Bond beams

Bond beams, introduced in Chapter 2, offer another approach to resisting horizontal inertia forces and transferring them sideways to bracing elements (see Fig. 2.19). In the absence of a floor or roof diaphragm a bond beam can span horizontally between lines of vertical bracing elements like shear walls. Although designers use bond beams frequently in masonry construction the same principle can be applied

Precast Shear Walls

▲ 4.16 A hidden eccentrically braced frame immediately behind precast concrete panels is offset from the ground floor braced frame. A short transfer diaphragm moves horizontal shear from the outer frame to the inner ground floor frame (Left). To resist the overturning-induced compressions and tensions from columns above, the first floor cantilever beams are very deep (Right). Apartment building, Wellington.

▲ 4.16 A hidden eccentrically braced frame immediately behind precast concrete panels is offset from the ground floor braced frame. A short transfer diaphragm moves horizontal shear from the outer frame to the inner ground floor frame (Left). To resist the overturning-induced compressions and tensions from columns above, the first floor cantilever beams are very deep (Right). Apartment building, Wellington.

to all construction types and materials. Where the distance between bracing walls requires a relatively deep or wide horizontal beam, a truss might be more suitable.

Imagine designing what is essentially a box-shaped form shown in Fig. 4.17(a). If a desire for a very transparent roof precludes a normal braced diaphragm, bond beams may be a solution for effectively creating a flexible roof diaphragm. Another solution is to cantilever the walls vertically from their foundations but very thick walls or walls with deep ribs at, say, 4 to 5 m centres, may not be acceptable architecturally (Fig. 4.17(b)). If bond beams are chosen, the wall thickness must be sufficient to span vertically between the roof level bond beam and the foundations. The bond beams are then designed to resist half of the out-of-plane forces on a wall and to transfer them to the two walls parallel to the direction of seismic force. By virtue of spanning horizontally rather than vertically, the orientation of bond beams is unusual. It is like taking a normally proportioned beam and rotating it 90 degrees to act horizontally. The relationship of the beam to the wall can be varied as shown in Fig. 4.17(c).

Wall transfers inertia force half up and half down

Direction of inertia force

Wall transfers inertia force half up and half down

(a) Box-shaped form

Bond beam transfers forces horizontally to resisting walls

No roof diaphragm - Resistance provided by wall

(a) Box-shaped form

Thickened wall

Constant thickness wall with tapered ribs

Constant thickness wall with tapered ribs

(b) Sections through two wall options where the wall resists out-of-plane inertia forces by cantilever action

Bond beam

Wall

(c) Bond beam in two different positions with respect to the wall

▲ 4.17 Vertical cantilever and bond beam options for resisting out-of-plane inertia forces on the walls of a box-shaped building.

Resistance from structural wall

Resistance from structural wall

Bond beam acting as arch

Inertia forces from wall acting at bond beam level

Curved wall

(a) Plan For x direction forces the bond beam acts like an arch

Resistance

Resistance

Deflected bond beam as it acts like a horizontal portal frame

(b) Plan For y direction forces the bond beam acts like a frame

▲ 4.18 A curved bond beam behaves differently depending on the direction of horizontal force.

If an exterior wall is curved, a bond beam can use the rounded geometry to transfer forces primarily by arch-action (Fig. 4.18(a)) . In the orthogonal direction the bond beam-cum-arch would be designed as a rounded portal frame (Fig. 4.18(b)).

Tension force in member transfers diaphragm Inertia force shear into shear wall

Collector or tie member

Tension force in member transfers diaphragm Inertia force shear into shear wall

Collector or tie member

Plan of diaphragm

Plan of diaphragm

Structural wall

Shear force from diaphragm acting on collector member

Collector drags ■■■--—forces back to structural wall

Exposed collector or tie beam x direction force resisting structure

Column

▲ 4.19 Collector or tie members transfer diaphragm shear forces into vertical structure.

▲ 4.21 Collector or tie beams connect into the end of a shear wall. They transfer inertia forces from a diaphragm on the right into the shear wall. Car parking building, Wellington.

Exposed collector or tie beam x direction force resisting structure x

Collector or tie member

End elevation

▲ 4.19 Collector or tie members transfer diaphragm shear forces into vertical structure.

Collector or tie member

Typical floor plan

▲ 4.20 A car parking building where collector members are needed to transfer y direction shear forces into the two shear walls.

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