Transfer diaphragms

The diaphragms discussed previously are sometimes termed simple diaphragms. They resist the inertia forces from their own mass and those of elements like beams and walls attached to them. Transfer diaphragms resist the same forces but in addition they transfer horizontal forces

Diaphragm chord

(a) Notch destroys continuity of chord

(c) Penetration where the diaphragm shear force is at its maximum

(c) Penetration where the diaphragm shear force is at its maximum

Structural wall

Direction of inertia force

Area where diaphragm shear failure is likely

(b) Bending moments can be carried across the notch by a continuous chord
(d) Optimal location of penetration

▲ 4.14 Diaphragm penetrations in various locations.

from one vertical bracing system above to another that is offset horizontally below. Transfer diaphragms are usually far more heavily stressed than simple diaphragms and consequently need to be considerably stronger and may not be able to accommodate large penetrations.

In the building shown in Fig. 4.15(a), y direction forces are resisted by structural walls at each end. One wall at first floor level is discontinuous. It does not continue directly from the roof to the foundations. Therefore, the horizontal forces at the bottom of that wall must be transferred through the first floor slab functioning as a transfer diaphragm into the offset wall at ground floor. The forces acting on the transfer diaphragm are shown in Fig. 4.15(c). If the floor is reinforced concrete, at the very least it may require extra reinforcement but additional thickening may also be needed. In this building the transfer diaphragm transmits all the horizontal shear forces from the wall above to the wall beneath. Since the diaphragm is incapable of resisting the overturning moment from the wall above, two columns, one

Inertia force in y direction

Inertia force in y direction

Resistance from offset wall

(a) Building form

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(c) Plan of first floor transfer diaphragm

Resistance from offset wall

Resistance from ground floor wall below

Inertia force from first floor wall

Most highly stressed region of transfer diaphragm

Structural wall

(b) Longitudinal section

(b) Longitudinal section

Overturning moment from wall

Compression in this column and tension in other

(d) Two columns are required to resist the wall overturning moment.

▲ 4.15 A transfer diaphragm provides a force path for horizontal shear forces from the upper wall to the offset ground floor wall.

y under each end of the wall, are required to resist the tension and compression forces from that moment and thereby stabilize the wall (Fig. 4.15(d)). If these columns are architecturally unacceptable the only other option is to provide deep transfer beams (Chapter 9).

Due to their less direct force paths that potentially lead to increased seismic damage, transfer diaphragms and any associated vertical structure need to be designed very carefully. Capacity Design procedures must be followed to prevent undesirable failure mechanisms. Although designers prefer to avoid transfer diaphragms, sometimes they are unavoidable and cause significant design complications (Fig. 4.16).

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