J

▲ 8.24 Seismic drifts and required separations in elevation and plan to avoid pounding.

▲ 8.24 Seismic drifts and required separations in elevation and plan to avoid pounding.

▲ 8.25 Concertina flashing between two buildings, San Francisco.

As demonstrated above, seismic separation gaps can become quite wide. The only way to minimize them is to design a stiffer structural system. An architect has to decide what is less problematic and more economical - larger vertical structural members or wider separation gaps. Each option yields a different usable floor area. The structural engineer might need to undertake several alternative designs before making a final decision. Mark Saunders calculates that the cost of floor area lost to separation gaps for a 40-storey building in San Francisco approximates 4 per cent of its value.8 Vertical gaps between separated buildings are usually flashed by a flexible concertina-type detail (Fig. 8.25). At roof level a typical detail allows free horizontal movement in two directions, towards and away from an adjacent building, and to-and-fro parallel to the gap length (Fig. 8.26).

▲ 8.26 Possible detail of a seismic separation gap between two buildings at roof level.

A seismic gap is also required where one building is separated into two independent structures. Detailing must allow for two-dimensional relative out-of-phase movements between blocks. Some codes don't require the seismic gap between the structures to be determined by simply adding the maximum seismic drifts of each block. They allow the two drift values to be combined in such a way that recognizes the low probability of both maximum drifts occurring at precisely the same time. Nevertheless, seismic joints are wide and expensive. The cost can be minimized by having them pass through diaphragm penetrations like stairs and elevators.

Satisfactory architectural treatment of seismic gaps is also required for junctions between floors, walls and ceilings (Figs. 8.27 and 8.28). Architects design and detail flashings and linings to cover the gaps in such a way as to avoid tying the separated structures together. Details must allow movement but it is permissible for them to be damaged in

A = max. movement apart B = max. movement towards each other

Detail allows plate to pop-up

Flooring Metal plate

Recessed fixing to concrete

Section

Fire blanket

Section

Fire blanket

▲ 8.27 A section through a generic floor level seismic gap. Dimension 'A' can be reduced if damage caused by the plate sliding off its left-hand seating is acceptable during smaller relative drifts.

▲ 8.27 A section through a generic floor level seismic gap. Dimension 'A' can be reduced if damage caused by the plate sliding off its left-hand seating is acceptable during smaller relative drifts.

▲ 8.28 A seismic joint between floors, walls and ceilings of two separated structures, San Francisco.

moderate shaking as they can usually be replaced easily. They are considered ' sacrificial ' in that their damage pre-empts more serious damage elsewhere. Gaps are frequently designed to be fire-resistant and are acoustically treated.

Seismic separation joints also raise various structural issues sometimes with significant architectural implications. For example, how are gravity forces to be supported on each side of a joint? One approach involves providing double beams and columns with the gap running between them (Fig. 8.29(a)). In another method (Fig. 8.29(b)) one floor system cantilevers. It is propped on sliding joints allowing free relative movement by the other structure. Obviously the horizontal overlap of the floor and the supports must allow for the structures to move towards and away from each other and should be dimensioned conservatively. The consequences of a floor slab falling off its supports are severe.

Seismic separation gap Column

Beam

(a) Plan of seismic gap with beams and columns on either side of the gap

Cantilever or corbel support to right-hand slab

(a) Plan of seismic gap with beams and columns on either side of the gap

Cantilever or corbel support to right-hand slab

-J

Beam

1

(b) Plan of seismic gap with beams and columns on one side

Gap Sliding joint

(b) Plan of seismic gap with beams and columns on one side

Gap Sliding joint

Section A-A

▲ 8.29 Two methods of supporting flooring at a seismic separation gap.

▲ 8.30 Collapsed bridge formerly spanning between two buildings. 1995 Kobe, Japan earthquake.

(Reproduced with permission from US National Geographic Data Center. Dr. R. Hutchison, photographer).

▲ 8.30 Collapsed bridge formerly spanning between two buildings. 1995 Kobe, Japan earthquake.

(Reproduced with permission from US National Geographic Data Center. Dr. R. Hutchison, photographer).

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