ANALYSI S OF EARTHQUAKE DAMAGE TO BRICK STRUCTURES

0 IAN PEI-FENG Professor Department of Architecture En g ineering, Be ijing Institute of Architecture Engineeri ng, Beijing, Ch ina .

";1:: -di.

QI AN DA Lect ur er Department of Architect ure En g ineering, Yunnan Institute of Technology , Kunming, Yunnan Provinc e , China .

ABSTRACT For many years the traditional viewpoint assumed t hat horizontal seismic force dominates in the dama g ing effect oE earthquakes on structures . We have ana l yzed earthquake damage of various kind s oE structures, ca used by ma ny strong earthquakes in our country and discover that t he pract ical vert i ca l seismic force is about ten, and severa l times bigger than the de fined value of our country ' s eart hqu a ke proof design code , so it plays the leading role . In this pa per, we a nalyze again several ea rthqu a ke damage phenomena which are genera ll y seen in brick chimneys , houses and cylindr i ca l wate r towers, to identif y the above viewpoin t , a nd to explain the earthquake damage .

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1. FOREWORD

The traditiona l viewpoint considers that hori zonta l seismic force pl ays the lcadin g role for st ructures damage . But the first author of this paper po int ed out in 1957 after he had studi ed brick chimne y ' s earthquake damage ?henomenon, and identified further through continuous analyses of earthquake damage of various types of structures in our count ry. He concluded that practica lly it is the vertical seismic force whic h plays the leading role for earthquakes , because this traditional viewpoint shows significant contrad i ction with real eart hquake damages both qualitatively and quantitatively . In the last 7 or 8 years, 20 more pap e rs and one book have been published . In referen ce (1), we analyzroma ny things and per sons being thrown up, transformer weighing more than 100 tons and train jumped off its rails in area of 8 or 9 earthquake in te nsity degree s , while furnlture and stone blocks dislocated in a n area of 6 or 7 degrees . We conclude Erom t hese facts t hat :

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(1)

where, Kv and Kh are the ratios of vertical and horizontal accelerations of the ground surface with gravitational acceleration respectively, K is

v the vertical dynamic coefficient.

According to the earthquake proof code of our country, 7 degrees earthquake intensity which increases to half decreases one degree.

K is 0.1 for area of varue as the intensity

We have analyzed earthquake damage of some tall slender structures, the results show that formula (1) is approximately convincible (5). Now we will prove again formula (1) by studying the exerted force relations in such ordinary stepwise slant joints, horizontal joints and dislocations of brick houses, chimneys and cylindrical water towers. Although to deduce destructive force inversely from earthquake damage can only make comparatively simple qualitative analysis, yet according to later analyzed results which give explanation to other earthquake damage situations it is more rational than the traditional viewpoint.

2. ANALYSIS OF THE THREE OFTEN SEEN EARTHQUAKE DAMAGE PHENOMENA

2.1 Stepped Slant Joints

An isotropic elastic body under action of pure shear, compressive shear and tensile shear, the destroyed section caused by principal tensile stress makinÔ angle with the compressive and tensile stresses exerted sections should be 45 , more or less than 45

0• As for brick mortar blocks, the strength of mortar

joints is lower than bricks, the strength of vertical joints is lower than horizontal joints, thus in case of sma1l exerted compressive or tensile force, under the action of compressive shear or tensile shear, stepped slant cracks may occur along mortar joints. The codes of alI countries define the vertical seismic force as smaller than the weight of structure in areas of nine or lower earthquake intensity degrees. Thus all should be under compressive shear condition and one always sees gently sloped slant cracks in seven to e ight degrees earthquake intensity areas. Furthermore, according to the ana1ysis of 14 brick cylindrical water towers (5), many brick chimneys in (4) and experimental results of three five-storey full sized model buildings of rnedium building blocks in Lanzhou, Shanghai and Hongzhou under action of earthquake, alI these structures should only be damaged by bending destruction, the dangerous section being at the bottom. But our investigations of numerous earthquake damages of brick chimneys show that they are destroyed at the middle to upper portions with many slant cracks and dislocations (Table 1). Except for one which had collapsed thoroughly with indiscriminating conditions, none of the 14 water towers had been destroyed at the bottom. They also showed many dislocations and slant cracks (Table 2). Slant cracks are the main damage phenomena in brick houses, where we see no damage caus e d by bending moment at the bottom. The above analysis show that although stepped slant cracks may be produced under action of compr e ssive or t e nsile shear, the damage must be decided by bending moment if horizontal seismic force plays the leading role. Moreover, there must be neither slant crack nor genr:ly sloped slant crack.

2.2 Horizontal Cracks

In brick houses, chimneys and cylindrical water towers (horizontal cracks) always appear in areas of 7-8 earthquake int e nsity degrees. Horizontal cracks are always penetrated thoroughly a t chimne ys and water towe rs. But for brick

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houses, horizontal cracks a lways appear in window stoo ls at projections of a roof or under the roof covers, anà they also penetrate thoroughly. Also some horizontal cracks appear at lower floors or other floors but few form thorough cracks . Whil e horizontal cracks at chimneys and water towers may be explained by bending moment yielding tensile stress, the following contradictions existo Cracks seldom exist at the bottom portion which is the most critical part to bending moment, but appear at the top portion of chimneys and roofs of brick houses where the bending moment is low. Chimneys and water towers alI possess poor stability capacity, if bending moment makes them fractures as the bending rigidity drops rapidly, and the P- effect is also fairly big. They are liable to falI down on one side. But our investigations of mor e than 100 chimneys with collapsed tops show fundamentally that brick pieces falI spreadingly around them. Collapsed brick cylindrical water towers a lso having their tanks falI down near their foundations even with a part pressed on foundations. Some brick c himneys have many horizontal cracks at the middle upper portion with l east distance less than 1m. At the Tangshan earthquake, bricks aro und the round cracks at chimneys had be e n smashed and fallen down, yet they stood still at their original vertical positions without inclincation or falling down (Photo 1).

2 . 3 Horizontal Dislocation

It i s seen from Table 1 and 2 that brick chimneys a nd cylindrical water towers beg in to ap pear horiz ontal disl oca tion situation in area of 7 and 8 degrees respective ly. Under action of e ight deg rees intensity earthq uake , (in factory buildings ) of one storey brick columns, horizont a l dislocation a lso a l ways appear ( 2 ) and (3) . Multi-storey brick buildings wo uld have slight horizontal dislocation at their upper stories even in area of seven degrees (3). There are three to four horizontal dislocation cracks at some brick chimneys, the dislocation is serious , in fact very dangerous. According to discussion on slant cracks this phenomenon should belong to tensil e shear dama ge , and vertical seismic force must be much greater than the grav itational pre ssure. Thus we see there is not any exaggeration in formulá ( 1 ) . Vertical seismic force exerted on multi-storey frame struc tures had been computed in reference (7) by put in strong earthquakes records and in (6) according to reaction spectrum theory, the results all show that the force exerte d i s bigger on top stories than bottom ones . Analysis of prima ry wave stress in c himneys in refere nce (4) a l so shows tha t the effect at the top is bigger than at the bottom . So there is no pec uliarity to explain why brick chimneys and building are a lways damaged at upper portion witho ut being seldom destroyed at the lower portion.

The analysis in reference (8) for earthquake damages of inverted perdulum structures s hows the rea l horizontal seism ic force is far less than assumed by traditional viewpoint one . It i s at the most 17% of our country's present code . Structurcs being pull e d apart as the vertical seismic force reaches its maximum va lue with no possibility that the horizontal seismic forc e also reaches i ts maximum value at the same time, if the l atter is very small or the vertical seismic force is not very large or exerts transiently, there may occur only horizon ta l cracks. Shear strength of mason blocks depends on the value o f compressive stress, when the compressive stress approaches zero or becomes tensile stress , the shear strength decre ases rapidly or vanishes thoro ugh ly. So in the top part of c himne ys and top floors of building s bearing very large vert i cal seismic force, small shear force.

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3. ANALY SIS FOR OTHER EARTHQUAKE DAMAGE PHENOMENA

3 . 1 Verti ca l Cra c ks

The r e a r e a lwa ys ve r t ical c r a cks appearing at brick c himne ys , hou ses a nd cy lindrica l wa t e r towe r s . In Table 2 , among s i x wate r t owers there are three with multi ver ti ca l c ra cks . (Photo 2) . The bri c k cy lind rica l wate r towe rs i nves tigat e d and di sc us sed in r efe r e nce (9) al so prese nt twen t y verti cal c r ac k s . Hori zontal se ismi c f orc e canno t i nduce ve rt ica l c r ac ks . Acco rding to (1), the max imum pre s s u re ma y at t ain ove r 3.4 times t he g r av itationa l press ur e as a s truct ure und e r t he acti on o f se i smi c for ce in nine degree s a r e a, t hu s i t causes mas on blo c ks to p ro duc e h uge l ate ral tensil e for c e to f orm ve rti ca l c ra c ks .

3 . 2 Bricks Smashe d a nd Pee led off

In a r ea of e i ght o r nine deg re es t he r e a l ways a pp ea r s br i c ks off at wall s unde r t he tru sse s of ordinary br ick bu i ld i ngs . These a re ev identl y induc ed by v e rtica l se ismic f orce .

3 . 3 Br ick Pi ec es or Members of St ruct ures Bei ng Th rown Away

smas he d o r pee led (4), ( 5 ) .

This phe nome non oc curs fr e quentl y a nd t wo exampl e s are g i ve n . During t he Tongha i earthquake in 1970 , a house in nine degrees area th rew the soi l bl ocks on to p o f i ts wall onto the roo f of anothe r hous e wi t h the same he i ght . Anot her t hree s t o r ey hou se in t he same a r ea tosse d its fo ur co lumn s on t he g r ound beyo nd the wa ll , the othe r two be ing t osse d ha l f and pre s se d on the wal l (the wa ll did no t f a lI down ) . ( 2) , (5) . Their ve rti ca l a cce l e rati on is many t imes b i gger t ha n th e hor izo nt al one .

TABLE 1 . Ma teri a ls of ea rthqua ke damage s o f some bri c k ch i mn e ys

7 8 9

Hori zon ta l cracks 37 11 31

Hor izont a l di s l ocat ion 25 6 5

Sl ant c r ac ks 22 3 9

Sma she d l.Jro ke n 3 2 O

To rsi ona l turning 3 1 3

Ve rti ca l c ra c k s 4 2 1

Undamaged 26 O

Note : D is the ear thqua ke in te ns it y

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: I

:

:

:

1

I I

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Photo 1. Photo 2 .

3 . 4 Wa ll Returns

During ea rthqua ke s, tops at outer wa lls turning co rner s of many brick houses frequently present ho rizont a l V- c r ac ks or even put out piecewi se (3) . Unde r a c tio n of horizonta l seismic force, bending mome nt and shear force are a li very small, so the traditional v iewp oi nt cannot give a sa tis fact ory e xplanat i on . We consider this i s du e to act ion of vertical seismic force whi ch serio u s l y dec r eases the shear strengt h of mortared bodi es eve n ca us es it to be l ost , thus to r s i on v ibrati on causes t o ps a t t he out er wa ll tur nings prod ucing bi gge r centri f uga I force a nd deserves the damage .

o

9

9

9

9

9

9

8

8

8

7

7

7

7

7

TABLE 2 .

,

H (m)

I 14

20

20

16

14

12 . 8

8

111. 5 ; ;

I 25 i i

i 18

I I 18 . 5 I

I 5 . 0 1

/11. 5

Y (m)

5 . 0 2·. O

2 . 0 1. 2

10 . 0 2 . 7

3 . 5 1. 6

4 . 0

1. 8

5 . 0 1. 2

1.7

7. O

Earthquake da mage distribution of bri c k cy lind rica l water towers .

(H : he ight o f cy lindric a l body , Y: he ight of the sect ion s .

damaged s itua tion

r ound c r acks hori zont a l disl ocat i on 2 cm

two vertical cracks

horizontal s lan t c rdcks 8 ve rtical c racks

r o und cracks i round cracks ho rizont a l c rack 2/3 c ircle \ many vertical crack~

horizont a l dis l ocatio n 5 cm round c r acks

hor i zo n tal dis l ocatio n 3 cm

a lI body smas hed broken

disl ocat i on of horizonta l s l a nt crack

! r ound cracks horiz ontal dislocation 5 cm ,

I

i fall e n down i

I I

i un damage d i I 1 r o"und cracks i I horizontal cracks

undamaged ,

slightly broken

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4. REFERENCES

(1) Qian Peifeng, On computations for certa in parameters of vertic a l earthquake force on engineering structures, ACTA Sinica, No. 4, 1983 (in Chinese with English abstract).

(2) Earthquake damages of the Haicheng Earthquake, Published by Seismological Publishing House, 1979 (in Chinese).

(3) Selected Compilation of the Tang-sheng Earthquakes, Earthquake Proof Investigational Conclusion Materiais, Chinese Architecture Engineering Publishing House, 1977 (in Chinese).

(4) Peifeng, Q., et ai, Vertical Earthquake load on chimneys, Proc. 7th WCEE, 1980.

(5) Qian Da, Q ian Peifeng, Analysis of the vertical earthquake force and the seismic hazard of the multi-layered frame words. J. Seis. Res., Vol. 7, No. 1, 1984 (in Chinese with English abstract).

(6) Qian Peifeng, Analysis of Structures Earthquake Proof, Seismological Publishing House, 1983 (in Chinese).

(7) James C. Anderson, Vitelmo V. Bertero, Effect of gravity loads and vertical ground acceleration on the seismic response of multi-storey frames, Proc., 5th WCEE, 1973.

(8) Qian Peifeng, To calculate horizontal and vertical seismic force inversely according to earthquake damages. To be published soon by Journal of Beijing Architecture Engineering Institute (in Chinese with English abstract).

(9) C heng Chaifif, Investigation and analysis of brick shells earthquake damages, Earthquake Engineering, Trend, 1982, (in Chinese).

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