Unreinforced masonry walls

Whether functioning as load-bearing infill walls or free-standing partition walls these heavy and brittle walls are vulnerable to out-of-plane forces. Many of these wall types collapse in earthquakes. They require

Description

Method

Comment

Concrete column: - steel jackets and straps

Section

Existing column (reinforcement not shown)

Grout or concrete

Steel encasement

Continuous steel angle Steel strap

Section

A steel jacket encases column confinement concrete, and increases shear strength and ductility.

Steel straps can be passed through holes in beams to confine and strengthen beam-column joints.

Concrete column: - composite fibre wrapping

Chamfered corner

Carbon or glass fibre bedded in an epoxy or other bonding material

Section

Composite fibre wrapping increases column confinement, shear strength and ductility.

Concrete column: - concrete jacketing

Vertical bar

Continuous (welded) horizontal tie

Cross-tie

Concrete

Section

Concrete jacket increases column confinement, bending and shear strength, and ductility. Vertical reinforcement must be anchored into the foundation and continuous through floor levels. Beam-column joints are strengthened by passing cross-ties through existing beams.

Concrete column: - strengthened with side walls

Section

Section

Side walls, if continuous up a column, can modify a dangerous weak column—strong beam moment frame, into a strong column—weak beam frame.

Concrete beam: steel hoops

Existing beam

Steel strap (fully welded)

Section

Hole cut through the slab

Existing beam

Steel strap (fully welded)

Section

Welded steel straps at close enough centres confine the beam and increases its shear strength.

If the beam corners are chamfered it is possible to use composite fibre hoop wraps.

▲ 12.9 Methods of retrofitting concrete columns and beams.

Description

Method

Comment

Post/mullion support to unreinforced walls

Vertical section

Unreinforced masonry wall

Steel post/mullion, UC or UB

Bolt grouted into wall

Vertical section

Posts require strong connections to upper and lower diaphragms. This method provides walls with out-of-plane resistance.

Concrete skin on concrete or unreinforced masonry shear walls

Vertical section

Existing wall

Grouted-in tie

Reinforcing bar

Sprayed-on (shotcrete) or cast-in-place concrete

Vertical section

A concrete skin increases the strength of an existing shear wall and provides out-of-plane resistance

Composite fibre or steel mesh overlay to unreinforced concrete or masonry shear walls

Fibre or steel mesh

Plaster bedding

Vertical section

Fibre or steel mesh

Plaster bedding

Vertical section

A double-skin overlay increases in-plane shear wall strength as well as out-of-plane resistance.

Fine steel mesh in cement plaster is also effective.

Individual fibre-strips can be used also.

Post-tensioned unreinforced shear walls

Vertical section

Post-tensioned tendons

Vertical section

Post-tensioning requires vertical holes drilled down walls into the foundation where tendons are well anchored.

Post-tensioning increases shear wall strength and out-of-plane resistance.

An alternative detail is to position tendons outside a wall on either side.

▲ 12.10 Methods of retrofitting unreinforced concrete or masonry walls.

support. Vertical structural members, usually steel posts or mullions, are inserted into the walls or fixed to one of their faces (Fig. 12.10). Frequent and strong connections from these members into the masonry are necessary. The posts must also be strongly connected top and bottom to diaphragms, which then transfer horizontal forces from the posts to principal seismic resisting systems elsewhere in plan of the building. New reinforced concrete columns can also provide out-of-plane resistance to unreinforced masonry walls. In a multi-storey building place these vertical elements, typically installed between 3 m and 5 m apart, above each other. Then, if an area of masonry is severely damaged, this new vertical structure can also act as props, preventing collapse of the floors of the building in the vicinity of the wall damage.

This new vertical structure then functions both as a mullion as well as a gravity prop.

Another method to provide out-of-plane resistance is to apply a thin coat of plaster reinforced by a steel or fibre mesh to each side of a wall. This creates a sandwich panel capable of spanning vertically between floor diaphragms. If a coating can be plastered on one side only, a thicker layer of up to 200 mm reinforced concrete, cast-in-place or sprayed as shotcrete can provide adequate strength (Fig. 12.1 1). Both types of layered strengthening also contribute to the in-plane strength of a retrofitted wall.

Existing diaphragms often require upgrading. Particularly in an unreinforced masonry building with wooden floors, it is not feasible to structurally improve existing low-strength diaphragms. In these cases, new diaphragms are constructed above or below existing floors or ceilings. One method that involves casting a new reinforced concrete slab over the existing flooring, provided that the floor joists can support the extra weight, adds undesirable additional weight to the building. A lighter alternative diaphragm is fabricated from structural steel to form a braced diaphragm or horizontal truss. Where a diaphragm resists and distributes a lesser amount of seismic force from surrounding walls to vertical structural elements a new plywood diaphragm can be laid over or under existing flooring or roof framing and diaphragm collector and tie members upgraded as necessary. Fig. 12.12 shows typical diaphragm retrofit solutions.

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