Seismic design and architecture 109

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Proposed seismic resisting structure

Moment frame

Remove column

0.6 x 0.35 m column and beam

Moment frame

Remove column

0.6 x 0.35 m column and beam

(a) Ground floor plan

Proposed seismic resisting structure

Bedroom Remove colum

Bedroom Remove colum

(b) First floor plan

Moment frame Shear wall

Collector/tie reinforcement

(c) Plan of first floor showing proposed structure

Moment frame below Shear wall below

(d) Plan of roof diaphragm y

▲ 6.26 Simplified floor and roof plans of the Villa Savoye showing the proposed seismic structure if rebuilt in a seismically active area.

out-of-plane inertia forces and structurally separated, as explained in Chapter 10. Due to their small diameter, the columns (or pilotis as they are usually called) are far too weak and flexible to function as members of moment frames assuming that there are beams, which in most cases there are not.

Now begins the difficult task of finding locations and space for the new structural systems. They need to be continuous from foundations to roof without drastically affecting existing spatial planning. One solution is to design two shear walls to resist y direction forces (Fig. 6.26(a)). From preliminary calculations performed by RESIST assuming a soft-soil location in Wellington, two ductile reinforced concrete walls 2.4 m long by 0.35 m thick are sufficient. If the bracing effect of existing Wall 1, which occurs only at the ground floor, is taken advantage of by making it structural the dimensions of the two new walls can be reduced. The rear wall passing through the chauffeur's space and the boudoir above has little impact on planning but the left-hand wall significantly reduces light into the maids ' room.

It is impossible to use shear walls in the x direction without destroying interior spaces or compromising the architectural form. The chosen approach therefore is to use four one-bay moment frames. Of all options they best suit the existing planning. Unfortunately, due to their 600 mm column depth they are intrusive in some spaces like the boudoir and disrupt the maids ' room. Its existing column which already disrupts that space would be removed but it is not nearly as inconvenient as the proposed column over three times its size.

The difficulty of inserting the proposed structure within existing subdivided space, as well as the rather unsatisfactorily forced and awkward outcome, clearly illustrates a point made previously. Preliminary spatial planning must be undertaken simultaneously with the development of rational seismic structure. If structural and planning requirements inform the development of each other, the outcome is a scheme where structure and interior spaces are harmoniously integrated. Otherwise, if these requirements are addressed independently the outcome as (in this example) leaves a lot to be desired.

The final step in the design of the relocated Villa Savoye is to assess diaphragm adequacy. Can the heavily penetrated diaphragms transfer inertia forces from all areas in plan to the proposed new shear walls and moment frames? For the first floor diaphragm, the answer is 'yes'. Both penetrations will not affect inertia forces being transferred into the shear walls. Since the right-hand shear wall is near the rear of the building, and shear forces cannot be effectively channelled into it where it is adjacent to the penetration, a collector is provided, as shown in Fig. 6.26(c). It consists of several continuous reinforcing bars anchored in the wall and embedded in the concrete slab beyond the length of the penetration.

The roof diaphragm is far more heavily penetrated (Fig. 6.26(d)). It is almost severed into two sections, A and B. A collector or tie member strong in both tension and compression is needed to connect both areas along the right-hand side. Then approximately half the inertia forces in the y direction from area B can be transferred to an area of diaphragm strongly connected to the rear shear wall. A similar tie is required along the left-hand side to transfer inertia forces from the left-hand side of area B into the shear wall on that side of the building.

Diaphragm performance in the x direction is more uncertain (Fig. 6.27). Note the considerable torsional eccentricity and moment in this direction. All the inertia forces from area B have to be transferred in shear through the 1.7 m wide strip between the smaller terrace and the circular stair penetration (Fig. 6.26(d)). Special calculations are required to ascertain if that highly stressed area can be made strong enough. Certainly many extra reinforcing bars are needed. Only then can the safe transfer of shear force from area B to the four moment frames be guaranteed. Another issue to be considered is the in-plan bending moment created about the weakened section when area B inertia forces act in the x direction (Fig. 6.27). Although some of that moment can be resisted by the two strips of diaphragm connecting areas A and B on either side of the stair they are probably too weak. The two y direction perimeter ties assist. The right-hand tie experiences compression and the left-hand tie tension for the loading direction shown. Both ties need to be checked to determine they can sustain the design compression forces without buckling.

In summary, it is possible to recreate the Villa Savoye in a seismically active area. But the proposed structural solution is far from elegant given the incompatibility of seismic resisting structure with existing interior planning. The case-study warns of the negative consequences that arise when architects procrastinate engaging with seismic design requirements.

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