STUDY OF MODES OF STRUCTURAL FAILURE DUE TO EARTHQUAKE

Shaukat Ali Khan*, University of Engineering & Technology Taxila, Pakistan

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32nd Conference on OUR WORLD IN CONCRETE & STRUCTURES: 28 – 29 August 2007, Singapore

STUDY OF MODES OF STRUCTURAL FAILUREDUE TO EARTHQUAKE

Shaukat Ali Khan*, University of Engineering & Technology Taxila, Pakistan.

Abstract

A devastating earthquake measuring 7.6 on Richter scale hit Pakistan at 8:50AM on October 08, 2005. The earthquake was unprecedented in the history of thecountry and caused very severe damages to the structures since they were notdesigned for that intensity. Its epicenter was reported about 95 kilometers north-west ofthe capital Islamabad. The main cities and towns of Pakistan seriously affected includeMuzaffarabad, Bagh, Rawalakot, Shenkari, Mansehra, Butgram, Balakot, GariHabibullah. A part of the Margalla towers Islamabad collapsed while many otherbuildings faced minor damages. The structures having major and minor damagesinclude brick-masonry, stone-masonry, block-masonry and RCC frame structures withbrick-masonry cladding as well as block-masonry cladding. The affected buildingsinclude single storey as well as multistoried structures. In hilly areas the buildingsgenerally have roof trusses.A study of damages to different structures is very important to adopt strategies forfuture construction and to develop design codes so that the earthquake resistantstructures could be designed. In this paper a study of damages mainly for the brick-masonry and RCC frame structures with block-masonry cladding are discussed.

Keywords: Block Masonry, Stone Masonry, Cladding

1. IntroductionEarthquakes mostly cause damage due to ground shaking. The shaking may be the result of tectonicmovement or volcanic activity. The surface of the earth consists of seven major tectonic plates and manysmaller ones. These plates move in different directions and at different speeds from those of theneighboring ones. The most devastating event recorded in the recent history of Pakistan on 8

thOctober

2005 is the result of the stress releases in the tectonically active zone due to collision of Indian plate withthe Eurasian plate. The major cities and towns affected were Muzaffarabad, Bagh, Rawalakot, Balakot,Shinkiari, Batagram, Mansehra, Abbottabad, Murree, Rawalpindi and Islamabad. [1]The area hit by the earthquake has to face serious loss of life and property depending upon theearthquake intensity and location of the epicenter. The main cause of loss of life is the collapse ofstructures. The affected areas have variety of construction e.g., load bearing masonry construction, framestructure, single storey dwellings or multistoried residential apartments, commercial or official buildings. Indeveloping country like Pakistan, the private houses are usually not properly designed and constructedwhile the multistoried commercial or official buildings are properly designed and constructed following theprevailing design codes and construction specifications.

Catastrophic natural disasters result in a major loss of life and tremendous amount of damage. Thevulnerability of society for these losses tends to rise by increasing population and investments in hazardprone areas, less resilience against disaster, changed land use and a higher dependency on vulnerablecommunication and services [2].In this earthquake about 80,000 persons died, and an equal number severely injured or disabled and over2.8 million persons have been left without shelter [3]. In recent events, earthquakes have also causedgreat damage and destruction of Mosques and other monumental buildings [4, 5, 6].The inherent compulsions do not allow shifting from the earthquake prone areas. Rather the man has theGod gifted qualities to fight the hazards and develop strategies and systems for safety. To developpositive future safety strategies, studies of the failure modes and mechanism are very important. Thispaper is dedicated to the study of failure modes for brick-masonry and RCC frame structures with blockand brick-masonry infill walls. The study area was mainly confined to AJK-University Muzaffarabad.

2. Site LithologyThe campus is located on a fairly narrow terrace at the foot of high mountains. The terrace ends with asteep slope into the river Neelum at a significant depth. The strata mainly consist of gravels (rounded tosub-rounded) with very little binder (fines). Serious sliding activity has been observed on all thesurrounding mountains standing around the campus. The dust at the slip-scar as shown in figures-1indicates active creep of the mountains. The slide may be deep seated and the entire terrace supportingthe campus buildings may be unstable.

3. BuildingsThe campus comprises of several newly constructed departments and residences. A brief of the buildingsis as follows:

The first building was completed in 1990 and the rest of the buildings are still younger. The departments mostly have double storey frame structure with brick-masonry cladding. The Main Boys Hostel comprises of double and triple storey wings. It is a frame structure with

hollow block-masonry cladding. The residences mostly have double storey load bearing brick-masonry structures.

4. Modes of FailureModes and degrees of damage varied among the buildings. The intensity of damage to all the buildings isquite severe and most of the buildings are none repairable and ultimately have to be demolished.However for the buildings (although quite small in number) with smaller degree of distress, detailed surveyneed to be conducted to decide the mode and extent of repair but definitely keeping in view the seismicityof the area. Different modes of failure are discussed as follows:

4.1 Failure due to Foundation SettlementThe main cause of failure for most of the buildings is the differential foundation settlement. Thedifferential settlement caused serious cracking of the walls, sagging of the floors and distress to thestructural frame due to development of additional moments/stresses. The administration block wasamong the buildings severely affected due to differential settlement. The administration block is adouble storey frame structure building with brick masonry cladding. The building is located right atthe foot of a hill.The administration block has two wings, with a front wing parallel and the second wing normal to theapproach road. The back portion of the second wing is expected to be supported on the rock/hardstratum, resulting in none to very small settlement. While the front wing, resting on a layer ofoverburden soil settled more. Serious cracking due to differential settlement and folding of the stepsof staircase connecting the two wings was observed.Shear movement of the porch column near was observed. The floor of the entrance lobby, probablysupported on a fill showed significant sagging. The intensity of damage due to differential settlementfor different buildings is shown in figures 2-10.

4.2 Shear Movement of the Structural ColumnsQuite large shear movement of the structural columns of the triple storey wing of the boys-hostel wasobserved. The hostel comprises of frame structure with hollow blocks masonry cladding. The hostelhas a double storey front portion and a triple storey back portion. Partial and total collapse of blockmasonry was observed at most of the points. The front double storey portion did not show majorsigns of distress to the structural frame. But the triple storey portion showed serious structural failuredue to shear movement of the ground floor columns. The modes of failure are shown in figures 11-15.

4.3 Shear Movement of the Column due to Cold JointShear movement of the top floor column of an under-construction triple storey building was observed.A triple storey frame structure building with brick masonry cladding was under construction. All thewalls were still without plaster. The columns for the third floor without roof top were projecting abovethe roof of the second floor. Shear movement of the column mainly due to cold joint was observedand is shown in the figures 16-17.4.4 Collapse of Block-Masonry CladdingTotal and partial collapse of the Block-masonry at many locations was observed as shown in figure18.

4.5 Alligator Cracking of the Brick-MasonrySerious alligator cracking was observed in the brick masonry walls of the campus mosque, mainlydue to greater wall length and height and lack of bond with the RCC structural frame. The mosquehas a single storey frame structure with brick masonry cladding. The spans are quite long with acomparatively higher ceiling level. The relatively long and high walls (9-inches thick) without lateralmembers (walls) badly cracked and a portion even collapsed. The buckling of main reinforcement atthe beam-column joint due to poor quality of construction (i.e., stirrups missing) was clearly visible.The walls supporting the extended Mehrab portion failed leaving the extended slab of the Mehrab inhanging position. The failure modes are shown in figures 19-20.

4.6. Buckling of main Reinforcement of ColumnsThe buckling of longitudinal column reinforcement due to lack of stirrups was observed as shown infigure 21.

5. Discussion1. All the buildings (except the mosque and the administration block) were locked and the

observations were made only from the exterior surfaces of the buildings.2. The intensity of damage observed, includes the effect of the main earthquake of October 08, 2005,

and also the after-shocks before the date of survey i.e., 24/10/2005.3. Scores of after-shocks of moderate to high intensity hit the area even after the date of survey,

which might have further aggravated the conditions (e.g., widening of the previous cracks anddevelopment of new cracks).

4. The buildings inspected were single, double and triple storey. Single storey buildings showed onlyminor damages and are repairable. The major damages to double and triple storey buildings aremainly due to localized weaknesses resulting from poor quality of construction.

5. Differential settlements are mainly the result of lack of vigilance of the supervisory staff in placingthe foundations. Uniform foundation design has been adopted for non-uniform soil support.Differential settlement might have been controlled if minor adjustment in foundation depth and sizewould have been adopted according to the actual soil conditions met during foundationexcavations.

6. Recommendations1. Most of the buildings, which were surveyed can’t be repaired and may have to be demolished.2. For repairable building, detailed survey should be made to decide the mode and extent of repair.3. For the repaired building it should be kept in mind that after repair the buildings are safe and stable

on long term basis considering the present seismic conditions of the area.4. The area seems to be geologically active. Detailed geological survey for stability of the area is

recommended before taking any decision what so ever regarding the campus.

7. Conclusions1. In hilly areas soil investigation for each building should be made separately with specific

recommendations to control the differential settlement.2. Most of the failures have been initiated due to weak zones resulting from poor quality of

construction.3. Shear failure of columns, only along a single plane, indicates that there exist some relationship

between the direction of earthquake movement and the plane of shear movement. Possibly theplane of shear movement is parallel to the direction of earthquake movements.

4. Double storey load bearing brick-masonry residences did not show any prominent sign of failure.

5. Conventional shape of the hollow blocks does not provide adequate strength of masonry bonds,reshaping is recommended.

6. Block masonry with the conventional shape of hollow blocks has low Earthquake resistance andshould be avoided.

7. Highest possible quality control is required for construction in seismic zones.8. Brick masonry performed better than block masonry.9. Cladding walls should be connected to the structural frame at adequate intervals for better

resistance against earthquake shaking.

8. References[1] EERI Special earthquake report December 2005.[2] J.G. Knoeff and G.J. Akkerman “Geotechnical contribution to risk reduction of natural hazards”PP-281[3] Report of Asian Development Bank and World Bank Islamabad, Pakistan, November 12, 2005.[4] Croci, G-1998, “The conservation and structural restoration of architectural heritage, computationalmechanics publication, 1998.[5] Croci, G-1998, “The collapse occurred in the basilica of St. Francis of Assisi and Cathedral of Noto,structural analysis of historical constructions-II, (Eds Roca, Gonzalez, Ofiate, Lourenco), CIMNE,Barcelona, 1998, PP 297-317.[6] Macchi, G-1998, “Seismic risk and dynamic identification in towers-A keynote lecture, ProcMonument-98, workshop on seismic performance of Monuments”, Lisbon, Portugal, 1998, K3-K17.

Fig: 1 Slip scar in the mountains surroundingthe campus (Dust is indicating that slide isstill active/creeping)

Fig: 2 Shear movement of the column nearplinth level (Administration block porch)

Fig: 3 Sagging of the lobby floor(Administration block)

Fig: 4 Cracks due to non-uniform foundationsettlement (Administration block)

Fig: 5 Cracks due to non-uniform foundationsettlement (Administration block)

Fig: 6 Cracks due to non-uniform foundationsettlement (Administration block)

Fig: 7 Differential movement of the landing ofstaircase, main-reinforcement is exposed

Fig: 8 Cracking of the masonry column due tonon-uniform settlement at the entrance of theDepartment of Zoology

Fig: 9 Cracking of the masonry column due tonon-uniform settlement at the entrance of theDepartment of Chemistry

Fig: 10 Cracking of the staircase due todifferential settlement at the entrance to theDepartment of Chemistry

Fig: 11 Shear movement of structuralcolumn and collapse of block masonry(Boys Hostel)

Fig: 12 Failure of the beam – column joint(Boys Hostel)

Fig: 13 Large shear movement of the groundfloor column due to poor reinforcementdetailing (Boys Hostel)

Fig: 14 Large shear movement of the groundfloor column (Boys Hostel)

Fig: 16 Shear movement of column due to coldjoint (Under Construction Students Hostel)

Fig: 15 Large shear movement of the groundfloor column due to poor reinforcementdetailing and weaker concrete (Boys Hostel)

Fig: 18 Failure of block masonry walls ofEnglish Department

Fig: 17 Shear movement of column due to coldjoint (Under Construction Students Hostel)

Fig: 19 Failure of brick masonry and hangingMehrab slab (Campus Mosque)

Fig: 20 Collapse of the back wall and debris(Campus Mosque)

Fig: 21 Buckling of the main column reinforcementdue to lack of stirrups (Campus Mosque)