Diaphragm materiality

The choice of diaphragm materiality depends upon the spans of the diaphragm and the intensity of inertia force to be resisted. While a house ceiling or roof diaphragm might span as little as 3 m and support the inertia force of light wood framing, a roof diaphragm over a sports stadium could span over 100 m and support heavy and high concrete walls. In the first example the diaphragm web might consist of single sheets of plasterboard or plywood nailed to wood framing and wooden chords. In the second, a braced steel diaphragm would be expected given that a wood solution would be too weak and concrete too heavy.

Where the roof and floors of a building are of reinforced concrete they, together with any perimeter beams, function as diaphragms. A cast-in-place floor slab is usually suitable as a diaphragm provided the structural engineer checks its strength for shear forces and bending moments. Where precast flooring is used, the precast units must be strongly connected so that they can work together to form an effective monolithic diaphragm. Usually a reinforced concrete topping slab provides a convenient connection method that also achieves a smooth and level

Topping Reinforcing Hollow-core slab steel slab

Beam

Section

▲ 4.6 Part section through a precast concrete and topping slab diaphragm.

floor surface (Fig. 4.6). Typically, the topping will be between 65 mm and 100 mm in thickness, contain reinforcing bars running in both directions and be cast over intentionally roughened precast concrete unit surfaces to achieve strong bonding between the fresh and hardened concrete.

Light-weight diaphragms consist of timber sheet products like plywood, particle board or wood cross-bracing (Fig. 4.7). In light steel construction, like that of the single-storey industrial building in Fig. 4.8 , diaphragms usually take the form of horizontal cross-braced frames or trusses. The diaphragm consists of two horizontal trusses. They resist horizontal y direction forces and transfer them to the vertical cross-braced frames. Note that this diaphragm cannot resist nor transfer x direction forces. The diaphragm layout in Fig. 4.9 is suitable for that direction. Instead of a truss at each end of the building we could use a total of three or four trusses. This would allow truss member sizes to be reduced. If only one truss forms the x direction diaphragm its member sizes will be large since fewer members resist the same force. If this is a problem it can be partially alleviated by increasing the truss depth from one to two bays. Structural members like purlins running in the x direction need to be continuous and strongly connected to the horizontal trusses in order to transfer inertia forces into them. Roof purlins are sometimes doubled-up to fulfil this function.

Plywood or particleboard

Nailing along sheet edges at close centres

Blocking under unsupported sheet edges

Plywood or particleboard

Nailing along sheet edges at close centres

Blocking under unsupported sheet edges

Floor joists

Wood chord

▲ 4.7 Section of a wooden floor diaphragm.

Floor joists

Wood chord

Direction of inertia forces

Direction of inertia forces

Intermediate moment frames resist gravity and wind forces only

Cross-braced frames at each end

Intermediate moment frames resist gravity and wind forces only

Cross-braced frames at each end

Diagonal diaphragm bracing across the full width

Diagonal diaphragm bracing across the full width

Direction of inertia forces

Gravity moment frame

Direction of inertia forces

Gravity moment frame

▲ 4.7 Section of a wooden floor diaphragm.

▲ 4.8 Structural layout of a single-storey light-industrial steel building.

▲ 4.9 Braced diaphragm and cross-bracing for x direction forces.

Member continuity and strength is also required in the y direction to form the truss chords. As well as functioning as the beams of gravity moment frames these horizontal members experience additional tension and compression stress during an earthquake when they take on a second structural role as diaphragm truss chords.

The final step of diaphragm design involves combining the requirements of the previous two diagrams (Fig. 4.10). Two of many possible

(a) Plan of diaphragm adequate for both x and y direction forces

Roof diaphragm

Chord strongly fixed to primary diaphragm

Chord (and beam)

Roof diaphragm

Direction of inertia force

Tension only bracing Light-weight addition

(a) Plan of diaphragm adequate for both x and y direction forces

Cross-braced bay in wall

Cross-braced bay in wall

(b) An alternative diaphragm configuration

▲ 4.11 A secondary diaphragm cantilevers horizontally from the primary structure to avoid vertical end-wall cross bracing.

Vierendeel truss

(beam under)

(b) An alternative diaphragm configuration

▲ 4.10 Diaphragm and wall bracing configurations suitable for both x and y direction forces.

▲ 4.11 A secondary diaphragm cantilevers horizontally from the primary structure to avoid vertical end-wall cross bracing.

Vierendeel truss

(beam under)

Direction of inertia force

Direction of inertia force

▲ 4.12 A vierendeel truss as a cantilevered diaphragm.

diaphragm configurations are illustrated. Now inertia forces in any direction have an identified force path. Depending on the levels of force these cross-braced diaphragms can consist of tension-only bracing or tension and compression bracing. Although tension-only bracing requires twice as many diagonal members they are far smaller in cross-section than any designed to resist compression.

Tension-only diaphragm bracing is also useful where connecting a lightweight glazed addition to a heavier and stronger primary structure (Fig. 4.11). Here the roof bracing, designed for wind as well as seismic force spans the width of the lean-to. As there is no vertical bracing within the end-wall of the addition the roof diaphragm needs to cantilever horizontally from the primary roof diaphragm.

Regarding the choice of diaphragm configuration the two types already illustrated are either opaque, as in the case of sheet-based wood or concrete diaphragms, or essentially transparent. Daylight penetrates trussed diaphragms with transparent roof cladding. If the exposed diagonals of normal truss diaphragms are unacceptable architecturally, vierendeel trusses are an option, albeit expensive (Fig. 4.12).

Horizontal Thrust Diaphragm
▲ 4.13 Horizontal forces from light-weight construction to the left are transferred through roof and first floor braced diaphragms to concrete masonry shear walls. Educational building, London.

Architects enjoy considerable freedom when configuring roof diaphragms. Ideally, a diaphragm should provide the most direct force path for inertia forces into vertical bracing elements yet it can sometimes provide opportunities to strengthen the expression of the architectural design concept or idea (Fig. 4.13).1

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