What wind loads on building canopy? 2

[structuresguy]

Hi

We are designing a building which has a very large canopy (30'deep x 40' long) at the main entrance. The canopy is completely free-standing and self supporting. The canopy is being designed by others, but we are responsible for the foundations. It is in a 125 mph hurricane zone.

Myself and the other engineers have been discussing wether to consider the canopy as an "Open Roof" per ASCE 7 or as a building overhang per FBC. The open roof concept would result in much lighter loads than the overhang. This has come up before when canopy specialists question our higher C&C loads listed on the drawings. Usually we get something like "What do you mean 125 psf uplift? ASCE says only 75 psf..... " etc

My personal feeling is overhang is more appropriate since it would deal with the wind suction on top and the wind pressure from below.

Can I get some other opinions from you guys?

Thanks

 

[RockEngineer]

With the canopy adjacent to the building I would tend to go with the overhang. The uplift will be higher adjacent to the building than for an open roof separated from any structures where the wind can just blow through. With a canopy 30 ft deep this may be too conservative. Maybe a transition between the two as you get further away from the building.

This is a good question and there probably isn't one simple answer.

 

[Ron]

Structuresguy....RockEngineer is right...there is no single answer!

I have analyzed hundreds of canopies under many, many different conditions, from very high wind loads to very high snow loads. In general, canopies such as the manufactured, extruded aluminum canopies/walkway covers are designed/analyzed as separate, free standing structures. In most cases, under ASCE 7, they are considered "Monoslope roofs over open structure" thus the Cf coefficient applies and you do not consider the canopy to be an "overhang".

There are conditions I would apply the overhang consideration. If the canopy structually is attached to the building, I would likely consider an overhang condition. In some cases, even its proximity might trigger consideration of the overhang condition. For the size canopy you have described, the overhang condition would not apply to the entire structure anyway...only a portion of it, even if designed as a MWFRS.

Other factors can have similar effects on the analysis. For instance, I use the importance factor of the main structure in my analysis. No, the canopy is not "important" in the context of the building failure, nor is it likely to be a shelter for anyone during a 125 mph wind event; however, it does bear impact on the structure and surroundings as it can become damaging debris if allowed to fail at a loading lower than the building itself would be expected to withstand.

Using a higher importance factor is a carry-over from a requirement for analysis for Florida schools. I have it in my pre-processing (load summary) spreadsheet and I rarely change it.

I notice a reference to FBC. If you are designing in Florida, your obligation as the EOR is to provide the criteria to the Delegate Engineer for the specialty design. Within reason, those can be anything you decide, though as you noted, you might get some argument (yes, I would probably argue with you at times!), particularly if you exceed the expected code requirements.

As for foundation design, these canopies can result in some large foundation requirements, due to their larger overturning moments (light structure, big wind!). Most of the time the foundation need is for mass, not bending or shear. The downward vertical loads are usually not bad...it's that "fly away" component!

 

[CTSeng]

I agree that if it is attached to the side of a building I would not consider it an 'open strucure' and then use some combination of overhang and edge zone values. However, wouldn't it then be a partially enclosed structure even if the main building is designed as enclosed?

 

[jheidt2543]

Ron,

Excuse me for changing the subject a little bit here, but you mentioned your "pre-processing (load summary) spreadsheet" and I wonder if you could describe it a little bit for those of us who would like to build one of our own. What is the format? Is it code specific or does it follow ASCE-7? Does it cover roof loadings only or wind, soil, floor loading etc. Do you taylor it to the project or type of structure? Do you work out both ASD and LFD?

If this should be in a thread of its own, maybe the board administrator can switch it into one.

 

[Ron]

jheidt2543...not much magic here. I use a spreadsheet to develop the unit loads on a canopy structure, usually a rigid frame bent. I have done three of them for this purpose, each geared to a different code or code aspect.

For example, the one I use most often is the one for ASCE 7. The spreadsheet uses typical wind and snow load input data (V,G,Cf,I,P,etc.) and computes different load cases based on the load combinations. The live load usually doesn't change unless there is a specification requirement to do so, and the dead load is pretty consistent for these structures by one particular manufacturer, so that rarely needs changing. The worst load case is then selected and highlighted for "unitizing" based on bent or beam spacing.

Almost all that I do with these is ASD. I'm still a believer in the ASD process. Limit state analysis under LRFD has merit and though mandated by some code provisions, it is my feeling that some analyses are better served in the ASD process.

The format is simple. I list the parameter, leave a cell for the input, then set up the calcs. I group each load parameter (wind, snow, seismic, dead, live)into a colored box and then accumulate all at the end. All on one page output. I'll see if I can post a pdf of one.

 

[EIT2]

Please excuse MY interruption also. I cannot find wind design load provisions for open structures.

I frequently design canopy structures for banks and entrance canopies. The design loads and applications that I incorporate are those found in the Wisconsin Enrolled Commercial Building Code Section 1609 for Wind Loads.

In this section are tables offering wind loads for roof overhang components and cladding. But nothing for open structures or canopies. I simply use these overhang design loads, apply appropriate factors, and apply resultant loads (pressures) in a purely horizontal, lateral direction toward the section profile as well as toward the underside of the structure for uplift.

Any comments or suggestions?

ASCE-7 shows coefficients for enclosed and partially enclosed structures. But, nothing for open structures.

I am interested in seeing diagrams for wind effects on open structures such as canopies.

Can anyone help?

Thank you!!

 

[jheidt2543]

EIT2,

The Wisconsin building code for commercial construction is the "Wisconsin Administrative Code, Chapters Comm 61 to 65, Wisconsin Commerical Building Code". In June of 2002, Wisconsin adopted the IBC 2000 code with their own amendments. Wisconsin also has a residential code. In the Commercial Code, Momm 62.1609 Determination of Wind Loads gives two options for wind loading, 1) follow IBC 2000 Section 1609.1.1 or under specified conditions you can use ASCE 7-98 Table 6-2. Both address open structures.

Ron,

Thanks for the note above. I am developing my own spreadsheets to summarize design loads. I end up working with both IBC 2000 and BOCA 96 so, I will likely have at least two basic spreadsheets. Then, I will modify the basic one to suit the project at hand. The nice thing about the spreadsheet is that on some projects you have two, three or more areas to calculate loads for, each in its own spreadsheet column. It makes a nice summary, once you fight your way through all the code requirements.

It is always nice to see how others are handling the same problems. Many hands make light work.........

 

[Ron]

jheidt2543...yeah, that's the way I do it also....different spreadsheet for each code. I also have to have a different spreadsheet for a gabled canopy roof as compared to the typical flat one. Different provisions apply.

I've made my spreadsheets "presentation ready" and include them in the calculation package. This makes it a bit easier for the EOR to check my work for compliance with his design criteria.

 

[ctg65]

Ron:
Could you post your spreadsheet for me to develop something similar.
Thanks

 

[SweetNlow]

A second round of support for 'Ron'. The basic design code for 'open roofs' he is refering to is ASCE7, upon which most other codes are based.

I only want to add that the type of structures that we allow to go through for permit at 75 MPH (90 MPH gusts) are fabric structures. Anything that if ripped apart could damage another structure or person, such as aluminum panels, I do not want to see flying.

 

[ERV]

Excellent software for wind loads per ASCE-7 is Wind Loads on Structures 2002. Published by SDG, Inc. Available through ASCE website.

 

[mdale]

Changing the subject alittle. You have talked about a canopy adjacent to a structure. Also, a canopy as an open structure. How about a canopy which stores RV's, etc... Eventhough there are no exterior walls, the wind would not necessarly pass by. The obstructions (RV's) would cause the wind to blow upward. So, would you use the wind pressures from the roof overhang components. Would you use the horizontal loads from the MWFR table (low roof angle)to design the cantilevered columns?

 

[SacreBleu]

mdale,
Speaking in a "aerodynamic" sense, it is relativly difficult to accurately quantify the uplift on a flat plate with wind moving parallel or slightly upwards. When wind is parallel, it will generate some uplift. Does ASCE-7 mention this?
Years ago, when we didn't have much in the Code to quantify the horizontal force on thin (not very much vertically projected area) canopies, we arbitrarily assumed a "guesstimate" friction value...otherwise the horizontal wind force was extremely low.

 

[dik]

SacreBleu... I think the uplift may occur from the wind being constrained by the building and consequentially pushing upwards (sort of like blowing into a bag where the bottom of the bag is the building). Adjacent to tall buildings, there's actually a downward component caused by pressure equilibrium.

Dik

 

[Structr]

I agree with SacreBleu...if he is speaking of an open canopy, away from any building.

dik... your comment is valid, especially the phenomenom of pressure equilibrium.