huangyhg

cracks on exterior bearing walls
a client claimed that he has noticed movement or kind of light vibration of his house that caused minor cracks on the exterior walls after the roofer has removed all the tiles for re-roofing and prior to the installation of new tiles for a period of 2 months, claiming that reducing the dead load on the roof have caused this vibration that resulted in minor cracks along the bottom as he was told by the inspector and the contractor.
i have a hard time to believe this.
please let me know if you have experienced this situation or if you agree with my client's statement.
what is the wall made of? are the cracks in the stucco?
when building new single family homes it is common practice to install the roofing material before doing the stucco(especially for heavy spanish tiles).
lina,
what type of wall?
what type of wall footing?
what kind of soil?
what did they say was the cause of the vibration? wind?
hard to believe!
the most common cause of cracks in walls is settlement of the footings due to poor soil, or to vibration of loose sands.
vibrations may be caused by driving of piles in the neighborhood, by heavy trucks passing close to the house, or by other type of construction in the area.
maybe you should get information on the subsoil and talk with a geotechnical engineer.
aef
there are plenty of examples where a large dead load is beneficial, espicially in counteracting tension forces. a masonry wall for example, subject to out of plane wind loading will perform better with a dead load which produces large compressive stresses, which in turn counteract the bending tensile forces. another example is a foundation mat subject to direct and bending loads. the larger the dead load the less tension stress. perhaps this is what is being refered to.
it is a masonry construction, large house about 6500 square feet with a high cathedral ceiling the house was without tiles for approximately 5 months, the client complaining that after the contractor have removed the tiles he felt that the house was moving or shaking on a daily basis without the presense of wind, storm, and any type of vibration.
i have a hard time to believe that reducing the dead load will cause movements without any cause.
it may happen that the diminished weight has some influence, since...
less, material, less damping
global rigidity (except if the roof tiles just relying in weight) diminished, the house being able to take mopre deformations somewhat precluded by the weight
joint movement more feasible, and maybe some loosening included
the cathedral-like push be less and the contrary adaptation under less weight may be incompatible -or made so by wind- with some fragile stucco staying without cracks
ishvaaag:
what about due to the decrease of load the angular frequency increases, therefore the frequency increases. this is a dynamic case (free vibration) due to static force. this case will cause movement of the second floor.
my analysis based on the following:
w = (k)^1/2/m if m (in this case dead load )decreases w (angular frequency increases. therefore, the frequency (f=w/2(3.1416) will increase causing movements.
please tell me if you agree with my approach
lina, steve1 and ishvaaag:
assuming that the removal of the roof tiles (spanish tiles?) modifies the natural frequency of the house, still it needs to be an exciting force to produce the vibration.
the client said that the vibration was felt without the presence of wind, storm or any type of (external) vibration.
what could have produced that vibration? to vibrate a masonry wall house, the exciting force should have been noticeable.
lina, in what part of the country is the house located? and what do you know about the subsoil?
aef
dlew:
thanks for your response.
the house is located in miami florida, was built in 1990 on clean well compacted soil.
what about the static force itself, again i am describing it as a free vibration and it is only as a result of the static force.
thanks for your help.
...of course the house will follow dynamics behaviour, and all other remaining equal with less mass will vibrate more rapidly.
if the movement came from statics ony, the potential loss of rigidity and damping may have caused the floor become a bit more bouncy...it may be also a case of enhanced sensitivity, seeing the reform.
respect noticeable walking or jumping vibrations being slender is enough. i lived 5 years in a steel framed building maybe 30 m long and a dozen stories tall in plan where some jump was felt well away of where made.
lina,
two questions:
1) the client said he felt the house moving or shaking on a daily basis. were the tiles removed, and the contractor was gone for 2 months until he came back to reinstall the tiles? did the vibrations stop after the tiles where replaced?
2) is the crack in the masonry wall or in the stucco?
there are many reasons why stucco cracks: shrinking, temperature changes, lack of adequate joints, etc., but if the house is 12 years old, the crack(s) should have developed a long time ago.
3) does the crack in the wall follows the joints in the masonry on a stair pattern? if so, that is an indication of a settled footing.
i live in florida too. when a heavy truck passes through the street, one may fill a very minor vibration. it could be acustic and/or soil transmitted. but that vibration is not strong enough to produce cracks on the wall, unless the soil settles under the footings.
i assume that when the contractor removed the tiles, a truck came pretty closed to the house, to be loaded directly from the roof to the truck. same when they unloaded the new tiles. a truck driving on sand (or on the driveway) will produce certain noticeable vibration; specially if the truck is heavy. that is a possible cause that may deserve investigation.
good luck!

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