Canopy Design 3

[GlobalPEC]

I need to design a steel canopy with concrete columns. It will be a proximately 25'x40'. I have never done this design, however I am very familiar with uplift, wind loads and so forth. My question is the follwoing: Is there a manual or design guideline for these type of structures?

 

[Ron]

Not aware of any design manual for canopies. I've designed hundreds of them.

They are designed as open structures, usually with monoslope roofs.

 

[GlobalPEC]

Thank you Ron for your input.

 

[Lutfi]

I would recommend the following steps and these are not all inclusive of every step:

1.Layout the canopy column and roof framing plan.
2.Study your applicable code. Various codes have certain requirements for the canopy. Florida Building code for example, has entire section dedicated to canopies. You need to determine if it is temporary or permanent type canopy.
3.Determine the canopy roof loads to include deal, live, wind and seismic as applicable.
4.Design canopy roof members for gravity (DL, LL, WL and SL).
5.Design canopy for lateral stability. You may consider vertical bracing or moment connections.
6.design foundations and provide adequate uplift safety factor (not less that 1.5)

If you are not an engineer, I strongly recommend that you seek one who is licensed in your area. If you are and engineer in training, you may want a senior engineer to review your design.




Regards,
Lutfi

 

[GlobalPEC]

Thank you very much Lutfi...I am trying to get as many inputs as possible prior to starting the design on Monday morning.

GlobalPEC

 

[Ron]

GlobalPEC...good info from Lutfi. Would add that you not forget to consider the lateral load on the faces of members. This is commonly neglected in open structures, but with your "footprint" you will likely only have 4 to 6 columns, so your face spaces (soffit beams) will likely contribute a fair amount of area with resulting load, particularly if you are in a high wind area.

Also, if you are using ASCE 7-02, and you use Figure 6-18, be sure to check both the distributed condition and the load application point as noted in the figure notes.

If you are not using a fixed base condition, you'll probably need knee braces or at least gussets at the column-beam connection.

 

[GlobalPEC]

Thank you Ron.
My canopy will have two columns and it will be used for a clinic entrance...Its for show . I was thinking about designning it as a cable stayed brigde with cantilever members supported by moment connections at the columns and cables for redundancy.
I read the IBC yesterday and found out that I need to design for a 20psf live load but I could not find anything about the uplift. Is it 125psf for open strcutures? seems high!!
The columsn will be fixed on the base. I have not decided whether they will be concrete or steel yet.

 

[Ron]

Not sure where you are located, but this is a common canopy application. Check out some of the manufacturers who supply these in aluminum...

http://www.dittdeck.com

Yes, 125 psf sounds quite high. Generally, you are looking at somewhere between 25 and 40 psf for uplift, even in relatively high wind areas.

20 psf is typical for the live load, but check the rain load depending on the type of roof/deck you will have. It can sometimes exceed the typical live load if you have damming or if your deck profile is deep.

 

[GlobalPEC]

Thank you Ron...I am in Oklahoma and I need to worry about high winds due to Tornadoes. No worries about rain loading, it should not collect any water.
Have you seen steel rods used to help with the uplift loading? I was thinking about using a 1" or 2" in diameter steel rods rather than cables, what do you think?

Thank you...GlobalPEC

 

[Ron]

GlobalPEC...Rods don't work well with uplift as they will be in compression and buckling is an issue (unless you're talking about tension rods from the bottom). I've designed lots of "hanging" canopies that have cantilevered decks with a hanger "rod" supporting the deck, angled from the wall. For these, we usually use a 2 or 3-inch diameter schedule 10 aluminum pipe, attached through a clevis device.

I did one about two weeks ago with double stainless steel cables (top cable to handle DL+LL...bottom cable to handle wind load) and turnbuckles.

Occasionally I've had an Architect refuse to budge on the "rod" issue....they wanted a certain profile and were not concerned about the structural aspects. In those cases, I had to design the deck to cantiliver with a moment connection in the deck plane, and using the rod only as supplementary support. That will only work if you have a cast-in-place concrete wall or a steel framed building....it won't work on masonry because the anchorage loads are too high. It throws a hellacious moment into the wall or soffit beam (torsion).

I'm not sure you can reasonably design an open structure canopy for tornadic winds. Large sail area...small connections.

 

[Loui1]

Does the owner ask for the design be made to withstand a tornado? That's unlikely. The wind design should be based upon ASCE7....for Oklahoma is 90mph 3 second gust wind speed. The wind pressures vary between structure type, height, and location. Its not as easy as, design it for ""psf. This is engineering, not McDonalds

If you have to design for a tornado, you'll have to read FEMA 361...and your design pressures will be astronomical 150 - 250psf, and that's only good for a Category 1 or 2 tornado. You cant design for any higher.

 

[RARWOOD]

Ron and others.

I deal with a lot of open structures which have gable and hip roofs. Do you use the force coefficients for a monoslope roof?

 

[Ron]

RARSWC...not for gable or hips, only for "flat" roofs or shed-type roofs. Otherwise, I use the appropriate coefficients for gables or hips with partially enclosed structures (since there's no table for open structure with hips/gables).

 

[GlobalPEC]

Loui1 and Ron, I am not designing for tornado wind loading nor did the owner requested such a thing. I was lookiong to be a little bit more conservative than the 80 mph norm.

Thank you all for your help

 

[RARWOOD]

RON

Correct me if I am wrong, my understanding is that the wind loads for partially enclosed strutures are higher than enclosed or open structures.

For the open stuctures I deal with, such as pinic shelters, the use of a higher conservative load, can really impact the cost of the structure.

I suppose what I really need to do is post a question about open structures on the wind forum.

 

[Loui1]

GlobalPEC, What does the engineer who is responsible for this design think? Is he/she in agreement with your design methodologies?

 

[Taro]

RARSWC, the 2005 ASCE 7 includes open structures with gable roofs.

 

[Ron]

RARSWC...you're right, but the code does not preclude the use of prudent engineering judgment in such cases.

 

[GlobalPEC]

Loui1, Last time I checked this website is supposed to help providing input to other engineers. My reply to you is the following...Don't provide any more input anymore if you are not ready to go along.

For everyone else, thank you very much for all your input and carrying the conversation with me.

GlobalPEC

 

[Loui1]

I do not take the subject of public safety lightly. If we see people in the position of endangering lives, then as licensed engineers are required to speak up. Please read Lutfi's response. You are asking elementary questions that a licensed engineer should not be asking....therefore leading me to believe someone else is stamping your drawing. If not, and you really are a licensed engineer who doesnt know how to calculate wind pressures, then I recommend you living up to the code and stepping aside, or let the Oklahoma licensure board discipline you.