Shear wall material and height

As noted previously, a shear wall can be constructed from any structural material. Provided its material strength is not significantly weaker than that of the surrounding structure its material of construction is likely to be structurally viable. Choice of shear wall materiality is also influenced by building height and other constraints as summarized in Table 5.1. Readers with the frequently asked question: ' How many shear walls of what thickness and length are necessary for my project?' must wait until the following chapter for an answer.

While still on the subject of shear wall materiality, unreinforced masonry warrants further discussion due to its extensive usage internationally. Its different manifestations in buildings include completely unreinforced masonry construction as in Fig. 5.14' as infill panels between reinforced concrete or steel moment frames, as confined masonry walls, or as non-structural partition walls. As discussed in Chapter 10 and illustrated in Fig. 10.4(a), a masonry infill panel within a moment frame functions as a diagonal bracing strut when a combined frame and infill system is loaded horizontally. Provided the frames and infills satisfy the requirements inTable 5.2 they may contribute positively to the seismic resistance of a building. In many situations, however, some of the listed requirements are architecturally unacceptable. In such cases, infills should be physically separated

▼ 5.1 Common shear wall materials, their typical ranges of height and general comments

Material

Typical height range (storeys)

Comments

Steel

4-8

Steel plate walls have occasionally strengthened existing buildings but otherwise are rarely used (Fig. 5.10).

Reinforced concrete

1-20

The most reliable material for medium to heavy construction. Its high level of ductility enables designers to achieve the shortest wall lengths. Reinforced concrete walls are usually cast in-place (Fig. 5.11). Precast concrete panels including tilt-up panels typically one- to three-storeys high, and other panels strongly connected together can also form monolithic reinforced concrete walls (Fig. 5.12).

Reinforced masonry

1-6

Popular where long wall lengths are achievable such as adjacent to boundaries. As the strength of masonry units and grout infill reduces, required wall lengths increase (Fig. 5.13). Construction quality control requires special attention.

Unreinforced masonry

1-4

Due to its inherent lack of ductility this material is prohibited in some countries and in others permitted only in less seismically active areas (Fig. 5.14).

Confined masonry

1-6

'Confined' masonry walls incorporate panels of unreinforced masonry within a reinforced concrete beam and column frame. They are discussed below.

Wood

1-5

Suitable for light-weight construction. Because of its relatively low strength and stiffness as compared to reinforced concrete or steel construction several long or many shorter walls may be required to resist seismic forces. Particularly appropriate for apartments and similar buildings where function dictates a cellular layout.

Plywood fixed to wood framing often forms the web. Chords can be from sawn or glue-laminated wood or even a more highly engineered wood product like laminated veneer lumber (LVL). Where horizontal deflections are critical steel chord members may be specified (Fig. 5.15).

Gypsum plasterboard or equivalent sheets are nailed or screwed to wood or steel framing to form shear walls in light-weight framed construction. A popular and cost-effective material for domestic construction in New Zealand, but the wall lengths required are longer than those using a stronger wood-based product such as plywood.

▲ 5.10 Steel shear walls during construction. Hospital,

Portland, Oregon.

(Reproduced with permission from KPFF).

▲ 5.12 A shear wall formed from numerous precast panels strongly connected vertically and horizontally. Apartment building, Wellington.

▲ 5.14 An unreinforced masonry wall under construction. Kanpur, India.

▲ 5.11 Reinforcing steel for cast-in-place concrete shear walls. High-rise office building, San Francisco.

▲ 5.13 A reinforced concrete masonry wall with attached columns under construction. Horizontal reinforcing, placed every third masonry unit is not visible. Office building, Wellington.

▲ 5.15 A two-storey wood shear wall with steel chords. Wanganui, New Zealand.

▼ 5.2 Recommendations for the safe use of masonry infill walls within reinforced concrete or steel moment frames

Discipline impacted by

Requirements of masonry infills and moment frames

the requirement

Architectural

Infills reasonably symmetrically located in plan to avoid

excessive torsion.

Infills continuous up the height of the building beginning

from the foundations.

If infills are penetrated openings should be the same

size and in the same position.

Structural

Infills should be physically connected to columns or

beams with reinforcement to prevent the masonry

collapsing under out-of-plane forces.

Moment frames are designed to resist horizontal forces

without the assistance of infills.

Special ductile detailing at beam-column junctions

and at the tops of each column adjacent to an infill

to prevent the infill diagonal compression strut causing

premature column damage.

from the structure or else the structure made so stiff and strong as not to be adversely affected by them. The positive and negative contributions of masonry infills to the sound seismic performance of buildings is a topic of on-going research and is discussed in more detail in Chapter 10.

The difference between an ' infill' and ' confined' masonry wall panel is that confined masonry is intended to play a structural role.3 Masonry is confined between columns and beams that are cast after the masonry is laid thereby thoroughly integrating it structurally and allowing diagonal struts to form within the masonry under horizontal forces (Fig. 5.16). This means large penetrations like doors that prevent the formation of diagonal struts are not permitted in confined masonry panels that are expected to provide seismic resistance. Confined masonry panels are both shear walls and gravity load-bearing walls. Beam and column confining members are not designed as elements of moment frames but rather as chords and ties of braced frames. The masonry acts as diagonal compression members (Fig. 5.17).

While confined masonry construction enables column and beam dimensions to be minimized due to an absence of seismically-induced bending moments significant architectural limitations beyond those of Table 5.2 are necessary. Because the seismic strength of a confined masonry building is reliant upon the masonry panels enough panels of sufficiently thick masonry need to be provided in each orthogonal direction to provide the necessary horizontal strength. In the example in Fig. 5.18 the cross-sectional area of structural walls along each axis is 8 per cent of the gross floor area. This compares to a value of

▲ 5.16 Confined masonry construction. Columns and beams are cast after laying the walls. Only walls without large penetrations and are continuous up the height of the building act as shear walls. Lima.

(Reproduced with permission from Ángel San Bartolomé)

Inertia force

Tension

Inertia force

Tension

Reinforced concrete frame

Compression strut formed in masonry panel

Tension Compression

Reinforced concrete frame

Compression strut formed in masonry panel

Tension Compression

▲ 5.16 Confined masonry construction. Columns and beams are cast after laying the walls. Only walls without large penetrations and are continuous up the height of the building act as shear walls. Lima.

(Reproduced with permission from Ángel San Bartolomé)

Elevation of a confined masonry wall showing internal 'truss action' forces

▲ 5.17 The forces within a confined masonry wall.

20 m

20 m

Ground floor plan

▲ 5.18 Plan of a four-storey confined masonry wall and concrete floor building in the most seismically active zone of Indonesia. The significant lengths and thicknesses of walls drawn to scale are required to satisfy structural requirements. All walls are constructed from brick masonry and are not penetrated.

3 per cent suggested for Iranian school construction up to three storeys high.4 Since the masonry of confined masonry construction can withstand only modest diagonal compression stress depending upon the quality of masonry units and the mortar, the height of this type of construction should be limited. One report, based upon Indonesian conditions, recommends these buildings not exceed four storeys in height.5

As for infill walls, researchers continue to develop seismic resistant guidelines for this common construction system. Infill walls are discussed further and in detail in Chapter 10.

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