Timber floor diaphragm overlay

New plywood sheet

Existing flooring


An overlay with plywood sheeting increases diaphragm stiffness and strength. Sheet-to-sheet and sheet-to-existing flooring connections require careful attention.

Braced Steel strap diaphragm

New flooring packed above straps

Steel strap


Diagonal steel straps in conjunction with existing floor members form a horizontal truss (diaphragm).

The straps can also be placed under the joists.

Diaphragm to wall connections. Use either A or B

Steel angle bolted regularly to wall

Plywood sheet overlay

Existing floor Existing joist

Steel strap bracing Steel channel

Where the steel angle or channel is continuous it also functions as a diaphragm chord.

Detail A is used when diaphragm strengthening is confined to above the floor, and B, below.

Vertical section

Concrete overlay diaphragm

New concrete overlay Reinforcing mesh

A reinforced overlay increases the strength and stiffness of an existing diaphragm.


▲ 12.12 Methods of retrofitting diaphragms.

by far the best seismic track record of the three systems. Construction is easier if walls are placed on the outside of a building where their new foundations (which are often required) are much easier to construct. If exterior walls are chosen, pay careful attention to their architectural impact on the existing building. For buildings with configuration deficiencies (like soft storeys) it may be sufficient to provide a'strong back' wall. This is a conventional shear wall, except that it can be pinned at its base or allowed to rock, thus greatly reducing foundation costs.13

▲ 12.13 External retrofitted concrete shear walls, Vancouver.

Figure 12.13 shows an example of a building retrofitted by external shear walls. Two pairs of reinforced concrete shear walls now resist all seismic forces parallel to their lengths. They free the seismically deficient I967 riveted steel moment frames from needing to resist seismic forces. The walls are strongly attached to each floor diaphragm and at their base are cast into a deep basement beam that reduces their tendency to overturn under seismic forces. Soil anchors at each end of the beam prevent it overturning.I4

In a second example (Fig. 5.8), an increasing number of windows up the height of the wall reflect the reduction in shear force. Large and deeply embedded tension piles under the wall prevent it from overturning and help transfer horizontal seismic forces into the soil.

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