ASCE 7-10 Wind Problems

[RustyDuster]

Hi all,

I'm having some issues regarding the ASCE 7-10 classifications for structures. Specifically, what chapter I should be designing with. A little bit about the background of the project as it's a bit odd. There is some concern about adding a 4 season porch to an existing house, and according to the new state Residential Code this needs to be designed using ASCE 7. This porch is supported on posts roughly 8 ft high (there is a walkout basement, and the porch is located on the first floor), and connected to the house by knocking out the ledger board and setting the floor trusses on the existing foundation wall. What I can't seem to figure out is what to classify the porch as; not rigid (wood frame), might meet the criteria of low-rise if you take into account the overall length of the house, but not just the porch itself.

Since this is a residential building I know they won't go for an analysis to determine the frequency of just a porch. Can anyone give me a bit of guidance?

 

[BadgerPE]

I assume you are looking at frequency to determine rigid vs. flexible. If that is your case, I will direct you to ASCE 26.9.2 Frequency Determination. This provision should help you with your frequency issue, if I assumed it correctly.

 

[RustyDuster]

That's just it. I won't be able to make a model to determine the frequency. This isn't a commercial building with a large budget, this is a family that wants an addition on their home. What I'm trying to determine is based on the type of structure, what wind loading method I can follow. If I'm boxed into the method that forces me to model the porch then I have to make the client aware of it. For something as basic as a porch I feel like making a model isn't necessary, and I'm missing something in the code to direct me to using a simplified wind loading method.

Maybe this will help to get the ball rolling. If I'm adding a porch to a building, can I take the overall width of the porch plus the building to classify the structure as a low-rise building? The porch isn't self-supported.

 

[BadgerPE]

I think you are way over-thinking this, but my guess is that your mean roof height is greater than your building's least horizontal dimension (unless you add the porch to it) so you are "violating" Note 2 of the building low-rise definition. If that is the case then you will need to make an engineering judgement call whether simplified methods can be used.

From your description, the porch is in fact self supported at the free end. At the building it is supported at a point of "infinite" rigidity (short concrete FND wall).

 

[RustyDuster]

I know for a fact I'm overthinking it, and you are right, with the porch on the building it would be considered a low-rise building. As I go back and look it seems like Part 2 of Chapter 27 might be the way to go. The definition of a simplified diaphragm makes sense for a wood framed addition. I did mistype when I said it isn't self-supported, it should have been is self-supported.

 

[medeek]

A picture would be nice to visualize your scenario.

I like to think of porch roofs the same way as decks that are attached to the house. The wind loading perpendicular to the porch will put a lateral load on the porch. The roof diaphragm will act in a similar fashion to a shearwall, with one side trying to tear away from the house and the other side in compression and pushing on the house. If its not a rectangular box with a simple gable roof I just go to Part 1 of Chapter 27, the all heights method. Calculating the wind pressures is really quite simple once you've set up a spreadsheet that will handle the pesky double interpolation required with this method.



A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[RustyDuster]

medeek,

It is exactly as you described it. A completely enclosed addition that is rectangular with a gable roof. Thanks everyone for your help.