cracks caused by earthquake

huangyhg

cracks caused by earthquake
is it possible that low intensity earthquake can cause only superficial cracks on plasters over concrete? if cracks on concrete appeared after an earthquake, i suppose the structural concrete has been affected thus cracks appeared on plaster. the crack maybe obvious only on the plaster but micro sized cracks may have developed through the structural concrete.
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cracks may have occurred in the concrete, but unless there are shear cracs, i would think you needn't worry about it.  concrete is meant to crack and will normally crack under service loads.  as long as teh reinforced section has the adequate capacity, there should be no concern (this may be invalid if the steel has yielded, but you would probably see larger cracks depending on the reinforcement ratio).
sometimes, the plaster cracks were existing prior to the earthquake but noticed afterward when the owner starts to look around for damage.  if the cracks do not look fresh, that is, have broken off edges, dirt and cobwebs in them, or paint in/briding the cracks, this is evidence of pre-existing cracking.  they could be shrinkage cracks on the plaster especially large areas with no expansion joints.  just some things to think about.
don phillips
thanks for your responses. the cracks looks fresh and appeared on 2nd floor hallway slab and on soffit of roof deck after a low intensity earthquake.
if your hallway slab is gypcrete, this is non-structural and of no concern other than aesthetic.  
if you have a friend who is a structural engineer and can come and take a look, he might be able to assuade your fears, or confirm them.   
mike mccann
mccann engineering
please explain the statement "concrete is designed to crack and will normally crack under service load". reinforced concrete is supposed to be firm and solid for us to say that it is structurally sound. concrete structure may still stand inspite of numerous cracks and other defects but theoretically it is not anymore safe to use.
any reinforced concrete   
if concrete cracks on the tension area then maybe it is ok as the steel reinf is the one resisting tension. nevertheless, cracks on concrete should not be left un attended and must be repaired to prevent further damage to concrete and steel reinf.
emanc,
if we repaired all cracks in concrete structures, all other structural activity would have to cease for us to catch up.  cracks may or may not be cause for concern, and that can really only be determined by structural engineers.  you are incorrect in making a blanket statement that "theoretically it is not anymore safe to use".
there is no construction ongoing, my query was about an existing building. my statement was only a personal opinion, if your own house have concrete cracks is it ok with you not to repair it?
my house does have cracks, and there is no need to repair them.  as i said before, it depends on the type of cracks.  my comment about structural activity ceasing was related to global activity, not your building.  suggest you need a structural engineer to assess the cracking and evaluate whether remedial action is required.