Discontinuous and offset walls

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Consider the building in Fig. 9.16 . At its upper levels y direction forces are resisted by shear walls at each end, but at ground floor level the left-hand wall, Wall 1, is discontinuous. Two perimeter moment frames x

▲ 9.17 Ground floor damage caused by a discontinuous wall. 1980 El Asnam, Algeria earthquake.

(Bertero, V.V. Courtesy of the National Information Service for Earthquake Engineering, EERC, University of California, Berkeley).

▲ 9.17 Ground floor damage caused by a discontinuous wall. 1980 El Asnam, Algeria earthquake.

(Bertero, V.V. Courtesy of the National Information Service for Earthquake Engineering, EERC, University of California, Berkeley).

resist x direction forces. When struck by a quake in the y direction, the ground pulses will distort the ground floor columns under Wall 1. Their ' softness' prevents Wall 1 from providing the seismic resistance perhaps expected of it and exemplifies the worst possible case of a soft storey. At the other end of the building the base of Wall 2, which is continuous, moves with the ground motion. Due to the more substantial overall strength and stiffness of Wall 2, as compared to Wall 1, Wall 2 tends to resist the inertia forces from the whole building. The two different wall drift profiles are shown in Fig. 9.16(d). Since Wall 1 resists almost no inertia force due to its discontinuity, yet Wall 2 is fully functional the building experiences serious torsion. To some degree, but limited by the modest lever-arm between them and their inherent flexibility, the two x direction moment frames try to resist the torsion. As the building twists about its CoR located close to Wall 2, the columns furthest away from the CoR are subject to large drifts and severe damage (Fig. 9.17).

(a) Ground floor plan showing walls replaced by moment frames

Columns to prevent Wall 1 overturning

New wall CoM

(a) Ground floor plan showing walls replaced by moment frames

Columns to prevent Wall 1 overturning r

New wall CoM

Wall 1 First floor continues transfer above diaphragm (b) Ground floor plan with a new wall added

Eccentricity

Wall 2

Wall 1 First floor continues transfer above diaphragm (b) Ground floor plan with a new wall added

Wall 2

/\

A

A

(c) Vertical section showing the transfer truss required if the columns under Wall 1 are omitted

(c) Vertical section showing the transfer truss required if the columns under Wall 1 are omitted

▲ 9.18 Alternatives to a discontinuous wall.

What are the solutions to this problem? Probably the best option is to make both walls non-structural. Form them from either light-weight materials or use non-structural cladding panels to achieve the required architectural characteristics. Using the same approach as the building of Fig. 9.16, provide new moment frames behind the non-structural walls (Fig. 9.18(a)). Another possibility is to introduce an off-set single-storey wall back from Wall 1 (Fig. 9.18(b)). As explained below, this solution, which introduces many architectural and engineering complexities, is best avoided. Chapter 4 discusses this less-than-ideal situation where a transfer diaphragm channels seismic shear forces from the base of the upper section of a wall across to the top of an off-set wall below. This situation applies to Wall 1. Two strong columns, one at each end of Wall 1 must withstand vertical tension and compression forces to prevent it overturning under the influence of floor diaphragm forces feeding into it up its height. As mentioned in Chapter 4, if those columns are omitted, the overturning-induced axial force can also be resisted by two deep transfer trusses or beams. They must remain elastic during the designlevel quake to prevent permanent downwards movement of the wall. As a rule-of-thumb, the truss depths should be between one and two times the cantilever span distance depending on the building height. In many cases such deep members, which must extend well into the body of the building to get sufficient vertical support to stabilize them, are not architecturally feasible (Fig. 9.18(c)). Since the trusses or deep beams create a strong beam-weak column configuration, ground floor shear walls in n

(a) Elevation of a partially (b) Structural fuse region discontinuous wall at ground floor level

■ n

(c) Structural fuse region at first floor level

(c) Structural fuse region at first floor level

▲ 9.19 A partially discontinuous wall and options for the location of its structural fuse or plastic hinge region.

Inertia force

T

TC

T

C

T

C

T T

C

T T

system.

Elevation of a staggered wall system

The indirect force paths of a staggered-wall the x direction will be required as well as the whole of the first floor slab being designed as a transfer diaphragm. Another reason the offset solution is not ideal is that torsion is introduced due to eccentricity between the CoM and CoR at ground floor level for y direction forces.

The danger posed by off-set walls supported on cantilever beams has been tragically and repeatedly observed during five Turkish earthquakes in the 1990s. After categorizing building damage a report concludes: ' Buildings having architecturally based irregular structural systems were heavily damaged or collapsed during the earthquake. Cantilevers of irregular buildings have again proven to be the primary source/cause of seismic damage. Many buildings have regular structural systems but [even if] roughly designed performed well with minor damage' '5

Figure 9.19 shows a less extreme wall discontinuity. A large penetration weakens the most highly stressed region of the wall creating an undesirable soft storey. Traditional engineering wisdom would advise approaches as outlined previously, such as designing the wall to be nonstructural, but given the sophisticated 3-D analysis and design tools available to contemporary structural engineers, a careful design might achieve satisfactory seismic performance.When approaching the design of an element with a discontinuity such as this, it is crucial that designers first identify the ductile overload mechanism (Fig. 9.19(b) or (c)) , and then using the Capacity Design approach as described in Chapter 3, ensure dependable ductile behaviour. One approach is to design for plastic hinging at ground floor level and detail the wall and unattached column accordingly with the wall above strengthened to avoid premature damage. Another approach is for the first floor section of wall to be designated the fuse region. This means the ground floor section and the wall above first floor will be stronger than the fuse so damage occurs only in that specially detailed area.

In an extreme example of a staggered-wall (Fig. 9.20), the same principles apply. After computer analysis in order to examine the indirect force path, a fuse region must be identified and detailed to accept damage before any other structural element in the force path is affected. Due to its structural irregularity and complexity, as well as the difficulty of applying Capacity Design principles, all of which drive up the cost of construction, this system is not recommended.

Staggered Setback
▲ 9.21 Typical setback configurations.

Transfer diaphragm Podium

(a) Setback building where podium structure supports tower base

Flexible or pinned columns tt

(b) Tower structure provides the podium with horizontal resistance

(c) Tower and podium structures separated

▲ 9.22 Different approaches to the configuration of a tower and podium building.

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  • SARAH FARBER
    What are discontinuous walls?
    1 year ago

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