Soft storeys

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Soft storey configuration describes structure where one storey of a building is more flexible and/or weaker than the one above it from the perspective of seismic forces. Rather than earthquake energy absorbed by ductile yielding of steel reinforcing bars, or structural steel sections in plastic hinge zones, or structural fuses throughout the whole structure as shown in Fig. 5.44(b), in a soft storey configuration earthquake energy concentrates on the soft storey (see Fig. 5.44(a)). Serious damage is caused especially to the columns of that soft storey. Once these structural members are damaged the nature of earthquake shaking is not to move on and damage other members. Rather, the quake intensifies its energy input and damaging power in that same storey. Often the structure above a collapsed soft storey is virtually undamaged. It has been protected by the sacrificial action of the soft storey. A soft storey building is doomed, since columns in the soft storey usually lack the resilience to absorb seismic damage and still continue to support the weight of the building above.

The sport of boxing provides an apt analogy for soft storey performance. Both boxing and earthquakes are violent. They pummel and injure. A boxer attempts to discover and then exploit an opponent's weakness. Once discovered, that is where the fury focuses and punches land. A ' softening-up' process, whereby injurious blows slow down reactions and lower defences, continues until the knock-out blow. So too with a soft storey. Once a soft storey is found, the quake focuses its harmful attention upon the relatively few vertical elements in that one storey until the building is either staggering or collapses.

Seismic Forces Architecture

▲ 9.1 Soft storey ground floor collapse of a four-storey building. 1995 Kobe earthquake.

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

▲ 9.1 Soft storey ground floor collapse of a four-storey building. 1995 Kobe earthquake.

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

▲ 9.2 A soft storey ground floor has disappeared in this three-storey apartment block. 1994 Northridge, California earthquake.

(Reitherman, R.K. Courtesy of the National Information Service for Earthquake Engineering, EERC, University of California, Berkeley).

▲ 9.2 A soft storey ground floor has disappeared in this three-storey apartment block. 1994 Northridge, California earthquake.

(Reitherman, R.K. Courtesy of the National Information Service for Earthquake Engineering, EERC, University of California, Berkeley).

Of all vertical configuration problems, the soft storey is the most serious and is by far the most prevalent reason for multi-storey building collapses. So many buildings, located in seismically active regions throughout the world possess relatively open ground floors and are at risk of a soft storey mechanism forming. A report on the 1995 Kobe earthquake observes that ground floor collapse was the most common failure mode in small commercial and mixed-occupancy buildings (Fig. 9.1). Regarding larger commercial and residential buildings, which in most cases appeared regular from the street, the report notes: ' Partial or full collapse of a single story of buildings was the common

"collapse" failure mode The particular story that sustained partial or full collapse varied from building to building ...'2 Soft storey collapses are a common occurrence during any strong ground shaking in a built-up region (Fig. 9.2).

Figure 5.44(a) illustrates ground floor soft storey behaviour caused by weak columns and strong beams. Before suffering damage, that storey may not have been any weaker than the storey above but seismic shear forces and bending moments in moment frame columns increase towards the base of a building. They reach their maximum values at the ground floor making it the most vulnerable. In the absence of the Capacity Design approach, columns are usually weaker than beams so columns alone sustain damage. Once ' softening-up ' begins during a quake the prognosis is very poor.

Soft storeys are also caused by other configuration irregularities as illustrated in Fig. 9.3. The ubiquitous soft storey caused by a combination of open ground floors and masonry infilled frames (Fig. 9.3(a)) is discussed in detail in Chapter 10. As mentioned above, if the soft storey irregularity is reasonably minor, a seismic code may permit the system to resist horizontal forces. However, the structural engineer must undertake special analyses and provide members within that storey with additional strength and ductile detailing. In more severe soft storey cases even the most advanced structural design cannot prevent poor performance in a design-level earthquake. So the questions arise: . Is it possible for a building to exhibit the visual characteristics of a soft storey for architectural reasons and still perform satisfactorily in a quake; and if so, how? .

Seismic Forces Architecture

(a) Stiff and strong upper floors due to masonry infills

(b) The columns in one storey longer than those above

(c) Soft storey caused by discontinuous column

▲ 9.3 Examples of soft storey configurations.

Strong Column Weak Beam Eurocode
▲ 9.4 A weak column-strong beam structure develops a soft storey at ground level once columns are damaged.

Fortunately for the aesthetic satisfaction of architects, building users and the public at large who appreciate slender columns and some degree of design variety, the answer is a resounding 'Yes ' . One of two strategies is employed: either separation or differentiation. Separation involves isolating from the force path those stiff and strong elements - like infill walls and deep beams - which cause adjacent elements - like columns - to be relatively more flexible and weaker. Differentiation describes a design approach that clearly distinguishes between gravity-only and seismic resisting structure and ensures that selected members primarily resist either seismic or gravity forces. The following examples illustrate the application of these strategies.

Imagine that you are designing a building whose façade is modulated by slender columns and deep beams (Fig. 9.4). How can

Spandrel Floor slab

Section through original spandrel beam f . ■ -

A separated precast concrete spandrel panel

Light framing and cladding forms spandrel

▲ 9.5 Options for non-structural spandrel panels.

Soft storey frame

Column Column of new two-way moment above frame

Column Column of new two-way moment above frame

Ground floor plan

New shear wall

New shear wall

Ground floor plan

▲ 9.6 Interior moment frames or shear walls designed to resist all horizontal forces due to the unsuitability of perimeter soft storey frames.

Ground floor plan

▲ 9.6 Interior moment frames or shear walls designed to resist all horizontal forces due to the unsuitability of perimeter soft storey frames.

you achieve this presumably architecturally desirable layout without creating a hazardous weak column-strong beam configuration? Initially, try applying the principle of separation. This means separating off the harmful excess strength of the beams from the frames in order to achieve a desirable weak beam-strong column moment frame. Remove the up- and down-stand spandrel elements often cast mono-lithically with beams. The beams become weaker than the columns and the moment frame becomes potentially ductile. A beam also becomes more flexible, so that aspect requires checking by the engineer. Perhaps the column dimensions will need enlargement to resist code-level seismic forces. Then clad the beams, now weaker than the columns, with spandrel panels. Any panel materiality may be chosen (Fig. 9.5). Panels are structurally separated from the moment frame to prevent them participating in force resistance and to avoid non-structural damage. Use one of the detailing approaches described in Chapter 11. In lieu of attached panels, spandrels can be fabricated from light framing and cladding attached directly to beams.

If, for any reason the strategy of separation is unacceptable, then consider differentiation as a solution. In this approach the seismically-flawed frame configuration remains on the façade but is relieved of any expectation of withstanding horizontal forces by provision of an alternative and stiffer system elsewhere in plan. The internal moment frames and shear walls of Fig. 9.6 resist all horizontal forces because they are stiffer and stronger. They are designed so as the perimeter frames need only carry gravity forces. The structural engineer might even intentionally soften-up columns of the seismically-flawed frames by introducing pins top and bottom to each column. This would prevent them attracting any horizontal forces at all. Whether or not that intentional softening is undertaken the perimeter frame must be flexible and possibly possess some ductility. It has to undergo the same horizontal drifts as the stronger alternative seismic resisting structure without distress while, at the same time, resisting gravity forces.

What if, as designer, you require a double-height floor at ground floor level, or anywhere else up the height of a building for that matter (Fig. 9.3(b))? Begin by accepting that the frames with such a flexible and soft storey must be excluded from the primary seismic resisting system. So keep their irregular configuration and design them to resist gravity forces only. Once again provide an alternative stiffer structure to resist all seismic forces (Fig. 9.6). The structural engineer will check that the soft storey frames can sustain anticipated horizontal drifts without damage.

Additional beam without a floor slab

(a) Provided beams without slabs

(a) Provided beams without slabs

Pin-ended beams support floor

Beam of the mega-frame

(b) Create a two-storey mega-frame by pinning the ends of beams on alternate storeys

Additional beam without a floor slab

Pin-ended beams support floor

Beam of the mega-frame

(b) Create a two-storey mega-frame by pinning the ends of beams on alternate storeys

▲ 9.7 Two methods of avoiding a soft storey where one storey is higher than others.

At least two other approaches are possible. First, introduce beams without floor slabs (Fig. 9.7(a)). This may achieve the intended spaciousness of the double-height storey yet avoid a soft storey by restoring the regularity of the moment frame. Now that the weight of the level without a floor is far less than that of the floor above, a special engineering analysis and design is required. If the idea of inserting beams to create regularity is unattractive, consider a mega-frame solution (Fig. 9.7(b)). The moment frame storey height is extended to two storeys. At alternate storeys floor beams are pinned at their ends to prevent them participating as moment frame elements. The main disadvantage of the mega-frame solution is that the frame member sizes are considerably larger than normal in order to control the increased drift and bending and shear stresses due to the increased storey heights. The columns must also be designed to resist mid-height inertia forces acting at alternate storeys.

Arnold and Reitherman suggest several other possibilities such as increasing the size and/or the number of columns in the soft storey or designing external buttresses to act as shear walls or braced frames for that storey.3

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