Parapet Wind Loads using IBC/ASCE 7-02

[sturr]

I design light gage steel curtain wall stud framing. Recently I have had to apply the 2003 IBC wind design for the first time. I chose to use the ASCE 7-02 analytical procedure because the IBC simplified method did not specifically address parapets under components and cladding. The ASCE method generated very high parapet load of about twice the typical wall load (much, much high than the old UBC loads), and I have verified my numbers using a "Guide" published by ASCE. This particular job has 9 foot cantilevred parapets.

My problem is this: This job has a "base" stud design (size, gage, and spacing) provided in the contract documents. I have found the wall design to be inadequate with respect to the parapets. After reporting the issue I was informed the IBC simplified method had been used and approved by the building department.

I don't believe the IBC simplified method can be used to determine parapet loading because it uses "net" pressures combining internal and external pressures. I believe parapets should be subject to windward and leward pressures simultaneously. Which is also why I believe the simplified method does not address parapets.

Can anyone out there back me up on this or provide some insight into this topic? Before I cause a major change order, I want to make sure I'm not way off here.

THANK YOU!

 

[DaveAtkins]

Good question. When I have designed parapets, I have NOT used windward plus leeward for the parapet, but have used the same load as the wall below. I'm not saying I am right. But will something as thin as a parapet get windward and leeward loads simultaneously?
One way to deal with this is to RECOMMEND that the more conservative approach be used, and to say if it is not, your firm is not liable.

DaveAtkins

 

[jike]

On page 45 of "Guide to the Use of Wind Load Provisions of ASCE 7-98", it suggests that MWFRS pressures for parapets can be approximated by using the coefficients in Table 6-11 and Equation 6-20 for a sign structure at ground level.

On page 49 & 50 of "Guide to the Use of Wind Load Provisions of ASCE 7-98", it suggests that C&C pressures for leeward or corner parapets can be approximated by adding positive and negative pressures wall pressures (Zone 4 or 5). It also suggests that C&C pressures for windward parapets can be approximated by adding positive pressures wall pressure (Zone 4 or 5)and negative pressures roof pressure (Zone 2).

 

[vmirat]

I'm not clear on what part of design your concerned with? Are you doing MWFRS or C&C for the parapet?

MWFRS: The windward and leeward pressures are combined in the simplified method into one coefficient, similar to the old UBC Method 1. Internal pressure is ignored since they cancel out.

C&C: The net pressure (internal and external) is applied normal to each building surface. Table 1609.6.2.1.(2) has both positive and negative values for pnet30. I believe these are net pressures which add external and internal. If you have a negative external and positive internal, you would have a net negative (away from exterior surface) pressure. This would be the leeward side of the parapet. If you have positive external and negative internal, you would have a net positive (towards exterior surface) pressure. This would be the windward side of the parapet.

So, I would say that the simplified method does consider windward and leeward pressures and is appropriate to use for parapet C&C. Of course, since the wind can come from either direction, the worst case senario will govern the design of your C&C.

 

[sturr]

Dave, thank you, I am leaning that way.

jike, thank you. I have the "Guide to the Use of Wind Load Provisions of ASCE 7-02". Your findings are very similar to mine with respect to the analytical procedure of ASCE 7-02. The additive pressures create the very high load I was refering to.

vmirat, thank you. The cantlivered parapet studs are considered components. These studs attach to the floor below the roof, and extend past the top of the roof. The parapet portion of the studs are not subect to internal building pressures like the wall below (although the cladding on the front or back could be). In fact the ASCE has an equiation used for the effect of parapet loading on the MWFRS, which does not include the internal pressure coefficient.

I agree with your intrepation of the net pressures in the IBC table. The analytical method uses the coefficients seperately, and yields similar yet undserstandably slightly lower loads. Adding the windward and leeward loads using the simplified method is probably appropriate. I the code simplfied simply state it!!

Thanks again.

 

[sturr]

Sorry about my last sentence above. I meant to say: "I wish the simplified method simply stated it!!"

 

[VTPE]

Below is an excerpt from the ASCE 7-02 commentary, chapter 6, section 6.5.11.5. It explains the ideas behind the calcs.
"The methodology chosen assumes that parapet pressures
are a combination of wall and roof pressures, depending on
the location of the parapet, and the direction of the wind,
see Figure C6-9. The windward parapet should receive the
positive wall pressure to the front surface (exterior side
of the building) and the negative roof edge zone pressure
to the back surface (roof side). This concept is based
on the idea that the zone of suction caused by the wind
stream separation at the roof eave moves up to the top of
the parapet when one is present. Thus the same suction
which acts on the roof edge will also act on the back of
the parapet.
The leeward parapet would receive the positive wall
pressure on the back surface (roof side) with the negative
wall pressure on the front surface (exterior side of building).
There should be no reduction in the positive wall pressure
to the leeward due to shielding by the windward parapet,
since typically, they are too far apart to experience this
effect. Since all parapets would be designed for all wind
directions, each parapet would in turn be the windward and
leeward parapet and be designed for both sets of pressures."

Take a look at the commentary, it is pretty helpful. Hope this helps.

 

[sturr]

Thank you VIPE. Trust me, I have read that many times. In fact, that commentary supports why I think that if someone chooses the simplified method, the windward and leeward loads should be combined for the parapet loading (when addressing studs like I described earlier).

Since I posted this issue, I have learned that the when the studs were designed for the contract documents, the simplified method was used, but the windward and leeward loads were not combined on the parapet studs.

At this point I feel comfortable with "sticking to my guns", and using the high parapet loads I calculated.

Maybe the next issue of the code(s) will address parapet loading in the simplfied method.

 

[sturr]

To let everyone know that helped me with this issue, I just found something in the IBC that might help anyone that is faced with these high parapet loads. In the 2003 IBC table 1604.3 footnote "f", the component and cladding wind loads can be reduced 30% when checking deflection. This helped my situation a great deal.

Thanks again.

 

[vmirat]

Good news! I just got my copy of Structural Engineer magazine and they have an article about ASCE 7-05. The 2005 edition includes parapet design provisions based on recent research.