Cracks on Exterior Bearing Walls 10

[Lina]

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.

 

[roscoe]

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).

 

[dlew]

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

 

[steve1]

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.

 

[Lina]

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.

 

[ishvaaag]

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

 

[Lina]

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

 

[dlew]

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

 

[Lina]

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.

 

[ishvaaag]

...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.

 

[dlew]

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!

 

[Ron]

Lina..most of the discussion seems to center on theoretical conclusions with respect to exciting forces and vibration response. All of these have merit, but I doubt that they are at play with your issue.

If the cracks are vertical or nearly so, they would have nothing to do with the dead load of the roof. Further, a 6500 sf house would likely exhibit a "box" stiffness effect, thus not having to rely much if any on mass damping of the roof tiles.

Since you have not had a wind event in the recent past that would be anywhere near the design loading for South Florida, I suspect your issues are more likely related to two things:
(1) Homeowners tend to start looking for problems all over a structure when there is a problem with any part of it. In this case, there must have been an obvious problem with the roof system, thus the owner gets into a mindset that there must be problems elsewhere. When cracks are seen, they are immediately perceived as problematic. It is likely the cracks were there before, just not noticed. As for the perception of vibration movement, DLEW is correct in that in the sands and near surface limestone you have in South Florida, it is easy to transmit surface vibrations from traffic and other sources in sufficient magnitude for perception, but not likely of sufficient magnitude to cause structure damage or even significant response.

(2) If the exterior walls are masonry or stucco, then look to the probability of shrinkage cracks. This is the most prominent cause of cracks in facilities in our sunshine state. Many of these cracks are exacerbated by the cyclic movement from both the dry and liquid varieties of sunshine prevalent in Florida!

Ron

 

[steve111]

exactly Ron,
highly unlikely that removing the dead load from a wood framed building with members at 24" o.c. or less, substaintial redundancies, multiple load paths etc., would affect the vibration characteristics of a wall, a floor or the structure for that matter. Dead load dampening long span owsj, long span composite wood I's, etc., yes a major factor. More than likely that the construction has caused some cracking that wasn't evident before and is now and the owner is unhappy. Renovations, especially residential, we always do a pre-construction survey of the structure, walk through with jpegs taken. ...

 

[mtroche]

Good afternoon, Lina:


There is basically two reasons for your problem:

1) The cracks are older and do not have something to do with the roofing works although the vibration that could cause the truck used to load the construction debris is not so significant to cause the cracks described. Vibrations from piling are a good example of the kind of vibration so heavy enough to cause so damage. Note that as old the concrete is as hard it becomes!!

2) If the carcks are not old ones, you should be take in account the effect of liquidfying of soil, in this case sand, caused by high water level in that soil. There have been many problems with liquid sand in Florida that have caused even excesive settlement of homes. Even if the soil is "well compacted" there will be a liquidfying problem as in the case of sand. So, check out what was the water level in soil on that days!!


Att,
MTroche

 

[Lina]

Let us forget about the cracks. My client stating that he feels vibrations like a very light earth-quake. What is causing these vibrations?
The tiles weigh about 4lb/sf and this is a large area (6500 sf)
Is it possible that reducing the dead load could in any way cause any movements or vibration.

Thanks

 

[carlorom]

Hi,

This is from an engineering point of view out of my league but read all this with interest. I am a Land Surveyor specialising in deformation surveying of structures.

Does the client perceive the vibrations still now. Then surely one should be able to measure it. From the description this sounds like a slow movement of low frequency. Hence one should be able to set up some kind of monitoring to observe this. Off hand I would say a Theodolite on a very stable base some distance away (where the vibration is not perceived) pointed to some target on the dwelling should show up this vibration.

Alternatively - set up theodolite near the house and observe to a far off object say a tower 3 miles away - if the vibration is there you will note that movement clearly through the telescope. Modern theodolites have electronic "bubbles" which show vibrations easily too. That should solve the issue of whether there is a vibration.

Then if cracks occur one can simply monitor these - take a few well chosen ones around the structure and establish differential movements around each either by guages or measuring to targets from a stable platform - similar to what i described earlier.

At least that will either corroborate or disprove clients claims instead of trying to solve the matter based on sheer theory?

Good luck.

 

[ishvaaag]

1) walking vibration

2) loose joints plus wind, movement or traffic

3) maybe the house is over a creeping landslide; this can not be discarded since he has got crack in his walls

 

[tabio]

Lina,
I have had similar complains in Miami... right after Hurricane Andrew. OUr conclusion a that time was that there were "events" that forced the homeowner to notice the cracks and feel the vibrations in the windows due to poor instalation and wind noises...

About ten years ago (after Andrew) we had one homeowner complaining about the cracks, and we appoached the problem and concluded that the cracks on the wall were due to poor mix on the mortar and the stucco mix. We were challenged, threatened to go to court and when we decided to keep with our theory after analyzing samples of stucco and mortar mix... the owner, his engineer and his attorney decided not to pursue more legal battles.