Masonry Wall Damage 1

[mann]

Antennas have been mounted to the walls of the penthouse of a masonry building. There is a crack in the mortor joint about four courses down from the roof. The antenna mounts consist of an angle at the top with two bolts and the same about 3' to 4' below the top. The crack is between these mounts. Also the top of the wall has rotated outward as much as 3/4" in one spot and is also pulling away from the pilaster. How can the damage be corrected? And how concerned should the owner be?

The roof framing appears to be W8x19's (built in the 60's I think) at about 15'9" on center spanning 15', metal forms with concrete complete the roof. I don't know what the connection between the wall and the floor and roof look like.

 

[Ron]

It sounds like the antenna was mounted in an area of masonry that was not reinforced. Masonry is pretty sensitive to eccentricity and moments caused by eccentricly mounted items. This could be a problem, both from a wall integrity problem and as a safety issue. If a chunk of wall falls out (which it could based on your description) then whatever is below could be adversely affected. If this occurs on an outside wall where it could fall to the ground, you must warn immediately. If the antenna and wall section can fall onto an existing roof, consideration should be given to how well the roof could handle such an impact. If it is suspected that the roof would fail under the impact, again, something should be done immediately.

The antenna should have been mounted only in an area of structure competence. Investigation must be done in order to establish this competence. In the event the wall is not competent, then alternate design must be done for the antenna, and also remediation of the wall to restore competence should be done.

This can likely be remediated as it sits. The antenna should be removed and then the tie beam can be jacked up slightly and the masonry pulled or pushed back into position. Some positive reinforcement of the masonry should then be done to establish a tie to the tie beam. A steel angle frame could be put on the inside of the wall that would span between the tie beam and the floor, to which the antenna could then be mounted on the outside of the wall with through-bolts, or plates/angles could be mounted on the outside with through-bolts, then the antenna mounted to the plates/angles.

Since the lateral stability of the wall has now been compromised, I would establish some vertical reinforcement in the wall and fill the cells on a reasonable spacing pattern considering the expected lateral loads (typically wind) for the area and height of the building.

Have fun!

I'm not sure what area this is in, so I don't know your code requirements. Perhaps it was designed and built correctly, just not to handle additional equipment mounted on the outside.

 

[Qshake]

A masonry wall is only as good as its components. Having said that I wonder what the condition of the mortar in the joints. This should have been investigated prior to placing the antenna to begin with. As Ron said, the masonry is sensitive to eccentric loads and with this application being at the top where there is little axial load in the wall to enhance the shear capacity its no wonder it cracked. Lucky not to have fallen on someone!

 

[Ron]

Q-you're right as usual. I forgot to mention the mortar but also an important component. In any repair the mortar should be re-pointed as a part of that repair.

I too am surprised they haven't lost a good chunk of that wall!

 

[yekta]

What kind of antenna is this? Please describe. So I can understand the impact of it.

 

[mann]

It is a panel antenna for a cellular phone company. About 12" wide and 7' long.