Solutions to problems caused by infill walls

Unfortunately, only three solutions are available: the first is often not feasible and the other two, while simple in theory, are difficult to achieve in practice.

(a) Symmetrical frame building with CoR and CoM co-incident

(b) Infill walls cause CoR to move to right creating an eccentricity between CoM and CoR

Horizontal movement of column furthest from CoR

t

Resistance 1

CoR

E/Q force

(c) A y direction earthquake force causes the building to twist about CoR

▲ 10.8 Asymmetrically placed infill walls cause building torsion that damages columns distant from the CoR.

The first solution (as discussed in Chapter 5) is for infill walls to be transformed into confined masonry construction that is fully integrated with the structural frame. As discussed in Chapter 5, confined masonry can play an important role in providing seismic resistance. However, this approach is valid only where designs comply with the numerous and architecturally restrictive criteria that apply to confined masonry construction.

Another alternative to overcoming the problems associated with infill walls is to provide very stiff primary structure. In this situation the less stiff infill walls do not attract horizontal forces. Reinforced concrete (RC) shear walls are the only structural system capable of achieving the necessary stiffness. Only such stiff structure can limit interstorey drifts to several millimetres per floor during seismic shaking. Commenting from a European perspective, Michael Fardis suggests that: ' The best way to protect an RC building from the adverse effects of heightwise irregular infilling is by providing shear walls that are strong and stiff enough to overshadow any difference between the infilling of different storeys. 3 Later he points out that the European seismic code takes a more permissive approach.4 It allows masonry infills to be present in frame structures provided certain rules are followed. For example, if the plan layout of infills is asymmetrical, possible adverse torsional effects must be investigated by extensive 3-D computer modelling. Where infills are distributed vertically in an irregular pattern causing a potential soft storey, affected columns are required to be far stronger and larger than usual. Where such columns are to be designed, and assuming they remain elastic in the design-level quake, their design forces may be of the order of five times greater than those of ductile columns.

Moment frame

Moment frame

Gap'

Separated infill wall

Gap'

Separated infill wall

This leaves the third and final option able to be summarized in a single word - separation. Based upon the relatively poor seismic performance of infill walls in past earthquakes, current practice in seismically active countries such as Japan, USA and New Zealand is to separate infill walls from their frames. Where a country's seismic design philosophy requires that non-structural elements escape damage in small earthquakes, and do not damage primary structure in a large event, separation becomes the most common solution. Separation gaps allow the frame to deflect freely without being impeded by the wall (Fig. 10.9).

between

Infill wall infill and

Gap with separation gaps columns and beam.

Infill walls require separation from the frame by gaps of sufficient width as calculated by the structural engineer. Separation gaps provide architectural detailing challenges. Issues such as acoustic control, weather tightness, fire protection and aesthetic qualities all need to be addressed.

▲ 10.10 Some typical architectural details of separation gaps between an infill wall and frame.

Resolution of these architectural details is commonplace in the countries listed above. Several typical details are shown in Fig. 10.10.

Two essential features of a seismically separated infill wall are: a clear vertical gap between the infill and columns (typically between 20 mm to 80 mm wide), and an approximately 25 mm wide horizontal gap between the top of the wall and the soffit of the beam above. This gap under a beam or floor slab must be greater than that element's expected long-term deflection, and also allow for the downwards bending deflection of a moment frame beam under seismic forces. Where provided, these gaps allow the floor above an infill to move horizontally to-and-fro without the infill wall offering any resistance in its plane.

Often the resolution of one problem creates another. Although an infill may be separated for tn-plane movement, where it is separated on three sides it becomes extremely vulnerable to out-of-plane forces or face-loads as they are sometimes called. It must be stabilized against

Steel angle

Reinforced infill

Recess formed in beam 200 mm long every 2 m along beam

Steel dowel or flat plate cast into wall, and central in recess -- — --■ Reinforced infill

▲ 10.11 Two possible structural details that resist out-of-plane forces yet allow relative movement between an infill wall and structure above.

▲ 10.11 Two possible structural details that resist out-of-plane forces yet allow relative movement between an infill wall and structure above.

▲ 10.13 Partial out-of-plane collapse of an unreinforced masonry infill wall. Commercial building, Tarutung, Indonesia, 1987 Sumatra earthquake.
▲ 10.12 A reinforced masonry infill wall separated from surrounding structure. Note the horizontal and vertical gap (to beam) and a galvanized steel bracket resisting out-of-plane forces yet allowing in-plane movement. Office building, Wellington.

these forces acting in its weaker direction, yet at the same time allow unrestricted inter-storey drift along its length. One of several structural solutions is required.

The most obvious approach to stabilizing an infill wall against out-of-plane forces is to cantilever it from its base. But this is not usually feasible for two reasons. Firstly, the floor structure beneath may not be strong enough to resist the bending moments from the wall. Secondly, the infill wall itself may not be strong enough or may require excessive vertical reinforcing. On the upper floors of buildings, elements like infill walls are subject to very high horizontal accelerations well in excess of 1 g, the acceleration due to gravity.

The preferred option is to design an infill wall to resist out-of-plane forces by spanning vertically between floors (Fig. 10.7). Through its own strength the wall transfers half of its inertia force to the floor beneath and the other half to structure above. Careful structural detailing at the top of the wall can provide sufficient strength to prevent out-of-plane collapse yet simultaneously accommodate interstorey drift between the top of the wall and structure above. Figure 10.11 illustrates some generic connections between reinforced masonry infill walls and concrete frames while Fig. 10.12 illustrates an as-built solution.

Where infill walls are constructed from unreinforced masonry - which is generally too weak to span vertically from floor to floor when withstanding out-of-plane inertia forces (Fig. 10.13) - one approach is to provide small reinforced concrete columns within the wall thickness (Fig. 10.14). Their function is not to support any vertical force but to stabilize the infill against out-of-plane forces. For a long panel, three or more intermediate ' practical columns ' (as they are sometimes called)

Separation gap

'Practical' column

Unreinforced masonry

Elevation of infill and frame

Cap over the end of the bars to allow beam deflection without loading the bars

Masonry infill

Concrete

Cap over the end of the bars to allow beam deflection without loading the bars

Masonry infill

Concrete

Wrap bars with tape to prevent bond with concrete in beam

Beam

Practical column

Wrap bars with tape to prevent bond with concrete in beam

Beam

Practical column

Detail at top of a practical column

▲ 10.14 Separated unreinforced masonry infill wall with 'practical columns' providing out-of-plane strength.

may be designed by the structural engineer.5 Support of this form is commonplace near the roof of a building where ground accelerations are amplified most strongly. Only the reinforcing bars that project vertically from these small columns connects to the underside to the beam. This detail, strong enough to resist out-of-plane forces, allows virtually unrestrained inter-storey drift in the plane of the wall. The ductile bending of the vertical practical columns bars will not provide significant resistance to that movement.

It is worth noting that some of the above separation difficulties can be alleviated by off-setting intended infill walls from the primary columns. Walls are therefore designed to run in front of or behind columns. No longer infills, they can be considered partition walls or exterior cladding and are discussed in Chapter 11.

Support at ends of top beam to transfer out-of-plane forces from wall but allowing frame to move freely towards or away from the wall tm

Reinforced concrete sub-frame (horizontal reinforcing not shown)

Detail A

Column

X.

k /

W/

Steel angle bracket bolted to column on either side of wall

Separated wall

Detail A

Detail A

Steel angle bracket bolted to column on either side of wall

Separated wall

Column Steel bracket

Separated wall

Detail A - plan section

▲ 10.15 The separation of a partial-height unreinforced masonry infill wall.

▲ 10.16 A separated partial-height infill in a moment frame. Due to the low height of the infill its out-of-plane inertia forces can be resisted by two small reinforced concrete columns. Office building, Pisco, Peru. (Reproduced with permission from Darrin Bell).

Partial-height infills also need separation from their adjacent columns yet be prevented from collapsing out-of-plane. A partial-height unreinforced infill wall can be enclosed within a reinforced concrete sub-frame that is then restrained out-of-plane by a steel bracket at each end or by some other structural solution (Figs 10.15 and 10.16). A clear gap must exist between the sub-frame and the primary structural frame.

An alterative detail is to provide a horizontal member, perhaps in the form of a channel section, to span between the columns of the main frame. The channel resists the upper half of the infill wall inertia forces and transfers them back to the main columns. The infill is free to slide

Steel channel attached to primary columns at each end is designed to support the out-of-plane forces on the wall

Wall free to slide in channel

Section

Steel channel attached to primary columns at each end is designed to support the out-of-plane forces on the wall

Wall free to slide in channel

Section

▲ 10.17 Alternative support to a partial-height infill wall. Depending on the wall height and the distance between primary columns the steel channel might require strengthening with welded-on plates.

▲ 10.17 Alternative support to a partial-height infill wall. Depending on the wall height and the distance between primary columns the steel channel might require strengthening with welded-on plates.

▲ 10.18 Full-height windows adjacent to columns suggest another means of creating vertical gaps between infill walls and columns. Library building, Kanpur, India.

in the direction of the channel and is of course separated by vertical gaps adjacent to the main columns (Fig. 10.17).

Another promising approach to the problem of infills currently under development is to ' soften' infills rather than fully separating them from frames.6 This entails using weak mortar between bricks and laying bricks between vertical studs and horizontal dwangs to subdivide walls into many small areas. Out-of-plane support is provided by the studs. In a damaging quake these infills respond by 'working' along the joints between the infilling and frame members. The sliding between masonry and framing dissipates a significant amount of seismic energy. Relatively weak and flexible, these softened infills are sacrificial elements that provide increased levels of damping to protect the primary structure. They may be a useful strategy to improve the seismic resistance of new and existing moment frames, particularly in developing countries.

Although the need for infill separations is usually seen as a problem to be overcome, it can also lead to innovations. In Fig. 10.18 the insertion of narrow and tall windows between column and wall suggests another possible response. Provided that clearances around the glazing can accommodate calculated inter-storey drifts this detail avoids the need for a specific separation gap between the infill wall and columns.

Staircases

Like infill walls, staircases damage the primary structure as well as being damaged by it. Where staircases are strongly attached to the structure, to some degree they act as structural members. Their inclination creates the potential for them to function as diagonal braces (Fig. 10.19).

Compression

▲ 10.18 Full-height windows adjacent to columns suggest another means of creating vertical gaps between infill walls and columns. Library building, Kanpur, India.

\

x

Stairs form compression struts (and tension ties) under horizontal force

Single-flight stairs

Stairs form compression struts (and tension ties) under horizontal force

▲ 10.19 Bracing action of stairs connected to structure.

Braces that form a triangulated framework are very stiff against horizontal forces as compared to moment frames. Therefore, staircases can attract unanticipated high levels of force (Fig. 10.20). If stairs are severely damaged, building occupants may be unable to exit a building after an earthquake.

Where stairs are positioned asymmetrically in plan they induce building torsion. With reference to Fig. 10.8 assume a staircase is located adjacent to the right-hand edge of the

▲ 10.20 Damage to stair support structure. Mexico floor plan. Before the staircase is constructed the building is

City, 1985 Mexico earthquake. completely symmetrical from a structural perspective, but if

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

we assume that the staircase stiffness is equivalent to that of two infill walls, it moves the CoR to the right, creating torsion as explained previously.

To avoid damage to both staircase and structure, the recommended solution is to separate the stairs by providing a sliding joint at each floor (Fig. 10.21). The sliding detail must allow inter-storey drift between floors in any direction without restraint. When inter-storey drifts do occur, the stairs slide on the floors below and therefore do not attract any seismic force apart from the minimal inertia forces arising from their relatively small self-weight.

Structural model of stairs with sliding joints (rollers)

Concrete stair flight

Concrete stair flight

Plastic sheet or equivalent to form a sliding plane

Detail of sliding joint

Detail of sliding joint

Plastic sheet or equivalent to form a sliding plane

▲ 10.21 Model and detail of a single-flight stair separation.

▲ 10.22 A staircase celebrating provision of interstorey drift through rollers at its base. Office building, Wellington.

Sliding joints which structurally separate stairs from primary structure are formed easily. Just break any likely bond between the stair and its base support; when the floor at the top of the stair drifts further than the floor below, the stair slides. The stair can be either pin jointed or rigidly cast into the floor above. Remember that all stair inertia forces, including forces at right angles to the direction of the staircase, must be transferred back to the main structure. Some stairs are separated with more sophisticated materials such as Teflon strips that bear on stainless steel plates. This combination possesses very low friction but is usually unnecessarily sophisticated for such a simple slip joint. Figure 10.22 illustrates movement detailing at the base of a staircase that an architect has chosen to celebrate.

The separation details for a switch-back or dog-leg stair with a half storey-height landing is more complex, as shown in Fig. 10.23'

▲ 10.22 A staircase celebrating provision of interstorey drift through rollers at its base. Office building, Wellington.

Idealized precast concrete switch-back stair in elevation

Upper flight fixed at end and landing hung

Lower flight fixed at base with landing hung to accommodate interstorey drift

Upper flight fixed at end and landing hung

Lower flight fixed at base with landing hung to accommodate interstorey drift

Line of fixity Upper flight

Tension hanger connection

Landing

Tension hanger connection

Landing

Horizontal gap Column

Horizontal gap Column

Plan

▲ 10.23 Idealized elevations and plan showing a method of separating a switch-back stair from the structure.

Was this article helpful?

0 0
Greener Homes for You

Greener Homes for You

Get All The Support And Guidance You Need To Be A Success At Living Green. This Book Is One Of The Most Valuable Resources In The World When It Comes To Great Tips on Buying, Designing and Building an Eco-friendly Home.

Get My Free Ebook


Post a comment