Dynamic strains in concrete and masonry walls

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Dynamic strains in concrete and masonry walls

C A N A D A

I I

I

1

1

₁₉₆₅

.

.

.

XN

CONCRETE

AND MASONRY WALES

DYNAMIC STRAINS

IN

by

R.

H.S.

In previous work ( 1 , 2) it has been shown that thc particle

velocity of _{basement walls} is _{the moat useful indicator of the onset}

of damage due to blasting operations.

A n

measure strains in the walls under study, but there w e r e not enough

results available at that time to provide a satisfactory correlation

between etrain and the other quantities being studied. The object of

the work described here was to obtain more information on the

relation between the peak particle velocity, the peak particle

acceleration and the maximum strain in the walls of a rectangular

tank, in order to learn more about the type of deformations developed

during blasting vibrations.

The results w e r e obtained by uaing a rectangular tank filled

with sand. A general view of the tank, looking westward, i s shown

in Figure 1. The external dimensions of the tank w e r e 8 x 8 f t in plan

and 6 _{f t high. The foundation of the} tank sat: on solid rock, and the four vertical walls were of different types of construction;

8-111.

the north wall,

9-in.

south wall and 1 0-in. block for the west wall.

The tank w a s filled with sand so that small charges of dynamite

could b e _{exploded in the body of the sand.} In this way it was possible

to measure the effect of the explosion on the four walls of the tank,

Vlbsation measurements w e r e made using three types of transducer,

which w e r e connected through.D.C. ampl3fiera to galvanometer^ fn _a multi-channel recording oscillograph. Particle velocity was measured

with velocity pickups (MB Type 1201, acceleration with Statham

accelerometers (Type A501), and strain with resistance strain gauges

of 6-in. gauge length. The strains measured were in all cases those

occurring in the outer (free) surface of the wall.

Different levels of vibration w e r e obtained by varying the

location a n d size of the charge, which consisted of ' F o r c i t e ' ' 40 per cent

explosive. The vibration measurements w e r e made at the centre of

each wall, A s a consequence of the previous work, the measurements

of velocity and acceleration w e r e taken only in the longitudinal direction

of the blast. Figure 2 _{shows the instrumentation of the}

_9-in.

The charge was placed

in

hole, which was generally

3

cavities in the sand w e r e eliminated b y pushing rods into the sand

a n d reconsolidating the material.

EXPERIMENTAL RESULTS

One set of records taken on the ?-in. concrete wall is

shown

i

n

the concrete tank walls. Figure 4 shows one s e t of measurements

taken on the 8-in. block wall. In the c a s e of the block walls,

measurements of strain were taken both on the block and across

block joints

.

The complete set of results for this work is given in

Figures 5 t o 11. Figures 5 and

6

velocity and strain and acceleration and strain for the 9-in. concrete

wall, and Figures 7 and 8 show the same factors f o r the 7-In. concrete

wall. F i g u r e s

4

strain relation, for the 8 -in. _{block ' m a l l . Only a few results w e r e}

obtained f o r the I 0 -in. block wall and these are shown in Figure I I. The results show the relation b e t w e e n velocity and strain across block

joints.

Figures 5 and 7 indicate that t h e r e is a good c o r r e l a t i o n

between peak longitudinal velocity and maximum s t r a i n for the concrete w a l l s . F i g u r e

6

a c c e l e r a t i o n a n d maximum strain in the c a s e of the 9-in. concrete

w,all. F r o m the velocity-strain results for the concrete walls it

appears that for a given velocity the strain in the 9-in. wall is slightly

smaller than that in the 7-in. wall. The discrepancy between the

strains in _{the two walls} decreases as the velocity i n c r e a s e s . Thus at

1 in. / s e c the discrepancy is around 17 p e r cent,at 3 in. / s e c it is 11

p e r cent,and at 10 in./sec the strains a r e almost identical.

The results for the 8-in. bloclc wall show that there i s

probably a slightly better correlation between velocity and s t r a i n than a c c e l e r a t i o n and strain. Nevertheless, the scatter for both s e t s of

results is far greater than is the case f o r the concrete walls. It is

interesting t o n o t e that, f o r a given velocity, the strain across the

block joints is n e a r l y ten times that on the block. Another interesting

feature of the results for the 8 - i n , block wall is that f o r a given

velocity the strains in the block i t s e l f a r e of the same order as the

CONCLUSIONS

In this

between particle velocity and strain.

_It

given velocity the strain is nearly independent of the type

of

construction. It seems, therefore, that strain could be used as

an i n d e x af damage to basement walls due t o blasting.

U

_a

velocity of 3 i n . / s e c is assumed to indicate the threshold of damage

to basement walls, then

the

in

of 35 t o 40 p in./in.

The correlation between acceleration and strain was also

good. Zn this case, however, the acceleration f o r a given strain

seems t o depend on the type of construction. Thus, i f

the

3 5 p in./ln. is taken as _{an index f o r the} threshold of d a m a g e , the

axperirnental r e sults indicate that the corresponding acceleration in

concrete walls will be 10 g and in block walls it will.

be

There is n o t enough information available t o predict the effect

of

of

brick

can develop in tension due to static loading is in the o r d e r of 100 p

in._{/in. Values of tensile strain f a r brick or block construction do not}

seem t o have been published, but typfcal values of _{the modulus of}

rupture range between

PO

The block walls always failed along mortar joints, due to the

failure of

the

velocity at which this _{failure took place w a s around 3 in. /sec. At}

this velocity level, the strains measured on indlufdual blocks w e r e

around 30

12

the strain an the blocks f a r a 1 velocity levels. T h i s indicates that a

block wall does not p e r f o r m as a monolithic body under the action of

lateral dynamic f o r c e s

,

between indfvidual blocks.

The concrete walls showed no visible signs of damage until

the particle velocity approached 10 i n . / s e c .

At

w e r e in the o r d e r of 100 p i n . / i n . , the strain at which concrete will

begin to crack in a static loading t e s t . The failure of the concrete w a l l s

was sudden, and consisted of large cracks originating at

the

a£ two walls. The way in which the concrete walls performed would

indicate that they w e r e well constructed and acted as monolithic

The results seem to demonstrate that concrete

walls

a

block

It

should

i

n

experimental tank w e r e probably of superior construction compared

with

walls.

_this

with the observations shown in Figure 7 of Reference 2. These it was

reported that minor damage, e . g . the opening or extension of old

cracks, began at about 3 i n . / s e c ; major damage, such as large cracks

in previously sound concrete, began at about 10 i n . / s e c . Thus,

the

results of

the

Previous studies have shown that

thick

behaved similarly t o concrete walls, but there is no such comparative

information for block walls. The present study indicates that 3 h . / s e c

is not a safe estimate of t h e threshold level f o r the onset

of

to block walls; a better estimate of

this

/set.

REFERENCES

1. Edwards, A .

T

.

T.

D.

of

Vol. 210, Sept. 1960, p. 5 3 8 - 5 4 6 .

2 . _Northwood,

_T.

_D.

_,

_R.

_T.

vibrations and building damage. The Engineer, VoP. 21 5 ,

F I G U R E

3

4 6 p in.

/in.

t-i

+

F I G U R E 4

Strain on Block

1 th

-

10

_sec.

*

S

trai

n

Across

Block

Joint

Longitudinal

Velocity of

Block

Longtudinal

Acceleration

of

Block

O N E

S E T

OF

MEASUREMENTS

T A K E N O N T H E 8"

B L O C K

W A L L

/'

1

1

I

I

I 1 1 1 1

I

F

I I

I

T a n k

R e s u l t s

for

E a s t

W a l l

19"

C o n c r e t e )

-

-

_o

_{R e s u l t s}

_{f r o m}

_{2 8 t h}

_{S e p t}

₁₉₆₄

-

3019164

-

i

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

F I G U R E

5

L O N G I T U D I N A L V E L O C I T Y A N D S T R A I N FOR

9"

W A L L

roo

₁

_{I I}

_I

_I

_I

_{I I}

1

I 1 1 1 1

I 1 1 1 '

East

W a l l 9 " C o n c r e t e

-

_-

R e s u l t s f o r

2819164

-

3019j64

_-

-

-

-

-

-

-

_-

-

-

_-

-

-

-

I

I

I 1

I

F I G U R E

4

L O N G I T U D I N A L

A C C E L E R A T I O N

A N D

S T R A I N F O R

9"

C O N C R E T E W A L L

-+

A

I

I

1

1

I

r

1 1 1

1

I

-

-

-

T a n k

Results

for S o u t h W a l l

-

R e s u l t s f r o m June/64

I

]

o

R e s u l t s o n

23rd &

25th

S e p t e m b e r 1964

-

sP

30/9164

./

-

-

-

u

-

-

-

-

-

-

-

-

I

I I

I

3

1

I

I l l

I

3

1

1

I I

I l l

10

'100

S T R A I N ,

/C

IN./IN.

F I G U R E

7

L O N G I T U

D l N A L

V E L O C I T Y A N D S T R A I N

W A L L

R e s u l t s

for S o u t h W a l l

2519164

-

r

I

I

I I

1

I

I

I l l

I

1

1

1

1

1 -1 -1.-

1 - 1

.

F I G U R E

8

L O N G I T U D I N A L

A C C E L E R A T F O N

A N D

S T R A I N F O R 7 ' X C N N R E T E W A L L

0 On !he Block

r A c r o s s B l o c k Joints

F I G U R E 9

0 On the B l a c k A c r o s s B b c k Joints 1 10 100 S T R A I N , +IN./I N . F I G U R E 10 L O N G I T U D I N A L A C C E L E R A T I O N A N D S T R A I N F O R 8" B L O C K W A L L B E 3 3 0 7 - A

T a n k

Results

for

Old

Block

W a l l

(West

Wall)

rn

A c r o s s & l o c k Joints

1

10

100

F I G U R E 11