Parapet Wind Loads

[ChiEngr]

Hello,

I have a question regarding parapet wind loading. I have a situation on a project where the application of parapet wind loads is not very clear per Code.

I am working on a project featuring an east building addition to an existing 100 year old building. The east wall of the existing building will be demolished in order to facilitate clear transfer between both buildings. As such, at the transition between the existing roof and the new roof there will be a built up curb of some sort, essentially an "interior" parapet. I am trying to not add additional wind load into the existing building structure (i.e., more than 10%), so I am curious as to how you all would handle the application of wind loads in this case. Attached is a sketch of how I am currently handling the wind loads, but I am wondering if it is too conservative (or not) to apply a leeward parapet pressure twice in this case.

I look forward to hearing your thoughts!

 

[GeorgeTheCivilEngineer]

When things don't fit neatly to 'code' situations then the question is what will happen in reality?

From a quick look, surely the internal parapet will be effectively shielded from peak wind pressures, if the new parapet is built up to the same level. So I would be inclined to reduce it. Is removal of the internal parapet an option?

 

[KootK]

When things don't fit neatly to 'code' situations then the question is what will happen in reality?

That I certainly agree with.

From a quick look, surely the internal parapet will be effectively shielded from peak wind pressures, if the new parapet is built up to the same level.

That I do not agree with. At any normal building and parapet proportions, I would expect the wind to return to a more or less steady state condition between parapets such that you would once again have to deal with turbulence at the interior parapet.

I am trying to not add additional wind load into the existing building structure (i.e., more than 10%)

Is it not possible to attribute the wind load on the curb to the new building rather than the old? The diaphragms of the existing building and the addition will be attached rather than allowed to move independently, correct?

 

[ChiEngr]

Koot,

I agree with your second point. Regarding your third point, it is an interesting situation. The new roof diaphragm is steel roof deck (flexible). The existing roof diaphragm is precast (rigid). They are tied together through the steel framing below the internal parapet. From a design standpoint, I am conservatively designing the new addition as being able to withstand all wind load directly applied to it. However, in actuality, I believe that all wind loads will be transferred from the flexible diaphragm across the roof interface into the rigid diaphragm. I am unsure as to how to "keep" that load in the new building addition lateral system (moment frames) and not get transferred to the existing building (shear walls).

 

[KootK]

However, in actuality, I believe that all wind loads will be transferred from the flexible diaphragm across the roof interface into the rigid diaphragm.

Sounds about right.

I am unsure as to how to "keep" that load in the new building addition lateral system (moment frames) and not get transferred to the existing building (shear walls).

With respect to the diaphragms, you could build discrete, horizontal trussing into the new roof diaphragm to allow its stiffness to compete with the precast diaphragm. This will be for naught, of course, with the moment frames in play. As you surmised, I suspect that most of the lateral load in one direction will wind up being resisted by the existing building if they are to be tied together.

 

[ChiEngr]

Sounds like I need to request that the architect reduce the parapet height.

 

[Rsd789]

I would recommend looking into ASCE 7 chapter 29. If I'm understanding the question correctly this sounds like it could be analyzed as either a solid free standing wall (29.3) or as a mechanical equipment screen wall (29.4)