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Why are Open-Ground Storey Buildings vulnerable in Earthquakes?

EarthquakeTip 21LearningEarthquake DesignandConstruction

Basic FeaturesReinforced concrete (RC) frame buildings are

becoming increasingly common in urban India. Manysuch buildings constructed in recent times have aspecial feature the ground storey is left open for thepurpose of parking (Figure 1), i.e., columns in theground storey do not have any partition walls (ofeither masonry or RC) between them. Such buildingsare often called open ground storey buildingsor buildingson stilts.

An open ground storey building, having onlycolumns in the ground storey and both partition wallsand columns in the upper storeys, have two distinctcharacteristics, namely:(a) It is relatively flexible in the ground storey, i.e., the

relative horizontal displacement it undergoes in theground storey is much larger than what each of thestoreys above it does. This flexible ground storey isalso called soft storey.

(b) It is relatively weak in ground storey, i.e., the totalhorizontal earthquake force it can carry in theground storey is significantly smaller than whateach of the storeys above it can carry. Thus, theopen ground storey may also be a weak storey.

Often, open ground storey buildings are called softstorey buildings, even though their ground storey maybe soft and weak. Generally, the soft or weak storeyusually exists at the ground storey level, but it couldbe at any other storey level too.

Earthquake BehaviourOpen ground storey buildings have consistently

shown poor performance during past earthquakesacross the world (for example during 1999 Turkey, 1999Taiwan and 2003 Algeria earthquakes); a significantnumber of them have collapsed. A large number ofbuildings with open ground storey have been built inIndia in recent years. For instance, the city of

Ahmedabad alone has about 25,000 five-storeybuildings and about 1,500 eleven-storey buildings;majority of them have open ground storeys. Further, ahuge number of similarly designed and constructedbuildings exist in the various towns and cities situatedin moderate to severe seismic zones (namely III, IVand V) of the country. The collapse of more than ahundred RC frame buildings with open groundstoreys at Ahmedabad (~225km away from epicenter)during the 2001 Bhuj earthquake has emphasised thatsuch buildings are extremely vulnerable underearthquake shaking.

The presence of walls in upper storeys makesthem much stiffer than the open ground storey. Thus,the upper storeys move almost together as a singleblock, and most of the horizontal displacement of thebuilding occurs in the soft ground storey itself. Incommon language, this type of buildings can beexplained as a building on chopsticks. Thus, suchbuildings swing back-and-forth like inverted pendulumsduring earthquake shaking (Figure 2a), and thecolumns in the open ground storey are severely

stressed (Figure 2b). If the columns are weak (do nothave the required strength to resist these high stresses)or if they do not have adequate ductility (See IIT-BMTPC Earthquake Tip 9), they may be severelydamaged (Figure 3a) which may even lead to collapseof the building (Figure 3b).

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Figure 2: Upper storeys of open ground storeybuildings move together as a single blocksuch buildings are like inverted pendulums.

Stiff upper storeys:Small displacement between

adjacent floors

Soft ground storey:

Large displacement betweenfoundation and first floor

Figure 1: Ground storeys of reinforced concrete

buildings are left open to facilitate parkingthis is common in urban areas in India.

Ground storey columns severely stressed(b)

(a)Inverted

Pendulum

Earthquakeoscillations

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IITK-BMTPC Earthquake Tip 21

Why are Open-Ground Storey Buildings vulnerable in Earthquakes? page 2

The ProblemOpen ground storey buildings are inherently poor

systems with sudden drop in stiffness and strength inthe ground storey. In the current practice, stiff masonrywalls (Figure 4a) are neglected and only bare frames areconsidered in design calculations (Figure 4b). Thus,the inverted pendulum effect is not captured in design.

Improved design strategiesAfter the collapses of RC buildings in 2001 Bhuj

earthquake, the Indian Seismic Code IS:1893 (Part 1) -2002 has included special design provisions related tosoft storey buildings. Firstly, it specifies when abuilding should be considered as a soft and a weakstorey building. Secondly, it specifies higher designforces for the soft storey as compared to the rest of the

structure. The Code suggests that the forces in thecolumns, beams and shear walls (if any) under theaction of seismic loads specified in the code, may beobtained by considering the bare frame building(without any infills) (Figure 4b). However, beams andcolumns in the open ground storey are required to be

designed for 2.5 times the forces obtained from thisbare frame analysis.

For all new RC frame buildings, the best option is toavoid such sudden and large decrease in stiffnessand/or strength in any storey; it would be ideal tobuild walls (either masonry or RC walls) in the groundstorey also (Figure 5). Designers can avoid dangerouseffects of flexible and weak ground storeys byensuring that too many walls are not discontinued inthe ground storey, i.e., the drop in stiffness andstrength in the ground storey level is not abrupt due tothe absence of infill walls.

The existing open ground storey buildingsneed to bestrengthened suitably so as to prevent them fromcollapsing during strong earthquake shaking. Theowners should seek the services of qualified structuralengineers who are able to suggest appropriatesolutions to increase seismic safety of these buildings.

Related - Earthquake TipTip 6: How Architectural Features Affect Buildings During

Earthquakes?Tip17: What are the Earthquake Effects on Reinforced Concrete

Buildings?

Reading MaterialIS 1893(Part 1) (2002), Indian Standard Code of Practice for Criteria for

Design of Earthquake Resistant Structures, Bureau of IndianStandards, New Delhi

This release is a property of IIT Kanpur and BMTPC NewDelhi. It may be reproduced without changing its contentsand with due acknowledgement. Suggestions/commentsmay be sent to: nicee@iitk.ac.in. Visit www.nicee.org orwww.bmtpc.org, to see previous IITK-BMTPC Earthquake Tips.

Authored by:C.V.R.MurtyIndian Institute of Technology KanpurKanpur, India

Sponsored by:Building Materials and Technology Promotion

Council, New Delhi, India

Figure 3: Consequences of open ground s toreysin RC frame buildings severe damage toground storey columns and building collapses.

(a) 1971 San Fernando Earthquake

Photo Courtesy: The EERI Annotated Slide Set CD, EarthquakeEngineering Research Institute, Oakland (CA), USA, 1998.

(a) Actual bui ld ing Building being assumedin current design practice

Figure 4: Open ground storey building assumptions made in current design practiceare not consistent with the actual structure.

Infill walls notconsidered inupper storeys

(b)

Figure 5: Avoiding open ground storey problem continuity of walls in ground storey is preferred.

Direct flow of forcesthrough walls

(b) 2001 Bhuj Earthquake

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