Hoop House Design 1

[jandlo]

I had a client come to me for structural analysis for hoop houses he's been building for nearly a decade with no problems. He will be using them in a situation where it will no longer be considered a temporary structure. Because of this the jurisdiction is requiring calcs showing it meets the governing building codes for permanent structures.

When running the numbers, the steel frame is over-stressed in some places on the order of 7x. I don't have experience with this structure type. Am I missing something? I'm using RISA3D. I have tried modelling the entire structure and a single frame, and still not coming close.

It's roughly 30'x80' and is built using 13-16 GA, 1.315" - 1.66" OD 50ksi pipe. The image below is the typical frame at 4' oc. They are mainly built in Michigan.

All suggestions are appreciated.

 

[DoubleStud]

Dont have experience but did you put a roller at one support or pin-pin?

 

[jandlo]

I originally had them both pinned, but changed them to both be fixed as they are typically set in concrete like a fence post. Of course this helped with deflections but not strength.

 

[hokie66]

I'd say that structure is VERY temporary.

 

[ALK2415]

what kind of cladding [or sheet covering] you expect to be applied?
is it for green house [plantation] ?
from there you could reduce your expected loading [mostly wind/rain loading]
Also
use cross tie rods/bars (pretension using turnbuckle) this will reduce your CHS to 60-80%
this will increase your truss stiffness [in both direction against lateral loadings]

 

[SteelPE]

My parents own a commercial greenhouse business.... Luckily for me, they haven't built a new one since I became licensed.

These things fall down all the time..... and when I mean all the time, I mean all the time. I would attribute 95% of collapses due to neglect. They are meant to be kept heated all the time to eliminate snow build up on the roof (yes, all day every day).... and even then, special care needs to be taken to eliminate snow loads from the roof. My parents clear off the snow from the roof after large storms so they don't lose their investment. They built their first greenhouse in 1984 and it is still standing to this day.

I had a client ask me just last year to evaluate a similar structure he had put up on his property to act as a salt storage area (the building official wanted him to either get an engineer to sign off on the structure or take it down). I told him I was not interested and told him to contact the greenhouse manufacturer and hung up the phone. I drive by the place almost every day, and his structure is still there, so I don't know who he got to sign off on it.

Most people use these an unheated structures.... which is not their intended purpose and why they collapse all the time. Check the structure as if it was heated 24/7, reduce your snow loads accordingly and see what that does to your stresses.

 

[jandlo]

This is helpful. I can likely reduce the snow load with the assumption that it will remain heated since it is a greenhouse. That does not help with wind loads which are resulting in the base members to be overstressed. Tie rods don't help much in that case either. Perhaps the flexibility of the frames allow the load to redistribute substantially under wind loads.

 

[ALK2415]

Tie rods works like pre-Stressed structure, counter the expected wind lateral forces, So they do matter
the total structure weight reduced just by adding them (have done this procedure multiple times with STAAD Software)

 

[XR250]

Seems kinda dumb to design a structure to be heated all the time. What happens when you run out of gas?
Same as designing a full bathtub basement and counting on a sump pump to prevent it from flooding.
I suppose the risk of this is mitigated by the inexpensive up-front cost of the spindly structure.

 

[JLNJ]

You get a reduction for it being a Type I structure as well. Supposedly when it fails it kills just plants (or animals).

You might be able to play around with the way the connections are modeled. For example, maybe the posts are fixed at the base and pinned at point "A".

 

[SteelPE]

To be honest, I am not even sure how you would analyze these structures for wind. I find it difficult to believe that they would collapse due to wind.... more like blow away. My parents structures have louvered fans in the gable ends. These are used for ventilation. However, they would open by themselves quite often when the pressures on the inside of the structure exceeded those on the outside of the structure.... meaning, I don't have any idea what you would use for an internal pressure coefficient (since it seems it would be difficult to have an internal pressure built up in the structure).

Plus, my parents structure is clad in two layers of opaque plastic.... with a small blower fan that runs 24/7 to keep the space between the two layers of plastic inflated (this is where the insulation comes from). Not sure if you have a similar situation with your project.

Seems like the whole project is just never ending.... and never worth the design fee.