C&C roof positive pressure near parapet 2

[srm01]

I am looking at the addition of a mechanical unit on a steel joist roof. The unit is resting on a steel beam on one long side of the unit and a joist on the other. ASCE7-22, Figure 30.3-2A, note 5 states there are different pressures near the parapets (see attached). The positive (downward) values are x~4. Is there a wind direction factor to this positive pressure? I.e. would the pressure on the "windward" parapet be negative in the roof zone and the "leeward" parapet see the positive - similar to Figure 30.6-1 (although roof pressure is shown as negative in the diagram for leeward, see attached)?

The way the numbers work out, and the positioning of the unit, beam, and joist if the positive roof pressure is only on the leeward side then joist reinforcements are not needed.

I was not able to find clarification in the commentary.

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1709931652/tips/CCF_000065_g9tagy.pdf[/url]

 

[jdcollins]

Figure 30.3-2A does not address pressures on the parapets, only the pressures on the roof. Essentially, if your parapet is 3ft or higher, your Zone 3 negative pressure just equals your Zone 2 negative pressure (or, Zone 3 goes away and becomes Zone 2). However, your POSITIVE roof pressures (pressures acting DOWNWARD) will INCREASE and will match the positive pressures you would calculate if you were calculating Zone 4 or Zone 5. Zone 3 positive pressures would equal your Zone 5, and Zone 2 would equal your Zone 4.

This will generally only affect your design if your unit is being placed on joists that frame into the wall where the parapet is located.

 

[srm01]

yes, this is the problem. the unit is placed in Zone 2. and about half of the joist is located in zone 2. would there be a positive pressure on the roof on the windward side of the building? I would think it would be negative.

I would only think there would be a positive pressure on the leeward side of the building...

Just curious, where you able to see the attachment (I can't see the attachment on my end so I have no way to know if it is viewable)?

 

[jayrod12]

Does the wind only blow in one direction? Regardless of windward or leeward, wouldn't each side of the unit/roof essentially get all potential pressures? I'm not familiar with ASCE so maybe I'm misunderstanding something.

 

[StrEng007]

Since you mentioned this is an addition, I'm not sure if you intended to say the roof was already designed for it's appropriate MWFRS and C&C pressures. Going on the assumption the roof design was previously completed...

The wind loads on the rooftop units and unit weights are going to have more importance than considering CC pressures in this situation. Rooftop equipment wind loads are found in ASCE 7-16, Section 29.4.1. This type of wind is MWFRS directional wind.

Your unit will experience horizontal and vertical (uplift) wind pressures. The previous editions of the Florida Building Code required these pressures to be applied simultaneous but recent versions has removed this language. Additionally the user is now directed to ASCE 7 for wind load calculations and it is not entirely clear if horizontal and vertical wind is to be considered at the same time. The way I see it, you'd have both since it would be hard to apply uplift to a rooftop unit without the horizontal cross wind.

Units will impart uplift, overturning, and shear forces to the existing supports (beams and joists). The design of these members should be investigated for these wind load reactions, in combination with the unit dead load, roof superimposed loads, and MWFRS wind pressures.

Personally, I wouldn't start combining C&C roof pressures with rooftop equipment reactions. I'd treat C&C as a separate load case.

 

[srm01]

jayrod12: yes wind blows in all directions. the unit is installed close to one parapet. steel beam supports unit close to parapet and joist supports away from parapet. i'm concerned about compression on the joist - which is where this roof pressure comes into play

StrEng007: existing roof design was completed by others a long time ago. they increased the size of the RTU recently. RTU wind loads determined by section 29. RTU loads plus wind pressure is causing a problem as described previously.

 

[StrEng007]

Perhaps there is a misunderstanding here. When you talk about parapets affecting the design, are the parapets being braced onto the roof framing members? Typically, the parapets are a continuation of the shell structure, and unless there are kickers on the roof, the MWFRS parapet wind loading should not affect beams that are used in the field of the roof.

[highlight #FCE94F]Your image never uploaded, so we cannot see the scenario you're discussing.[/highlight]

Either way, if you are now seeing overstress in your roof framing members (this is most apparent if the unit weight is making your framing member fail), you'll need to reinforce them. You'll also need to make sure that your connections don't fail under the increased shear demand.

I mention roof framing failure with unit dead load to specifically avoid getting lost in the application of MWFRS winds and C&C winds. As I mentioned before, if you're considering the affects of your unit in combinations with wind, I'd be looking into MWFRS.

Check out Section 5.5 of the link below. It has some design examples for reinforcing open web steel joists:

https://vulcraft.com/catalogs/JoistGirder/DesigningwithVulcraft3rdEdition2019.pdf

 

[srm01]

StrEng007: I was finally able to get the images on the original post.

I am not worried about the parapet itself just its effect on the roof pressures. I am looking at unit and roof deadloads and wind loads. The question is if the windward side of the roof behind a parapet can see a positive wind load.

the roof is 25' in height so Zone 2 is 15' wide.

Schematic:

 

[jdcollins]

Not sure if this is acceptable per the code, but my interpretation of this condition would include the higher positive wind roof pressures (downward) on the area between the unit and the parapet when you are analyzing the wind in the direction right to left in your sketch (leeward parapet). When analyzing the wind blowing left to right, I would probably assume the normal positive wind pressures (assuming no parapet).

 

[kissymoose]

The question is if the windward side of the roof behind a parapet can see a positive wind load.

Yes, Fig. 30.8-1 only gives you loads on the parapets. The only reason it mentions the roof wind load (p7) is to tell you what to use for the leeward side of the windward parapet. Use Fig. 30.3-2A for CC wind loads on the roof.
Also, Chapter 29 should have all you need, you don't need to get into Chapter 30 for RTUs.