How to design a greenhouse shed

[Usman Altaf]

Hi All,

I am designing a greenhouse that has steel as main supporting members and polythene sheet as cladding. Have a look at this image.
My Client wants a minimal design. The governing loading is wind load. 

We all know that polythene sheet has significantly less strength as compared to steel and designing the steel frame assuming that the wind pressures are fully resisted by polythene cladding is not wise and would lead to a over design.

 

I have following 2 questions:

1. Am I right that I should reduce the wind intensity/ wind forces on the steel members by a coefficient due to the presence of polythene sheet vs Steel, as it will burst/ puncture far earlier before resisting all the design wind pressures.

2. How to measure wind pressure sustaining capability of a polythene sheet. I knows its thickness (200 micron) and I know its tensile yield strength (20MPa). I know the spans across which its supported on the steel (300mm and 500mm). Is the method for calculating tensile stress similar to method used to determine tensile stress in steel sheet supported on all four sides (the coefficient method). or is there another method. Please guide. 

Thanks

Usman

[Ayesha]

11 hours ago, Usman Altaf said:

1. Am I right that I should reduce the wind intensity/ wind forces on the steel members by a coefficient due to the presence of polythene sheet vs Steel, as it will burst/ puncture far earlier before resisting all the design wind pressures.

This is tricky. The sheet would be punctured only if a sharp object tears through it. Surface area wind might not be able to puncture plastic. It might cause some permanent deformation (strain based) but the thing might stick there.

Enclosed or partially enclosed sheds also have internal pressure that adds to members in addition to externally applied pressure. 

There is no horizontal and vertical bracing provided for the structure . The structure might be unstable. There should be some bracing no matter how small or big the wind load is.

11 hours ago, Usman Altaf said:

2. How to measure wind pressure sustaining capability of a polythene sheet. I knows its thickness (200 micron) and I know its tensile yield strength (20MPa). I know the spans across which its supported on the steel (300mm and 500mm). Is the method for calculating tensile stress similar to method used to determine tensile stress in steel sheet supported on all four sides (the coefficient method). or is there another method. Please guide. 

You can get these values from the manufacturer. If the manufacturer that you are using has no data like that you can google to see what other manufacturers provide an establish a basis for your structure design.

Out of curiosity, how are these poles connected with the structure below? How are you resisting the tension due to wind.

 

 

 

[Usman Altaf]

7 hours ago, Ayesha said:

This is tricky. The sheet would be punctured only if a sharp object tears through it. Surface area wind might not be able to puncture plastic. It might cause some permanent deformation (strain based) but the thing might stick there.

Enclosed or partially enclosed sheds also have internal pressure that adds to members in addition to externally applied pressure. 

There is no horizontal and vertical bracing provided for the structure . The structure might be unstable. There should be some bracing no matter how small or big the wind load is.

You can get these values from the manufacturer. If the manufacturer that you are using has no data like that you can google to see what other manufacturers provide an establish a basis for your structure design.

Out of curiosity, how are these poles connected with the structure below? How are you resisting the tension due to wind.

 

 

 

Got your point.
Yes.

there is bracing provided in alternate/ end panels.

Manufacturer of materials normally share the mechanical properties. I have those with me. I was curious if method of determining plate stresses will be applicable or not.

The poles, i mean the columns have a proper concrete base, base plate and anchor bolts. They are not appearing in the picture.
Thanks for your response. 

 

So to conclude your point, you are opting that i should not reduce the wind intensity and total force applicable on frame elements?

[Ayesha]

On 4/23/2020 at 12:00 AM, Usman Altaf said:

So to conclude your point, you are opting that i should not reduce the wind intensity and total force applicable on frame elements?

You shouldn't do any reductions and also your members look too flimsy. You should check KL/r.I doubt they would pass.

 

[Nustian371]

Just to add a bit more to what is said above, these structures are a combination of Strut and ties capable of full design loads as per codes, they take tension and compression only with pinned connections. 

[Ayesha]

2 hours ago, Nustian371 said:

Just to add a bit more to what is said above, these structures are a combination of Strut and ties capable of full design loads as per codes, they take tension and compression only with pinned connections. 

This would be true if it was a pure space truss. The V in between will take the loads as compression tension couple, but the bottom and top chord will have flexural stresses as they are "bending".

 

[Simple Structures]

1. The wind loading should not be reduced, even with fabric structures - the give (flexibility) in the polythene  even could create dynamic loading (flapping of the sheet) causing both pressure/suction quickly, one after the other.

2. I can see that one gable end is closed. If the other end is fully open, then you can get is known as a "dominant opening effect" which increase wind pressure inside the structure. You see this is large aircraft hangers with large open doors. If both ends are open then the dominant opening effect is greater - for your structure you do not need to worry.

3. The issue will be wind uplift on the foundations - ensure weight of foundation is heavier than net total wind uplift... this is key check. I am assuming you have concrete beam strip foundation that the structure is bolted, and the factor of safety against wind uplift varies from 1.2 - 1.4 depending on code used.

4. The structure you have is a "tied-arch" in my view. You can use either a computer analysis or empirical formulae book for various wind loading - you can undertake a simple design using Roark's book.

5. For lateral (side-way) movement and sway I can see you have have a knee brace to provide lateral stiffness at the the eaves location (where horizontal tie / column meet).

This alone may not be enough and you may need to provide base fixity (4-bolts fixed base) at base of column and foundation junction.

6. Wind loading on tied arches can vary - here are a few permutations for you to thinking.

 

[Simple Structures]

Correction:

*You can see this IN large aircraft hangers with large door opening.....