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12Architectural Detailingfor Earthquake Resistance

12.1 Introduction

A large part of the damage done to buildings by earthquakes is non-structural. Forinstance, in the San Fernando, California, earthquake of February 1971, a total of$500 million worth of damage was done to the built environment of which over halfwas non-structural. The importance of sound anti-seismic detailing in earthquake areasshould need no further emphasizing. The choice of a suitable structural form is crucial,involving full collaboration at conceptual design stage between architects and engineers(Chapter 8).

Buildings in their entirety should be tailored to ride safely through an earthquakeand the appropriate relationship between structure and non-structure must be logicallysought. For the effect of non-structure on the overall dynamic behaviour of a buildingsee Section 8.3.8 (pages 2468), where the question of full separation or integrationof infill panels into the structure is discussed.

Architectural items such as partitions, doors, windows, cladding and finishes needproper seismic detailing; many non-seismic construction techniques do not survivestrong earthquake motion as they do not provide for the right kinds or size of move-ments. Detailing for earthquake movements should, however, be considered in con-junction with details for the usual movements due to live loads, creep, shrinkage andtemperature effects. As with so many other problems, it is worth saying that goodplanning can provide the right framework for practical aseismic details.

An ironic example of the inadequacy of a non-structural item comes from the SanFernando earthquake; a modern fire station withstood the earthquake satisfactorilywith regard to its structure, but the main doors were so badly jammed that all thefire engines were trapped inside. Arnold (1991) notes that engineers tend to empha-size structural damage in earthquakes, but in certain situations earthquake damageto non-structural components will greatly exceed the cost of structural damage. Forexample, in an analysis of a new 27-storey condominium building in Los Angeles,Shipp and Johnson (1990) estimated that in a Maximum Credible Event the building

Earthquake Risk Reduction D.J. Dowrick 2003 John Wiley & Sons, Ltd ISBN: 0-471-49688-X (HB)

458 Architectural detailing for earthquake resistance

would suffer structural damage of just over $1 million compared to non-structuraldamage of just under $6.7 million, relative to a total construction cost of $42.8 million.This estimated cost is for direct economic loss only, excluding indirect losses ofrevenue and building use. Moreover, costly damage to non-structural elements canoccur in earthquakes of moderate intensities which would cause little or no struc-tural damage.

In the last two decades of the 20th Century, useful work on non-structural detail-ing has been carried out, in particular, by Massey (1992), Arnold (1984) and Arnoldand Reitherman (1982), to whom reference should be made to supplement the follow-ing discussion.

12.2 Non-structural Infill Panels and Partitions

12.2.1 Introduction

The recommendations of this section should be applied in conjunction with nor-mal design considerations regarding creep, shrinkage and temperature effects whichoverlap, but are generally less exacting than the seismic design requirements forinfill panels.

In earthquakes all buildings sway horizontally, producing differential movements ofeach floor relative to its neighbours. This is termed inter-storey drift (Figure 12.1), andis accompanied by vertical deformations which involve changes in the clear height hbetween floors and beams.

Any infill panel should be designed to deal with both these movements. This canbe done by either (1) integrating the infill with the structure, or (2) separating the infillfrom the structure. A discussion of both systems of constructing infill panels follows,while further guidance on the aseismic effectiveness of some types of partitions maybe found in Rihal and Granneman (1984), while the need to avoid accidental formationof soft storeys in infilled walls is discussed by Dolsek and Fajfar (2001) and also inSection 8.3.8 (page 265).

(Not to scale)

Drift

Beam

A

B h

Column

1

2

Integratedpanel

Separatedpanel

Figure 12.1 Diagrammatic elevation of structural frame and non-structural infill panels

Non-structural infill panels and partitions 459

12.2.2 Integrating infill panels with the structureIn this case, the panels will be in effective structural contact with the frame such thatthe frame and panels will have equal drift deformations (Panel A in Figure 12.1). Suchpanels must be strong enough (or flexible enough) to absorb this deformation, andthe forces and deformations should be computed properly. Where appreciably rigidmaterials are used the panels should be considered as structural elements in their ownright, as discussed in Sections 5.4.6 and 10.4.5. Reinforcement of integrated rigid wallsis usually necessary if seismic deformations are to be satisfactorily withstood.

Integration of infill and structure is most likely to be successful when very flexiblepartitions are combined with a very stiff structure (with many shear walls). Attentionis drawn to the fact that partitions not located in the plane of a shear wall may besubjected to deformations substantially different from those of the shear wall. This isparticularly true of upper-storey partitions.

Light partitions may be dealt with by detailing them to fail in controlled local areasthus minimizing earthquake repairs to replaceable strips (Figure 12.2).

Finding suitable flexible construction for integral infill may not be easy, especiallyin beam and column frames of normal flexibility. These may experience an inter-storeydrift of as much as 1/100 of the storey height in an earthquake.

12.2.3 Separating infill panels from the structure(See Figure 12.1, Panel B.) For important structural reasons, this method of deal-ing with non-structural infill is likely to be preferable to integral construction whenusing flexible frames in strong earthquake regions. The size of the gap between theinfill panels and the structure is considerably greater than that required in non-seismicconstruction. In the absence of reliable computed structural movement, it is recom-mended that horizontal and vertical movements of between 20 mm and 40 mm shouldbe allowed for. The appropriate amount will depend upon the stiffness of the structure,and the structural engineers advice should be taken on this.

This type of construction has two inherent detailing problems which are notexperienced to the same extent in non-seismic areas. First, awkward details maybe required to ensure lateral stability of the elements against out-of-plane forces.

Replaceable lining

Stru

ctur

al w

all

Seismic movement gap

Column

Figure 12.2 Lightweight partition detailed so that earthquake hammering by the structure willdamage limited end strips only


Secondly, soundproofing and fireproofing of the separation gap is difficult. Moderatesoundproofing of the movement gap can be achieved with cover plates or flexiblesealants, but where stringent fireproofing and sound proofing requirements exist, theseparation of infill panels from the structure is inappropriate. Designers should becareful in the choice of so-called flexible materials in movement gaps; the materialmust be not only sufficiently soft but also permanently soft. Both polysulphide andfoamed polyethylene are not flexible enough (or weak enough) in this situation.

It is in fact difficult to find a suitable material; Mono-Lasto-Meric is both per-manently and sufficiently soft, but is not suitable for gap widths exceeding 20 mm.Foamed polyurethane is probably the best material from a flexibility point of viewand will provide modest sound-insulation, but may have little fire resistance. A fire-resistant possibility is Declon 156, a polyester/polyurethane foam which intumesces infire conditions.

Figures 12.3 to 12.6 show some details used for separated infill panels. Note thatgreat care has to be taken during both detailing and building to prevent the gaps

Sealant

Sealant

Soffit

150 mm

SkirtingFloor

Vertical section

Plan sections

Plasterboard

Cornerreinforcement

Conc

rete

colu

mn Metal

stud

Cornerreinforcement

75 mm

Metaltrim

Attenuationblanket

Stru

ctur

al w

all

Figure 12.3 Light partition details for small seismic movements (i.e. suitable for stiff-framedbuildings or small earthquakes

Metal angles Metal dowelgrouted intohollow block

Flat metal strapnailed in perpend

(a) (b) (c) (d)

Slot A

A

AA

Sealant

Figure 12.4 Separated stiff partitions: top details for lateral stability of brick or block walls(see Section 12.2.2)

Cladding, wall finishes, windows and doors 461

Difficult to keepgap clear ofmortar

Stabilizing buttress

Infill wall

ColumnStructuralwall

Figure 12.5 Separated stiff partition: plan view of stabilizing buttress systems

Plaster bead

Plaster

Figure 12.6 Plastering detail to ensure preservation of gap between partition and structure

being accidentally filled with mortar or plaster. Figure 12.6 shows a detail which helpsprevent plaster bridging the gap. Further details suitable for small seismic movementsmay be found elsewhere (Arnold, 1984; Massey, 1992).

12.2.4 Separating infill panels from intersecting servicesWhere ducts of any type penetrate a full-height partition, the ducts should not be tiedto the partition for support. Support should occur on either side of the partition fromthe building structure above. If the opening is required to be sealed because of fireresistance or acoustics, the sealant should be of a resilient non-combustible type topermit motion of the duct without affecting the partition or duct. It is important forboth seismic and acoustic considerations that the duct be independently supported byhangers and horizontal restraints from the building structure.

Further discussion of ducts is to be found in Section 11.3.6, and for some remarkson the required properties of gap sealants around ducts, see discussion on infill panelsin Section 12.2.3.

12.3 Cladding, Wall Finishes, Windows and Doors

12.3.1 Introduction

The problems involved in providing earthquake-proof details for these items are thesame in principle as those for partitions as discussed in the preceding section. Theirin-plane stiffness renders them liable to damage during the horizontal drift of the build-ing, and the techniques of integral or separated construction must again be logicallyapplied.


BeamGap filled withflexible sealant(section 8.2.3.)Co

lum

n

Spandrel

Gap

Figure 12.7 Detail of external frame showing separation of spandrel from columns to avoidunwanted interaction

12.3.2 Cladding and curtain walls

Precast concrete cladding is discussed in Section 10.3.12. Suffice it here to point outthat in flexible buildings, non-structural precast concrete cladding should be mountedon specially designed fixings which ensure that it is fully separated from horizontaldrift movements of the structure. Brick or other rigid cladding should be either fullyintegral and treated like infill walls (Section 5.4.6), or should be properly separatedwith details similar to those for rigid partitions (Figure 12.4, 12.5) or for spandrelssuch as shown in Figure 12.7.

External curtain walling may well be best dealt with as fully-framed pre-fabricatedstorey-height units mounted on specially-designed fixings capable of dealing with seis-mic movements in a similar way to precast concrete cladding, as mentioned above.

12.3.3 Weather seals

Weather seals that may be damaged in severe earthquakes should be accessible andsuitable for replacement.

12.3.4 Wall finishesBrittle or rigid finishes should be avoided or specially detailed on any walls subjectedto shear deformations, i.e. drift as applied to Panel A, Figure 12.1. This applies tomaterials such as stone facings or most plasters. In Japan it is recommended thatstone facings should not be used on walls where the storey drift is likely to be morethan 1/300.

Brittle veneers such as tiles, glass or stone should not be applied directly to theinside of stairwells, escalators or open wells. If they must be used, they should bemounted on separate stud walls or furrings. Preferably the stairwells should be freeof material which may spall or fall off and thus clog the exit way or cause injury topersons using the area.

Heavy ornamentation such as marble veneers should be avoided in exit lobbies. If aveneer of this type must be used, it should be securely fastened to structural elements

Cladding, wall finishes, windows and doors 463

using appropriate structural fastenings to prevent the veneers from spalling off in theevent of seismic disturbance.

Plaster on separated infill panels must be carefully detailed to prevent its bridgingthe gap between panel and structure (Figure 12.6) as this may defeat the purpose ofthe gap, resulting in damage to the plaster, the infill panel and the structure.

12.3.5 Windows

It is worth observing that in the 1971 San Fernando, California, earthquake, which caused$500 million worth of damage, glass breakage cost more than any other single item.

Window sashes should be separated from frame action except where it can be shownthat no glass breakage will result. If the drift is small, sufficient protection of the glassmay be achieved by windows glazed in soft putty (Figure 12.8), where the minimumclearance c all round between glass and sash is such that

c >w

2[1+ (h/b)] (12.1)

The failure mode of hard putty glazed windows tends to be of the explosive bucklingtype, and should be used only where sashes are fully separated from the structure, forexample when glass is in a panel or frame which is mounted on rockers or rollers asdescribed in Section 10.3.12. Further discussion of window behaviour in earthquakesmay be found elsewhere (Osawa et al., 1965).

12.3.6 Doors

Doors which are vital means of egress, particularly main doors of highly populated andemergency service buildings, should be specially designed to remain functional aftera strong earthquake. For doors on rollers, the problem may not be simply a geometricone dealing with the frame drift , but may also involve the dynamic behaviour ofthe door itself.

c

Beamw

Column

(Drift not to scale)b

c h

Figure 12.8 Detail of external frame with window glazing set in soft putty


12.4 Miscellaneous Architectural Details

12.4.1 Exit requirements

Every consideration should be given to keeping the exit ways clear of obstructions ordebris in the event of an earthquake. As well as the requirements for wall finishes anddoors outlined in Sections 12.3.4 and 12.3.6, the following points should be considered.

Floor covers for seismic joints in corridors should be designed to take three-dimensional movements, i.e. lateral, vertical and longitudinal. Special attention shouldbe given to the lateral movement of the joints.

Free-standing showcases or glass lay-in shelves should not be placed in publicareas, especially near exit doors. Displays in wall-mounted or recessed showcasesshould be tied down so that they cannot come loose and break the glass front duringan earthquake. Where this is impracticable, tempered or laminated safety glass shouldbe used for greater strength.

Pendant-mounted light fixtures should not be used in exit ways. Recessed or surface-mounted independently supported lights are preferred.

12.4.2 Suspended ceilings

In seismic conditions ceilings become potentially lethal. Individual tiles or lumps ofplaster may jar loose from the supports and fall. Ceiling-supported light fixtures mayloosen and drop out, endangering persons below. Thus, alternatives to the standardceiling construction procedures should be considered. A thorough review of the seismichazard from suspended ceilings and detailing recommendations has been given byClarke and Glogau (1979), while studies on dynamic response behaviour have beenmade by Rihal and Granneman (1984).

The horizontal components of seismic forces to which a ceiling may be subjected canbe allowed for in several ways. A dimensional allowance should be made at the ceilingperimeter for this motion so as to minimize damage to the ceiling where it abuts thewalls: one way of doing this is to provide a gap and a sliding cover (Figure 12.9). Someceiling suspension systems need additional horizontal restraints at columns and otherstructural elements, such as diagonal braces to the floor above, in order to minimizeceiling motion in relation to the structural frame. This will reduce hammering damageto the ceiling, and tiles will be less likely to fall out. The suspension system for theceiling should also minimize vertical motion in relation to the structure.

Lighting fixtures which are dependent upon the ceiling system for support shouldbe securely tied to the ceiling grid members. If such support is likely to be inadequatein earthquakes, the light fixtures should be supported independently from the buildingstructure above. Diffuser grilles, if required for the air supply system, should also behung independently.

In seismic areas, a lay-in T-bar system for ceiling construction should be avoidedif at all possible, as its tiles and lighting fixtures drop out in earthquakes. In both the1964 Alaska and the 1971 San Fernando earthquakes, the economical (and thereforepopular) exposed tee grid suspended ceilings suffered the greatest damage. Evidentlythe differential movement between the partitions and the suspended ceilings damaged

References 465

Alternative freeend detail

Cover plate ormoulding

Ceiling

GapRestraint

Structure

Figure 12.9 Details of periphery of suspended ceilings to prevent hammering and exces-sive movement

Q-decktype flooring

Fixture yoke

Box type lightfixture protection

ConcealedZ-spline system

Indirectsuspensionsystem

Figure 12.10 Two details of suspended ceiling construction providing movement restraint andsecure tile fixing (after Berry, 1972)

the suspension systems, and as the earthquake progressed the ceilings started to swayand were battered against the surrounding walls. This damage was aggravated whenthe ceilings supported lighting fixtures, and in many instances the suspension systemswere so badly damaged that the lighting fixtures fell.

Damage to ceilings can also occur where sprinkler heads project below the ceilingtiles. One way of minimizing this problem is to mount the heads with a swivel jointconnection so that the pipe may move with the ceiling. Figures 12.9 and 12.10 givesuggestions for seismic detailing of suspended ceilings.

ReferencesArnold C (1984) Non-structural issues of seismic design and construction. Earthquake

Engineering Research Institute, Oakland, California.


Arnold C (1991) The seismic response of non-structural elements in buildings. Bull NZ Nat SocEarthq Eng 24(4): 30616.

Arnold C and Reitherman R (1982) Building configuration and seismic design. John Wiley &Sons, New York.

Berry DR (1972) Architectural seismic detailing. State of the Art Report No 3, Technical Com-mittee No 12, Architectural-Structural Interaction. IABSE-ASCE Int Conf on Planning andDesign of Tall Buildings, Lehigh University.

Clarke WD and Glogau OA (1979) Suspended ceilings: the seismic hazard and damage problemand some practical solutions. Bull NZ Nat Soc for Earthq Eng 12(4): 292304.

Dolsek M and Fajfar P (2001) Soft storey effects in uniformly infilled reinforced concreteframes. J Earthq Eng 5(1): 112.

Massey W (1992) Architectural design for earthquakeA guide to the design of non-structuralelements. New Zealand Nat Soc Earthq Eng.

Osawa Y, Morishita T and Murakami M (1965) On the damage to window glass in reinforcedconcrete buildings during the earthquake of April 20, 1965. Bull Earthq Res Institute, Uni-versity of Tokyo 43: 81927.

Rihal SS and Granneman G (1984) Experimental investigation of dynamic behaviour of buildingpartitions and suspended ceilings during earthquakes. Proc. 8th World Conf on Earthq Eng,San Francisco V: 113540.

Shipp JG and Johnson MW (1990) Seismic loss estimation for non-structural components inhigh-rise buildings. Proc. 4th US Nat Conf on Earthq Eng. EERI, Oakland, California.

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