Wind Loading On a Shade Structure

[jonnywalker]

Seems like a simple question but the loads that I am coming up with are huge! The structure is a simple cantilevered column that supports cables with fabric spanning between the cables.

Attached is a plan view of what is happening. On one side of the 60' span the columns are 18' and the other the column is 12'. The client doesn't want the cable so sag more than a couple of inches...

Ive gone into the ASCE 7 code and come up with a 13psf load @ the top half and 3.5psf for the bottom half of an open structure with a pitched roof. This load is being applied to a tributary area of 150 sq ft. Ive gone back and read my structrual analysis books but it seems to only address uniform loading...Ive also gone back through some old forums for some help....

My question is: is this the correct way to come up wind loading on this type of structure? I imagine there will be flutter, and neither method is valid for a structure with flutter.

If not this code would i use ASCE55 for Tensile Membranes Structures or ASCE74 for Transmission Lines? Would it be considered a tensile membrane? Ive also looked at transmission lines and investigated SAG10... Thanks for your time! JW

 

[MikeHalloran]

Won't those edge-supported fabric panels pool some water when it rains?



Mike Halloran
Pembroke Pines, FL, USA

 

[msquared48]

Yea Mike, but that will detract from the flutter... initially.

I think that 2" is way too little here - going to give high tensions to be resisted.

What is the snow load for the area? Any icing problems?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[jonnywalker]

No snow. No ice. Its in San Diego.

 

[Ron]

Depending on the character of the fabric, you might get some flow-through, but probably not much...still picks up most of the wind load.

Your cable span is long and your criteria tight...I would expect high loads on your columns. Further as MikeHalloran noted, with only a 5' fabric width, I would expect a lot of flutter, particularly from lateral wind load.

Run the numbers under ASCE 55. Keep in mind that you still have to weave your way back through ASCE 7 for code purposes.

 

[csd72]

johnnywalker,

Your biggest problem is your clients expectations - you need to manage them!

2" is far too small for this type of structure.

I would sell it as 'once every 10 years the deflection will exceed x inches but in most days it will be much less than that'

 

[SAIL3]

In addition to all the valid concerns already pointed out I would add the following:
The wind loading looks low to me, and looks like it was based on a flat panel.I see no restraint in the design that would ensure this.As a result, these panels could deform, twist in the wind that could give much higher lift and drag coefficients.
I don't see much restraint in the direction transverse to the cables so the the fabric could take on an airfoil shape etc.
Anyway, trying to develop any meaningful tension in the cables
off a cantilevered column is probably not the most efficient way
to achieve this. At least, I would run the cables over the columns
and anchor them at grade.
Flutter in fabric is not a good condition and could fray the fabric in short order.
Too many variables here to prevent this and any other random
vibration or resonance to occur and almost impossible to predict.
Any chance of using fiberglass panels?. They would, at least, have
some rigidity and may have a better chance of holding there overall shape during wind loading, whose magnitude is based on assuming a deflected shape and the correct lift and drag associated with that shape.
I guess you could use tear-away fabric(if it exists) and limit the loads that way or let nature do it for you at possibly much
higer loads and consequences.

 

[jonnywalker]

@Ron: I actually thought of looking into flow through the fabric last night, did research and came up with a number to reduce some of the area the wind would be acting on. I got that idea after reading MikeHalloran's comment....but the forces are still too large for a cantilevered column to support.

What confuses me is that Ive looked at a transmission line design that spans 700' and the tension in the cables are only in the 3000# range. My loads (due to just wind loading) are about the same even with multiple feet of sag and even higher with less sag.

@msquared48 and csd72: You are right...their needs to be more sag in the line, which I have tried, but the wind loads still end up being really high and produce a huge moment at the base of the columns.

@SAILS3: I think ultimately guy lines are going to be my best bet to achieve the desired cable tension. Not what they want but Im not sure I see any other way.

It is a 13psf load applied over a tributary width of 5'/2 for a 60' span....however I agree with you and think this is for a flat panel, but if the cables are tensioned enough and the fabric pulled tight couldnt it be considered close to a flat panel? I do see what your saying though. Ill look into fiberglass panels as well.

Thanks for the responses. Greatly Appreciated.

 

[MikeHalloran]

There used to be some shade structures over the sidewalks on the departures level at the Fort Lauderdale airport. They had cables sewn into the periphery of the fabric. They also had taller poles at the center of each support, so the cross section of the fabric at a plane transverse to the span formed an inverted V, and the transverse cross section at span center formed an inverted C. The cables were in tension, pulled down by crossbeams at the supports, so that the elevation view perpendicular to the span showed curvature in the top/peak, concave up, and curvature in the edges, concave down, and concave outward in plan view. The general shape of each span of fabric resembled a hyperboloid, with no upward facing hollows to collect rain. I think the design was copied from a larger sunshade structure in an airport in the Middle East.

The cables and supports, all very sturdy, are still there. The fabric was shredded by a hurricane, then removed. I'm guessing there was no money budgeted for replacement of the fabric after major weather events.



Mike Halloran
Pembroke Pines, FL, USA

 

[csd72]

The suggestion by sails3 of using the fabric tearing strength as a limit to design is a good one. We have used this method for fabric signs in various parts of the eastern states. You can get the fabric strength from the manufacturer and then I usually add 20% on to allow for variability.

Also remember that the cantilever deflection of the supports will also increase the deflection and therefore reduce the required tension.

Have you considered an arrangement like a cable stayed bridge with the column acting as a compression strut only (though you may need to tilt them outwards to get this to work).

Also, if this is a shade structure then there may be some porosity to the fabric which will help reduce the wind loads.

 

[zerosequence]

Hi jonnywalker,
Perhaps you can adopt a suitably coloured shade cloth that will look resonably opaque from the ground but lets most wind through.
Chris