Masonry

Seismic considerations for masonry cladding are similar but less complex than those for infill walls (Chapter 10). Although external masonry walls may not be placed between columns and so by definition are not infill walls, they share some of the same undesirable seismic characteristics. Therefore, unless very stiff shear walls resist horizontal forces in the direction of their lengths, non-structural masonry walls should be separated from the main structure. The strategy of separation also

▲ 11.1 Unseparated masonry cladding damaged by the interstorey drift of a flexible internal steel frame. Mexico City, 1985 Mexico earthquake.

(Reproduced with permission from R.B. Shephard).

▲ 11.1 Unseparated masonry cladding damaged by the interstorey drift of a flexible internal steel frame. Mexico City, 1985 Mexico earthquake.

(Reproduced with permission from R.B. Shephard).

▲ 11.2 Collapse of exterior wall. 1983 Coalinga, California earthquake

(Reproduced with permission from R.B. Shephard).

▲ 11.2 Collapse of exterior wall. 1983 Coalinga, California earthquake

(Reproduced with permission from R.B. Shephard).

Wall B

Top of wall after interstorey drift

Wall B restrained against out-of-plane in x direction so at its top it follows the movement of the floor above

Crack where Wall B pulls away from wall A

Wall A separated from floor above so does not move in x direction

(a) Unseparated corner-interstorey drift in x direction

(a) Unseparated corner-interstorey drift in x direction

(b) Separated corner detail

▲ 11.3 Plan of two walls forming an exterior corner. Because both walls need out-of-plane support where they connect to the structure above, interstorey drift damages the corner (a) unless a vertical separation gap, yet to be treated architecturally, is provided (b).

prevents indirect damage to the primary structure by eliminating potential configuration problems (Chapter 9) as well as inplane cracking and more severe damage to the walls (Figs 11.1 and 1 1.2). Details of out-of-plane restraints like those of Fig. 10.11 are appropriate. A vertical separation gap at wall corners is required to control damage in those areas (Fig. 11.3).

Brick veneer is also a very popular cladding system. Unfortunately, it also doesn't have a good earthquake track record.3 As an example, considerable the veneer damage that occurred during the 1989 Newcastle, Australia, earthquake (Fig. 1 1.4). Veneers support their own weight but, as their name implies, a veneer must be tied back to internal structure. Reinforced masonry walls or vertical wood or steel studs provide out-of-plane resistance by transferring inertia forces from the veneer to floor and ceiling diaphragms (Fig. 11.5). In wooden buildings, veneer ties are embedded in horizontal mortar joints and screwed to vertical posts or studs.

The two most important design principles for achieving the best seismic performance from a veneer are:

• Tie the veneer strongly to the structure for both tension and compression forces

• Veneer tie spacing should comply with code requirements. In New Zealand, ties are typically placed no further apart than 600 mm horizontally and 400 mm vertically,4 and y

Wall A

▲ 11.4 Damage to brick veneer due to corroded veneer ties. 1989 Newcastle, Australia earthquake.

(Reproduced with permission from R.B. Shephard).

▲ 11.4 Damage to brick veneer due to corroded veneer ties. 1989 Newcastle, Australia earthquake.

(Reproduced with permission from R.B. Shephard).

▲ 11.5 Inertia forces acting on a veneer are transferred through ties to studs and then to diaphragms above and below.
▲ 11.6 Veneer on a multi-storey wood framed building. Each storey-height of veneer is supported on a steel angle bolted to framing. Hotel, Tongariro National Park, New Zealand.

• Use veneer ties that are flexible in the direction of the plane of the veneer itself unless the primary structure is at least as stiff as the veneer in that direction against horizontal forces. This allows the primary seismic resisting structure to deflect horizontally without loading and damaging the often stiffer veneer.

Relative flexibility between veneer and structural framework leads to a concentration of damage where veneer panels meet at corners. Damage prevention, necessitating wide vertical separation gaps at corners, is usually deemed impractical or aesthetically unacceptable for most buildings and so damage in those areas is often accepted as inevitable.

Due to its hazard if it were to fall from a building, the maximum height of veneer panels is limited by some codes. Where brick veneer is used for cladding multi-storey buildings, steel angles or other means of supporting the weight of the veneer are provided at each storey (Fig. 11.6). Increased earthquake accelerations up the height of a building might necessitate reduced tie-spacing and perhaps special horizontal reinforcement for additional safety.

Cladding panel

J

Movement

connection

—1

Interstorey drift

(a) Panel hung from the top

Interstorey drift

(a) Panel hung from the top

(b) Panel supported at its base

Interstorey drift

----- Movement connection

Drift of structure behind panel

Bearing connection

▲ 11.8 Precast panels attached to a reinforced concrete frame building. Note the connections near the tops of columns and the plane of horizontal separation between panels below the beam soffit. Office building, Wellington.

(b) Panel supported at its base

▲ 11.7 Fully separated storey-height panel, top-hung (a) and supported at its base (b).

▲ 11.9 Slotted steel connection welded to a plate cast into the column (a) and the completed detail allowing movement (b).

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