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138 STRUCTURE AS ARCHITECTURE▲ 7.12United Airlines Terminal, Chicago, USA, Murphy/Jahn, 1987. The main concourse.▲ 7.13Beam–column junction.Materiality and constructionSome architecture is characterized by a strong expression of structuralmateriality and construction. Each structural material possesses featuresparticular to its own materiality. For example, thinness of section, flangedcross-sectional shapes, potential for extreme slenderness in both compressionand tension, and the ability to accommodate significant penetrationsin members are characteristics unique to steel construction.Concrete, in a plastic or even completely fluid state while still fresh, canharden in moulds of almost any shape and display many different surfacetextures. Other signatures of concrete include negative details at constructionjoints and form-tie recesses. Timber materiality on the other hand isbest expressed by its natural grain and colour, typical rectilinear crosssectionshapes and connection details that respond to its relative softnessand anisotropy. Certain structural configurations such as vertical andhierarchical layering of horizontal joists and beams, and relatively closelyspacedbeams and posts are also trade-marks of timber construction.This section, which illustrates structures whose detailing not onlyexpresses building materiality and construction, but celebrates it, beginsby considering a structural steel building whose materiality becomesapparent at first glance.The structure of the United Airlines Terminal concourse and departurelounges, Chicago, utilizes a limited vocabulary of two steel sections, theI-beam and the tube (Figs 7.12 and 7.13). Highly penetrated I-beamsform the irregularly shaped beams of portal frames that articulate and