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Old 12-07-2020, 21:32   #1
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Design suggestions please for very large bimini, Materials and supports

I am designing a bimini 40ft wide and 19ft long. The center line will be supported by the boom. I have the following design questions.


Question 3 is the most important one.

1. Currently I have designed it to have 4 panels of cloth 59 inches wide. I am being quoted WeatherMax 80 by one vendor and Ferrari Stamoid by another. I am leaning away from the Ferrari Stamoid since it seams to be a PVC film and not a fabric. Any experiences with either?

2. I can design the slope from 10 degrees up to 45 degrees. The problem with the higher the slope, the more material and greater the wind load. However, a high slope has a faster water run off and looks cool. I have no problem calculating wind loads based upon finding the exposed vertical surface area.

Is it valid for me to use the exposed vertical surface area? I recall when caught in a typhoon in the Philippines that i could feel a lot of downward force of a high wind. However, the vessel is now in Langkawi Malaysia. I have never seen any thoughts on calculating downward wind forces and everyone seems to only talk about vertical forces.

Another problem I am finding is where to find record high wind conditions. Anyone have an idea where to find record winds for a given area?

3. The flatter the slope the less vertical wind resistance, however, the longer it will take water to run off. I can't seem to find any rule of thumb to calculate the weight of the water running off a slanted roof. I found that if I assume 2mm thick film of water during a monsoon downpour, that the weight of the water on the canvas is actually more force than a 35 knot wind. However, I just pulled 2mm out of thin air.

4. I am not a mechanical engineer, however, used beam calculators to try and find the deflection of the support under load. If any kind sole could check my work I would be very grateful. What I calculated seems too good to be true.

A. I found FRP pipe that is 50mm OD and 5mm wall thickness. The manufacturer never heard of Young's Modulus of Elasticity however, I found a website saying use 17 for FRP.

B. I used another website to calculate the Area Moment of Inertia and found for this pipe a value of 181000
https://www.engineersedge.com/calcul...re_case_12.htm

C. I used 15 degree slope case. I calculated wind load at 35 knots, weight of the fabric, 2mm of water on the canvas, and weight of the beam to be 130 kg per panel. This 130 kg seems reasonable to me.

D. I calculated the beam deflection as being only 8.5mm using this online calculator
https://www.epsilonengineer.com/even...alculator.html


This deflection seems to me too good to be true. Things I can do to improve support is to add a proverbial skycrane support coming down from the mast to support the canvas midway. This would reduce the unsupported length from 20.7 ft down to 10.35 ft and reduce design weight by half. Deflection of the beam when I enter these numbers is only 0.26mm which again seems too good to be true.

Note: I am intending to support the first and last of the 5 beams midway since these will see an additional load. The canvas will be held down using tiedowns leading fore and aft. I need to counter these two forces with an upforce since the tie downs will add a downward force.

Thoughts are greatly appreciated.
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Old 13-07-2020, 17:22   #2
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Re: Design suggestions please for very large bimini, Materials and supports

Given the way a certain group of posters here like to offer suggestions, I'm surprised no one yet has.

So let's give it a bump and see if there's any interest.

I have no official qualitications, other than building lots of stuff for a long time, so what follows are just "thoughts" --- and questions.

Do you plan on making the bimini foldable or otherwise easily removable?

The calculated load at 15 degrees seems at least reasonable, as long as there is only a 2 mm layer of water on it and the angle of incidence remains 15 degrees. I don't think that represents a real-world scenario though, because the wind doesn't stay parallel to the ground; it can vary from below horizontal to perpendicular to the earth's surface. Additionally, a 5/64" layer of water on a 15 degree surface during the monsoon season seems about 2-3 times underestimated, but that is a metric where the real value may be counter-intuitive.

Regarding the 'deflection' of a 2" FRP tube with .2" walls, almost certainly the weight of the pole itself, if supported only on the ends, would cause it to deflect by more than 8.5 mm. If you mean that after taking into account that 'initial' deflection, it only deflects 8.5 mm under the stated load, well that seems, as you say, too good to be true. To put it in perspective, imagine a weight of a pound and a half, which is the amount of load 2 square feet of the bimini material will support, as per your calculations, hung from the center of your 20' fiberglass tube. I'd guess the deflection would be more on the order of 10 cm, not 10 mm.

Without knowing more about your intentions and design, I'd lean toward a steeper angle to get the rain off faster, and the use of strong tension to keep the material as taut as possible, to the same end.


As far as finding the maximum wind, someone at the Malaysian weather service should be able to steer you in the right direction, or at least tell you if such records exist.
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Old 13-07-2020, 21:30   #3
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Re: Design suggestions please for very large bimini, Materials and supports

I located my error. The moment of inertia is not a dimensionless value. 181000 is the value for this pipe mm4 not m4. Plugging in the correct units into the calculator indicates these 50mm pipes with 5mm thick walls would not hold up to the load.

I am not keen on stainless steel but may have to go that route.
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Old 13-07-2020, 21:41   #4
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Re: Design suggestions please for very large bimini, Materials and supports

My next step is to talk to manufacture of the piping to see if they will make me a custom profile closer to 50mm OD and 25mm ID. This will still require me to add support from the mast, midway so the span is not 20ft by only 10ft. This also reduces load to 1/2 for each section.
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Old 13-07-2020, 21:45   #5
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Re: Design suggestions please for very large bimini, Materials and supports

Another thing to consider with a small angle (20 degrees or less) is the significant lift that would be produced in a strong wind. Shallow angle roofs usually fail in wind by being lifted off their building, not by lateral or downwards forces.
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Old 14-07-2020, 05:09   #6
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Re: Design suggestions please for very large bimini, Materials and supports

Have you considered anodized aluminum tubing for support? Light, and remarkably stiff
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Old 14-07-2020, 05:16   #7
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Re: Design suggestions please for very large bimini, Materials and supports

Quote:
Originally Posted by pbmaise View Post
My next step is to talk to manufacture of the piping to see if they will make me a custom profile closer to 50mm OD and 25mm ID. This will still require me to add support from the mast, midway so the span is not 20ft by only 10ft. This also reduces load to 1/2 for each section.
12.5 mm wall thickness is way overkill. 3 mm should be plenty, and will add much, much less weight.
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Old 14-07-2020, 08:38   #8
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Re: Design suggestions please for very large bimini, Materials and supports

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Originally Posted by jimbunyard View Post
12.5 mm wall thickness is way overkill. 3 mm should be plenty, and will add much, much less weight.
The manufacturer I am talking to makes pultruded fiberglass reinforced tubing. It is not where near as strong as steel hence the wall thickness of 12.5 mm is not even doubling the strength compared to 5 mm.

I may very learn that the price of custom pipe will be so much higher that I should switch to metal and aluminum is a good suggestion. Getting them shipped here will be fun.
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Old 14-07-2020, 10:53   #9
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Re: Design suggestions please for very large bimini, Materials and supports

Thought you had dispensed with the idea of frp tubes and was looking at stainless.

Thickening the wall on a fiberglass tube (or any tube) will increase stiffness only up to a point, from then on one's just adding weight.

Pulltruded fiberglass tubes are generally the weakest overall. A careful consideration of individual constructions and properties from tubes of various manufacturers may yield a more ultimately efficient choice.
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Old 15-07-2020, 07:01   #10
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Re: Design suggestions please for very large bimini, Materials and supports

I found a manufacter in China that can make me 100mm poles from carbon fiber 4mm wall. This will span the full 20.7 ft and flex only 3 inches under a full thunderstorm load. They will look good too.
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