Calculating Load Capacity of Steel Cage with A-frame Roof

[diy-er]

I have a steel cage 7 ft by 11 ft and 7 ft high. It's made out of standard 1 in square steel vertical tubing. Each vertical tube is spaced 18 inches apart all the way around the cage. All vertical posts are welded to a 2 in L-bar all the way around the top, and the bottom.

I'm trying to calculate the load this structure can carry if I install an A-frame style roof on it.
It's across the 11 ft width, with center roof support in the middle, by means of a 2x4 across the 7 ft span with vertical 2x4's 16 in apart carrying the roof. Similar 2x4's laying across both ends, where the roof ends.

All I find online is load calculations of horizontal square tubing, nothing on the vertical load. Any help is appreciated.

 

[Lnewqban]

Welcome, @diy-er !

Perhaps your main issue is not the ability of the cage to support the weight of the roof, but wind loads (anchorage of roof to cage to ground) if the roofed cage is going to be exposed to weather.

 

[dlgoff]

2x4's

 

[berkeman]

I'm trying to calculate the load this structure can carry

What keeps this structure from racking? You did not mention any diagonal members or shear wall structure.

What is this enclosure going to be used for? What kinds of things can happen if it fails?

Can you upload some pictures or mechanical drawings to help us see what you are asking about? Thanks (use the "Attach files" link below the Edit window)

 

[berkeman]

standard 1 in square steel vertical tubing

Oh, and what is the wall thickness of your steel tubing?

 

[diy-er]

Welcome, @diy-er !

Perhaps your main issue is not the ability of the cage to support the weight of the roof, but wind loads (anchorage of roof to cage to ground) if the roofed cage is going to be exposed to weather.

Thanks for the reply. It's on the east side of the house , the prevailing wind is from the west, so barely ever touches the structure. I do need to know if it can support the roof, so I can get a building permit for it. Thanks

 

[hutchphd]

Snow load? Roof pitch? You will need to be much more specific and a good annotated drawing is the place to start

 

[diy-er]

What keeps this structure from racking? You did not mention any diagonal members or shear wall structure.

What is this enclosure going to be used for? What kinds of things can happen if it fails?

Can you upload some pictures or mechanical drawings to help us see what you are asking about? Thanks (use the "Attach files" link below the Edit window)

Thanks for the reminder, yes it has flat crossbeams 1 in wide. a cross from each corner on all 4 sides.

 

[Lnewqban]

Thanks for the reply. It's on the east side of the house , the prevailing wind is from the west, so barely ever touches the structure. I do need to know if it can support the roof, so I can get a building permit for it. Thanks

The building department review will require a wind lift calculation, resulting in strapping specifications.
Normally, uplift forces due to wind are normally greater than down-weight.

Regarding the cage’s weight carrying capability, you will need to calculate the weight of the built roof, acumulated snow or water, as well as the locations of the point through which that weight is transferred down from roof to cage.

 

[berkeman]

I do need to know if it can support the roof, so I can get a building permit for it.

It's great that you are going to pull a permit for this; that is something we like to see our DIY users do here at PF.

It still would help a lot to see a drawing of this structure. What kind of documentation do you need to submit to the Building Department to pull the permit? How will this be anchored to the ground? I assume based on the size that it is for storage and not human habitation, right?

 

[diy-er]

What keeps this structure from racking? You did not mention any diagonal members or shear wall structure.

What is this enclosure going to be used for? What kinds of things can happen if it fails?

Can you upload some pictures or mechanical drawings to help us see what you are asking about? Thanks (use the "Attach files" link below the Edit window)

Basically it was used to ship CNC machine from Taiwan to the US. The company I worked for was throwing it out... I was able to bring it home. It will double as a mudroom and greenhouse, hence all glass panels around 3 sides. Unfortunately no drawings are available, so I am trying to 'calculate' it... Thank you

 

[diy-er]

Oh, and what is the wall thickness of your steel tubing?

0.12

 

[berkeman]

Thanks for the picture; it's a big help. Several other replies so far mentioned snow loading, and from the pitch of your house's roof it looks like you do get snow where you live. What shape were you planning for this addition's roof? I would think that it would need to be a single inclined piece with the same pitch as your house's roof and angling up into the east side of your house, right? You would want the snow to be falling off the extension's roof toward the east, and not building up between the top of a peaked roof and the east side of the house.

 

[berkeman]

BTW, can you anchor the structure and the peak of the angled roof to the east exterior side of your house? That would give you a lot better structural integrity and help to weather seal the roof peak against the side of your house.

 

[diy-er]

Snow load? Roof pitch? You will need to be much more specific and a good annotated drawing is the place to start

pitch is 3/12, it will be metal roof, warm underneath, so mostly no snow will stick

 

[diy-er]

The building department review will require a wind lift calculation, resulting in strapping specifications.
Normally, uplift forces due to wind are normally greater than down-weight.

Regarding the cage’s weight carrying capability, you will need to calculate the weight of the built roof, acumulated snow or water, as well as the locations of the point through which that weight is transferred down from roof to cage.

it is more a reverse engineering question, I need to know what the steel construction can carry. That way I can figure out what kind of roof I can safely build on top of it. Thanks

 

[diy-er]

Thanks for the picture; it's a big help. Several other replies so far mentioned snow loading, and from the pitch of your house's roof it looks like you do get snow where you live. What shape were you planning for this addition's roof? I would think that it would need to be a single inclined piece with the same pitch as your house's roof and angling up into the east side of your house, right? You would want the snow to be falling off the extension's roof toward the east, and not building up between the top of a peaked roof and the east side of the house.

double incline, downslopes facing south and north, indeed same angle as existing roof line.

 

[diy-er]

It's great that you are going to pull a permit for this; that is something we like to see our DIY users do here at PF.

It still would help a lot to see a drawing of this structure. What kind of documentation do you need to submit to the Building Department to pull the permit? How will this be anchored to the ground? I assume based on the size that it is for storage and not human habitation, right?

It will not be for human habitation, it is basically just to keep the weather out of the house, and it will be housing a lot of plants, like a greenhouse. Did you see the picture I attached in an earlier post? The building dept needs to know if the structure can carry the load of the roof (and snow...). I'm trying to figure out what each vertical post can carry, the way they are assembled as described in the post. Thanks again

 

[berkeman]

Did you see the picture I attached in an earlier post? The building dept needs to know if the structure can carry the load of the roof (and snow...). I'm trying to figure out what each vertical post can carry, the way they are assembled as described in the post.

TBH, the frame looks pretty flimsy to me, but I'm an EE not an ME. What will the sides be made of for this greenhouse? Just plastic sheets that offer no support or maybe Lexan sheets that can be part of a shear wall structure plus add compressive support? I guess at a minimum if you are using plastic sheets for the walls of this greenhouse, you could make the "roof" a thick plastic sheet that you pull from the structure's east wall up to just under your house's roof. That will give a nice tall angle for dropping off any snow load, and will not add weight to the structure that it has to support with those thin wall beams.

 

[Baluncore]

The wall height is 7 foot = 84 inches. The 1"x1" vertical columns are therefore 84 times longer than their width. That is greater than about 60, so there is a column stability problem. Buckling failure of the columns will be critical and must be analysed. Check the degree of freedom at the column end attachments. Design in some cross-connections between columns that will reduce the free length of columns to prevent buckling.

The walls must support the structure without the cladding in place. The walls must survive any probable side force applied. What might those forces be in this case ?

 

[diy-er]

TBH, the frame looks pretty flimsy to me, but I'm an EE not an ME. What will the sides be made of for this greenhouse? Just plastic sheets that offer no support or maybe Lexan sheets that can be part of a shear wall structure plus add compressive support? I guess at a minimum if you are using plastic sheets for the walls of this greenhouse, you could make the "roof" a thick plastic sheet that you pull from the structure's east wall up to just under your house's roof. That will give a nice tall angle for dropping off any snow load, and will not add weight to the structure that it has to support with those thin wall beams.

The steel structure is stronger than it looks. It will have used aluminum frame full glass panel patio doors screws to the inside of the structure.

 

[berkeman]

It will have used aluminum frame full glass panel patio doors screws to the inside of the structure.

Sorry, I'm a bit dense at the moment. You mentioned "greenhouse" which implies transparent roof and side panels. Are you a time traveler with access to transparent aluminum?

 

[diy-er]

Sorry, I'm a bit dense at the moment. You mentioned "greenhouse" which implies transparent roof and side panels. Are you a time traveler with access to transparent aluminum?

roof is sheetmetal and will have A frame roof with 3/12 pitch on it both slopes on the 7 ft sides.