Jonathan Ochshorn's Structural Elements for Architects and Builders, Third Edition
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Chapter 5 — Reinforced concrete: Construction systems

Reinforced concrete buildings are built twice: first as an "inverse" building in which the desired content is made void and the space around this content is actually constructed; and then again as the real, intended structure consisting of concrete reinforced with deformed steel rods. Of course, the first building is not really a building at all, but rather the formwork in which the reinforced concrete building is cast. Still, the construction of extensive formwork as the inverse condition of the intended concrete structure has significant ramifications, not only in terms of structural costs but also in terms of formal constraints that are imposed on the concrete design by the necessity of first constructing the inverse forms from some other material.


Historically, lumber was the primary material used to create forms into which concrete is placed, or cast. Now, other materials are also used, especially metal (reusable) forms and plywood (rather than boards), but also plastics and fiberglass. Formwork must be structurally able to withstand the lateral pressure of the "wet" concrete before it cures (hardens). Metal formwork ties are often provided for this purpose, leaving small circular marks in the surface of the concrete that may be organized and detailed for aesthetic purposes in so-called "architectural" concrete (i.e., concrete where the architect and client care about the surface qualities as in Figure 5.5a), or simply filled with grout in more utilitarian applications (or where the sloppiness of the finish is actually consistent with the aesthetic intentions as in Figure 5.5b).

pattern of formwork tie holes
Figure 5.5: The pattern of formwork tie holes (a) can be considered as part of the architectural expression, as in the Johnson Museum, designed by I.M. Pei & Partners, or (b) can be filled solid in a utilitarian manner as in this stair adjacent to Gates Hall, designed by Morphosis Architects (photos at Cornell University by the author, Sept. 2014)

These ties can be configured so that they simultaneously support reinforcing bars, which are placed in the forms before the concrete is cast (Figure 5.6).

pattern of formwork tie holes
Figure 5.6: Formwork ties are inserted through the formwork to keep the two form surfaces aligned properly, to keep them from deforming due to the lateral pressure exerted by the concrete before it has cured, and to support reinforcing bars; these ties are shown (a) from the side and (b) from the top of a form before the concrete has been placed (all images screen-captured from the Construction of Milstein Hall Part 2 Substructure video by the author at

Traditional formwork structures consist of ordinary dimension lumber arrayed in a gridded pattern of soldiers (vertical elements) and wailers (horizontal elements) that support two wooden surfaces (boards or plywood) between which a concrete wall is cast (Figure 5.7).

Typical formwork for reinforced concrete wall
Figure 5.7: Typical formwork for reinforced concrete wall consists of soldiers and wailers supporting plywood surfaces, with formwork ties providing resistance to lateral pressure of the "wet" concrete

Reusable metal forms are also commonly employed (Figure 5.8) and, in applications where concrete walls need to be insulated in any case, insulating concrete forms (ICFs) — consisting only of rigid insulation held together with metal ribs instead of formwork ties — can be used instead of wood- or metal-based systems of formwork (Figure 5.9). In such systems, the insulation-as-formwork remains in place permanently.

Reusable metal forms
Figure 5.8: Reusable metal forms, these based on a 2-foot module, are assembled for a residential foundation wall (photos by the author)
Insulating concrete forms (ICFs)
Figure 5.9: Insulating concrete forms (ICFs) consist of rigid insulation tied together with metal ribs (photo by the author)

When reinforced concrete elements are exposed, the surface quality of the concrete may be problematic, since it is the cement — the least visually interesting component of the concrete — that rises to the surface. Many compensatory strategies have been employed to turn concrete into something more visually compelling. To remove the mottled gray surface of cement and expose aggregate, the surface can be sandblasted or acid etched; or the materials comprising the formwork itself can be carefully chosen to impart on the cement a mirror image of whatever the forms were made from — examples include the carefully spaced wooden boards shown in Figure 5.5a, or a rougher, "brutalist" aesthetic deriving from the use of construction-grade lumber, plywood, or even metal forms. Alternatively, form liners can be inserted into formwork to impart a texture or pattern onto the concrete that is independent of the formwork material itself (Figure 5.10).

Form liner being pulled from concrete
Figure 5.10: Form liner is peeled away to reveal ribbed concrete surface, Milstein Hall, Cornell University, designed by Rem Koolhaas-OMA (from the Construction of Milstein Hall Part 2 Substructure video by the author at

Conveying and placing concrete

Concrete is moved from mixer to formwork by various means, including wheelbarrows, buckets, pumping, or mere gravity (Figures 5.11 and 5.12).

Examples of concrete being placed
Figure 5.11: Concrete can be placed in many ways including (a) by wheel barrow, (b) using pumps, and (c) down a chute directly from the ready mix truck (photo a by author; other images b and c screen-captured from construction video by the author — see
Concrete being pumped
Figure 5.12: Concrete for a slab is pumped from a remote vehicle

A danger in such movement is segregation, where heavier aggregate settles and water rises. Concrete is placed or cast rather than poured, although the latter term has insinuated itself into common construction vocabularies, and cannot be entirely avoided. In any case, try to say "cast-in-place" instead of "poured-in-place." To make sure that concrete has reached all parts of the formwork, it is often vibrated with special tools (yes, vibrators are used — see Figure 5.13). This prevents honeycombing (where voids appear after the formwork is removed). Concrete should be protected from moisture loss (evaporation) for at least 7 days, by sprinkling water on its surface, or by covering it with sheets such as polyethylene.

Concrete being vibrated
Figure 5.13: A vibrator is inserted into concrete from the top of the formwork

Frames and slabs

Reinforced concrete can be cast into an infinite variety of shapes, subject only to the laws of statics and the difficulty (expense) of creating the forms into which the concrete is cast. At one extreme can be found reinforced concrete structures whose forms are entirely unique, idiosyncratic, and expressive; at the other extreme are concrete structures made entirely from rationalized and reusable forms organized to produce buildings with relatively simple and repetitive geometries. In the latter category are framed structures consisting of columns and slabs, either with or without the hierarchically intermediate girders and beams characteristic of wood and steel structures. Reinforced concrete girders are never really omitted in such systems, but they can be subsumed within the thickness of floor or roof slabs so that the structure appears to consist only of a slab resting directly on columns. Figure 5.14 shows common structural slab systems for reinforced concrete-framed structures.

Concrete slab types
Figure 5.14: Reinforced concrete slabs take many forms including (a) 1-way slabs spanning between beams supported by girders, (b) 2-way flat plates in which all beams or girders are subsumed within the slab thickness, (c) 2-way flat slabs with articulated drop panels and/or column capitals to improve resistance to punching shear, and (d) 2-way grid (waffle) slabs created with reusable formwork with a pre-determined module (arrows indicate the direction of slab reinforcement)

Examples of one-way slab systems and grid (waffle) slab systems can be seen in Figure 5.15. Procedures for the design of reinforced concrete columns, one-way slabs, beams, and girders will be discussed later in this chapter.

One-way concrete slab and two-way grid slab
Figure 5.15: (a) One-way slabs span directly between reinforced concrete girders supported on square columns, Teagle Hall, Cornell University, photo by the author; (b) two-way grid slabs are supported on cantilevered girders, Robert Purcell Community Center, Cornell University, photo by the author