wood-framed or of brick construction, can withstand amplitudes of about 400 μ. From the results of such tests, working rules were adopted enabling the operator to determine the maxi- mum charge that could safely be fired at certain distances, the maximum amplitudes of vibration permitted ranging from 125 μ to 400 μ.
It is not easy to define, with abso- lute certainty, what constitutes damaging vibration, nor does any uni- versally accepted criterion exist that may be used to compare the effects of vibrations of differing amplitudes and frequencies. In assessing the possibility of damage many factors should be taken into account, for example, the additional stresses set up by the vibra- tion, the size and type of building, the fatigue properties of the materials, and the possibility of resonance.
unpleasant. Vibration from blasting is usually of short duration and it would appear that higher amplitudes or velo- cities can be tolerated than would be the case for sustained, steady-state, vibrations, e.g. from machinery.
The data given in Fig. 4 for fre- quencies from 5-50 Hz, the range most commonly encountered in buildings, is the most reliable information at pre- sent available for the estimation of structural damage from vibrations and is based on a review carried out in 1961. So far as is known, no case of damage has ever been reported for vibrations in Zone I of the chart. For Zone II, cracking in load-bearing struc- tural units is likely only when reserves of strength have been used up prior to the dynamic loading. For purposes of comparison, the danger-zone according to DIN 4150 (1939) and maximum velocities of 50, 75 and 100 mm/sec have also been indicated on the chart. There is probably little risk of damage if the quantity A2 is less than 50 mm2/sec3.
In these earlier tests, and in similar tests more recently carried out in Sweden, the UK and elsewhere, dama- ge was caused deliberately by firing charges far in excess of those normally charges far in excess of those normally permitted. Large amplitudes, ranging from 250 μ to 5000 μ were involved.
μ Current opinion is that the maxi- mum velocity ('peak particle velocity') is a more realistic criterion for damage than is the simpler criterion of maxi- mum amplitude and this is in general accord with an 'energy ratio' criterion suggested in 1949. The latter indicates that structures are safe if the peak particle velocity does not exceed 80 mm/sec and that there is danger if the value exceeds 120 mm/sec.
It is often difficult enough to per- form a static stress analysis for a com- plete structure; analysis of the dyna- mic stresses set up by vibration in a masonry structure is virtually impossi- ble.
The criteria that assist in determin- ing the effects of vibration include those of maximum amplitude, velo- city, acceleration and energy, as well as the use of the human sensitivity scales. A comparison with cases where no damage has been produced by vi- brations of known characteristics may often be useful and information avail- able concerning vibrations and shock from blasting, pile driving and earth- quakes is also relevant. The various scales of earthquake intensity and associated acceleration values should not however be applied, without modi- fication, to vibrations of buildings due to more usual sources of excitation.
Swedish information is that there is minimal damage at maximum velocity minimal damage at maximum velocity of 70 mm/sec, slight damage at 110 mm/sec, moderate damage at 160 mm/sec and serious damage at 230 mm/sec. A Canadian Building Digest* issued in 1965 gives the threshold of damage due to blasting for a horizon- tal or vertical velocity of about 75 mm/sec, as recorded in the foundation mm/sec, as recorded in the foundation wall nearest to the blast.
Much information is available on the effects of blasting vibration. Some early work in the USA indicated that conventional types of property, either
100
50
30
20
frequency (Hz)
0-5
4
5 8
10
2
4
6 B
K*0.5
K*01
2
Cases where vibration has caused definite damage to buildings have been extremely rare and the generalisation can be made that vibration must be- come unpleasant or painful to the occupier long before there is any possi- bility of damage to the building itself. Nevertheless, complaints are often re- ceived that minor damage such as the cracking of plaster ceilings, brickwork and window glass, as well as the loosening of roofing tiles, has been caused by some particular source of vibration. That such complaints are so common may be due to the estreme sensitivity of the human body.
For vertical vibrations of frequen- cies 5-40 Hz, a maximum velocity of 75 mm/sec corresponds to a Dieckmann K-value of 60, and such a vibration would normally be deemed extremely
* Blasting and Building Damage, Canadian Building Digest 63 (1965), NRC, Canada.
46 8
2
4
6
K#50
K#5
*K=10
K#1
amplitude (μ)
Fig. 3. Dieckmann values: horizontal vibrations
103
K=100
When a new source of heavy vibra- tion arises and is such as to disturb the occupier of a building he may become concerned about the structural effects of such vibration. He may then assume that cracks which have escaped notice for some time have been caused by the vibration. There are many causes of cracking in buildings and cracks should not be attributed to the effects of vibration until other possible causes have been eliminated. For example,
much of the cracking in plaster ceilings must be attributed to movements of the plaster itself or of the timber sup- ports due to changes in moisture con- tent and such cracking often occurs in areas known to be free from external sources of vibration.
The earlier work in the USA in connection with blasting vibrations in- cluded tests on floors and ceilings
Far East BUILDER, November 1970
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