,pre-assembled or otherwise, has its own context-dependent radius of viability.Centralizing their separate manufactures in a comprehensive plantmay simply mean that many components end up getting shipped fartherthan their value density warrants, or not far enough to give their productionmachinery a viable market.However, understanding the principles and evolution of logistics will helpus envision new ways of systematizing buildings and components. Someof these are in evidence today, some are yet to be realized. Together theysuggest new kinds of construction economy. They also support a more elegantintegration of systems, and a better orchestration of visual elements,intervals, and proportions.

Volume Editors
John Quale, Rashida Ng & Ryan E. Smith

ISBN
978-0-935502-85-5