[brentron]

I just finished designs for a workshop I'm planning to build on a 8.5'x14' concrete pad in my yard. I wanted to have a few non-standard things to give it a little dramatic flare and make it feel larger than it is:

• Oversized eaves on the side (2' instead of standard 1' or 16")
• Oversized front overhang
• Steep roof pitch
• Large portal opening in front

For those extra things, I wanted to ask and see if y'all think the way I have framed it will work.

Some details about the framing:

The shed is 24" on center, the front and back are 2x6 while the non-load-bearing sides are 2x4.

The roof rafters are 2x8s and are 24" on center lined up with the framing.

The side overhangs are supported by four 6x6 cantilevers that sit on top of the top plates. Each 6x6 cantilever is 9' long: 2' in the air holding up one rafter and 7' on the top plate held down by 4 rafter.

The portal opening is supported with three 2x10s. The framing around the portal opening has triangle blocking to help resist lateral forces. There are also straps tying the 2x10s to the jack/cripple studs, as well as to the concrete pad.

Hurricane ties are used for the rafters and in some strategic places to help prevent wind from lifting the large front overhang up.

Here is the sketchup file if anyone is interested:

3dwarehouse.sketchup.com/model/d16fdffa-3131-4866-b1dd-bb8cf8cba84c/8x14-Shed-Workshop

 

[Nealtw]

You can build the 2x4" overhang with out the poss holding them up unless that is wanted in your design?

 

[brentron]

You can build the 2x4" overhang

Not sure what you mean. The side overhangs are 2 foot long and are built with 2x8's, not 2x4s. Is that what you meant?

 

[Nealtw]

Not sure what you mean. The side overhangs are 2 foot long and are built with 2x8's, not 2x4s. Is that what you meant?

You can build the ladder to reach the second rafter and support that with the side wall

 

[brentron]

You can build the ladder to reach the second rafter and support that with the side wall

Ah, I see. I considered that, but was concerned that it would reduce the strength of the big front overhang by either putting notches in a rafter or by increasing the distance between rafters to 3' or 4'.

 

[Nealtw]

Ah, I see. I considered that, but was concerned that it would reduce the strength of the big front overhang by either putting notches in a rafter or by increasing the distance between rafters to 3' or 4'.

The side wall replaces the rafter over the wall so there is no strength lost.

After building the front and back walls and bracing them up straight and plumb
Just park a rafter over the side wall location, use that to locate the position of the top plate for the of the wall below that.

 

[brentron]

The side wall replaces the rafter over the wall so there is no strength lost.

Ah, but there is no side wall under the 4' front overhang.:wink2:

 

[Nealtw]

Ah, but there is no side wall under the 4' front overhang.:wink2:

And your point is? :biggrin2: There is no support under the front corner either way.

 

[brentron]

And your point is? :biggrin2: There is no support under the front corner either way.

Lol, that's my point. The only thing holding up the overhang on the high end the rafters, so I am worried about removing a rafter to support the side overhangs.

 

[Nealtw]

Lol, that's my point. The only thing holding up the overhang on the high end the rafters, so I am worried about removing a rafter to support the side overhangs.

Fair enough, you do add in what you get form the fascia but if you are not comfortable with that don't do It. :wink2:

 

[brentron]

Fair enough, you do add in what you get form the fascia but if you are not comfortable with that don't do It. :wink2:

I'll look into it some more.

 

[HotRodx10]

The following are only suggestions; if I'm overstepping on your or someone else's design, I apologize.


You could eliminate the diagonals in the front wall by providing adequate resistance by diaphragm action of the sheathing (as a shear wall, see the APA guide). My very rough guessimate puts the shear force from wind at around 2000 lbs. (I can run through how I got that, if you're interested) Per Table 1 of the guide, 15/32" OSB with 8d nails (#8 construction screws will be similar) at 4" spacing provides an allowable shear capacity of 510 lbs per foot of width. If the front walls are each at least 2' wide, you should have adequate capacity with that configuration. If you're concerned, you could put sheathing on both sides of those walls; it would still seem to be far less labor than the diagonals.



My biggest concern would be the adequacy of the straps from the studs to the foundation. You'll need sufficient anchorage to the slab, tensile strength in the straps, and shear strength of the screws into the studs, to transfer the force from the studs to the foundation. Again, using a guess of 2' for the width of each of the front walls, I get a force of about 3000 lbs. Shear strength of a #10 construction screw is in the ballpark of 300 lbs, so you'd need at least 10 of those screws in each strap into the studs.

 

[brentron]

Thanks @HotRodx10, you are way more knowledgeable than me about this! A few questions.

You could eliminate the diagonals in the front wall by providing adequate resistance by diaphragm action of the sheathing

I'm not familiar with this. Is this a way of saying that the sheathing can keep the rectangular frame from becoming a parallelogram?

Shear strength of a #10 construction screw is in the ballpark of 300 lbs, so you'd need at least 10 of those screws in each strap into the studs.

The concrete is already poured so I'm not sure if a normal construction screw will work. I was planning on using concrete anchors, though now I'm looking it up and I see there are concrete screws that might work. I'm not sure I could get 10 in, so maybe I should just use a couple of these, if I'm reading the specs right just they are rated over 2,000lbs each.

it would still seem to be far less labor than the diagonals.

It's an uninsulated shed and I'm planning on just using T1-11 shed sheathing. I suppose I could put some plywood or OSB under that... 🤔 but the blocking really doesn't seem like that much work, assuming it's adequate. Chop saw at 45˚, cut a bunch of lengths, nail them in bottom to top.


And yes, I am curious how you calculated the forces!

 

[HotRodx10]

I'm not familiar with this. Is this a way of saying that the sheathing can keep the rectangular frame from becoming a parallelogram?

Exactly. With adequate connection to the framing, the sheathing can provide significant resistance to racking of the structure (the rectangle becoming a parallelogram).

The concrete is already poured so I'm not sure if a normal construction screw will work. I was planning on using concrete anchors, though now I'm looking it up and I see there are concrete screws that might work. I'm not sure I could get 10 in, so maybe I should just use a couple of these, if I'm reading the specs right just they are rated over 2,000lbs each.

My comment about the construction screws was aimed at the connection of the strap to the sheathing and/or wall studs.

The holddown anchors you linked to would probably work, if you have an anchor bolt in the concrete with enough capacity. I assumed you'd be drilling bolts or screws into the side of the slab, where their resistance would equal the shear capacity of the steel shank of the fastener. If you use the holddown you linked to, the challenge is getting the needed capacity from the anchor bolt in the concrete, considering the minimal distance to the edge of the slab you'll have to work with.

I estimate it would take a 1/8" x 1" mild steel bar to provide the 3000 lbs tensile capacity I estimated. If it could be attached with a couple of anchor bolts embedded horizontally into the side face of the slab, and extended far enough up the stud to get the 10 screws into the stud, you should have a very sturdy connection.

All of this is based on a few conservative assumptions, however - 100mph wind speed and no redistribution of the wind load to the other walls. If the roof diaphragm is fairly stiff, as it should be, for the front wall to bend, the other 3 walls would have to rack somewhat as well, essentially twisting the entire structure. The reality is the whole structure works together, but as a structural designer, I don't generally count on resistances I can't quantify, even if I am fairly sure they are significant.

And yes, I am curious how you calculated the forces!

I assumed that the wind load (40psf) on the end of the shed is split evenly between the front and back walls, and the wall is 12' high - 40psf * 8.5/2 * 12 = 2040 lbs. The center of that load would be at center of the wall height - 6' up from the slab, creating a rotational 'moment', (aka torque) of 2040 lbs * 6' = 12,240 ft-lb. Each of the sections of the front wall carries half that moment, so 6120 ft-lb. Assuming the width of each front wall is 2' (the lever arm for the moment), the force at each end of the wall section is 6120 ft-lb / 2 ft = 3060 lb (up at one end of the wall section and down at the other). As I said, it's a rough approximation, based on several assumptions. If the wall section are wider and/or the building is shorter than I assumed, the forces would be less.

 

[brentron]

Fair enough, you do add in what you get form the fascia but if you are not comfortable with that don't do It. :wink2:

Hi Neal, I reworked the designs a bit, is this what you're suggesting? With this design, it looks to me like the long front overhang (the high one) doesn't have enough support since it's missing a whole rafter.

 

[huesmann]

You might wanna check ASCE 7-05 with all that overhang area and steep angle.

 

[HotRodx10]

I'm not really fond of that rework of the roof framing. You've doubled the effective overhang on the sides, which are supported almost entirely by fascia or rim boards on the front and the back edges. (The short ones in the overhang are not continuous, so they contribute very little to support of the overhang) You should have rafters that run front to back as far out to the end of the wall as you can get them, to shorten the cantilever supported by the fascia. You may need to double up those rafters, since they essentially support the entire cantilever.

It might look odd visually, but structurally the best way to support the cantilever is with beams running along the top of the front and back walls, extending out to support the outside rafters. It's similar to your original framing, except the beams can be as deep as you need them without raising the roof off the walls.

Similar to how it's better to have the tie-down points of the wall farther apart, deeper beams are stronger than wider beams of the same (or even greater) volume. For instance, in the strong direction, a 2x6 is stronger than a pair of 2x4s side by side.


You need to follow the load path, i.e. where doe the weight of, say, a couple of feet of snow go? It goes to the deck first, then to the adjacent rafters. From there it goes to whatever supports those rafters, and so on. In the case of the reworked framing, a load on the outer 2' of the cantilever goes to the edge rafter, and then to the corners, where all the load is concentrated on the end of the fascia board. From there, it flows to the next rafter and down that next rafter to the wall. So, all the weight applied to the outer 2' of the roof on the front half of the roof is carried by the edge rafter as a simple span, the cantilever of the fascia board, and then the cantilever of the next rafter. The longer the cantilever, the more stress in the boards.

 

[Nealtw]

Hi Neal, I reworked the designs a bit, is this what you're suggesting? With this design, it looks to me like the long front overhang (the high one) doesn't have enough support since it's missing a whole rafter.

If you turn the 2x4s on the flat you can put a 2x6 rafter under them on the outside of the wall..

 

[brentron]

You should have rafters that run front to back as far out to the end of the wall as you can get them, to shorten the cantilever supported by the fascia.

I'm not sure I understand what you're saying here. Do you mean the way I had it originally? Are you thinking of the front as the side with the portal opening or the side with the small door? We might be thinking of this 90˚ off from each other, which is maybe why I'm confused. I think of the high wall as the "front."

You may need to double up those rafters, since they essentially support the entire cantilever.

Slightly confused by this too... the way I see it, the rafters are the cantilever. Or do you just mean the whole part of the roof that sticks out is the cantilever? In any case, I definitely have been worried about having enough strength in the rafters for the cantilevered overhang (the high end, which I think of as the front.)

It might look odd visually, but structurally the best way to support the cantilever is with beams running along the top of the front and back walls, extending out to support the outside rafters. It's similar to your original framing,

I'm missing how that's different from my original design.

except the beams can be as deep as you need them without raising the roof off the walls.

If they're on top of the walls won't any height raise them off the wall?

 

[brentron]

@HotRodx10 In thinking more about your post and trying to understand, I put together another design to see if I can capture what you're saying. Is this what you're saying? Putting a 2x10 (or 12 or whatever) on the face anchored into the top, rather than actually sitting on top of the top plate?