T Nation

Engineers---A Little Help Please

Looking to have a pergola/shade structure built. Would like to know if the following is doable/safe/etc. Have been trying to find out online, etc but no definitive answers…

  1. pressure treated lumber grade #1
  2. 4 corner posts, 8x8, 18’ on center, cemented in ground 3-4’
  3. header beam consisting of (2) 2x12x20’ notched and bolted into each set of posts
  4. rafters consisting of 2x10x20’; rafter on each side of post bolted to post, 20 additional rafters spaced between this post/rafter set-up; rafters will most likely be notched to “sit” on header beam
  5. 8x8 cross bracing from post to header beam
  6. will be constructed in NE Ohio, if needed for weather variables
  7. pergola will sit about 13’ from ground to provide unobstructed view from inside house

Any information/insight would be greatly appreciated. Thanks.

Correct question to ask online:
Excuse me Mr. Engineer, did you like Inception?

Mediocre question to ask online:
Excuse me Mr. Engineer, how much weight can I put on a 2x4?

Cheap bastard question to ask online:
Excuse me Mr. Engineer, I’m looking to have a pergola/shade structure built. Would like to know if the following is doable/safe/etc. Have been trying to find out online, etc but no definitive answers…

  1. pressure treated lumber grade #1
  2. 4 corner posts, 8x8, 18’ on center, cemented in ground 3-4’
  3. header beam consisting of (2) 2x12x20’ notched and bolted into each set of posts
  4. rafters consisting of 2x10x20’; rafter on each side of post bolted to post, 20 additional rafters spaced between this post/rafter set-up; rafters will most likely be notched to “sit” on header beam
  5. 8x8 cross bracing from post to header beam
  6. will be constructed in NE Ohio, if needed for weather variables
  7. pergola will sit about 13’ from ground to provide unobstructed view from inside house
    Any information/insight would be greatly appreciated. Thanks, because I’m cheap and don’t like paying tradesmen for their time.

SHOULD BE FINE…just built similar although i would use 2 x 6 rafters it will redece weight dramatically and 2 x 10’s will look strange.

i spanned 18’ with 2 x 10’s doubled up exactly as you describe…main posts 6 x 6 sandwiched between rafters just as you describe…approx 12’ off teh ground…i was worried about rigidity until i got the frame work up and started tying it together…i weigh 275 and could walk anywher on it with no problems.


pic

6 x 6 posts
2 x 10 mains
2 x 6 rafters

i will be aqdding lattice and flowering vines next spring for additional shading and aesthetics

[quote]Enders Drift wrote:
Correct question to ask online:
Excuse me Mr. Engineer, did you like Inception?

Mediocre question to ask online:
Excuse me Mr. Engineer, how much weight can I put on a 2x4?

Cheap bastard question to ask online:
Excuse me Mr. Engineer, I’m looking to have a pergola/shade structure built. Would like to know if the following is doable/safe/etc. Have been trying to find out online, etc but no definitive answers…

  1. pressure treated lumber grade #1
  2. 4 corner posts, 8x8, 18’ on center, cemented in ground 3-4’
  3. header beam consisting of (2) 2x12x20’ notched and bolted into each set of posts
  4. rafters consisting of 2x10x20’; rafter on each side of post bolted to post, 20 additional rafters spaced between this post/rafter set-up; rafters will most likely be notched to “sit” on header beam
  5. 8x8 cross bracing from post to header beam
  6. will be constructed in NE Ohio, if needed for weather variables
  7. pergola will sit about 13’ from ground to provide unobstructed view from inside house
    Any information/insight would be greatly appreciated. Thanks, because I’m cheap and don’t like paying tradesmen for their time.[/quote]

And here you are on a free site for weight training and rehab information.

“Mr. Orthopedic surgeon, why does it hurt when I do this? My back hurts when I do this? My shoulder hurts when I do this? I am too cheap a bastard to go to the MD, get any imaging and go see a profressional.”

“Mr. Strength Coach, what can I do to increase my bench, squat, dead, lose weight, get jacked? Cause I am too cheap to pay someone for their time.”

See where I am going with this. I wasn’t asking you to design a new World Trade Center. Just what I am sure is a simple question for an engineer.

Thanks I appreciated your informative read.

[quote]morepain wrote:
SHOULD BE FINE…just built similar although i would use 2 x 6 rafters it will redece weight dramatically and 2 x 10’s will look strange.

i spanned 18’ with 2 x 10’s doubled up exactly as you describe…main posts 6 x 6 sandwiched between rafters just as you describe…approx 12’ off teh ground…i was worried about rigidity until i got the frame work up and started tying it together…i weigh 275 and could walk anywher on it with no problems. [/quote]

Thanks for the reply and information. Looks very similar to what I was thinking.

My reason for the larger size lumber is that it will be 14’+ off the ground to allow unobstructed view from french doors/transom windows. Similar to what you have in the picture, but the patio is 3’+ below the doors.

Thanks again.

http://www.awc.org/calculators/span/calc/timbercalcstyle.asp

Go here to figure your spans. With a pergola what you are actually looking at is more of a ceiling joist where the dead load is only the weight of the joist itself. Don’t put in rafter or you will get a much greater span.

My guess would be that 2x6’s will probably start to sag in very short order, not to mention how bouncy they will be. Play around with the span chart and I bet you’ll find 2x8’s are sufficient though. However, if you are planning on notching them I would definitely stay with the 2x10’s as you are creating a real weak point here. If you go with something smaller you can always just put some hurricane ties on instead. You also might consider some cross-bracing.

A true (2)2x12 beam and notched posts will be much stronger than the example more pain posted, but 18’ is an awful long beam. Probably fine for a purgola, but you’d hate to see it sag. You may want to go for a 3-2x12 or add a post. Beam calculations are more difficult than joist span so you can’t really just look up a chart.

I think you are probably fine, the only thing about your plan that really worries me is your hole depth. First you need to check where the frost line is for NE Ohio. Second, you have to remember that you’re going to have 13’ sticking out of the ground and there will be a lot of leverage at this point. You get some soggy ground and streamline winds and those joists are just going to be windbreaks. I could easily see a purgola going over in such conditions. If you’re going to be that tall, I’d probably go a bit deeper. Also definitely make sure to use a good form so you don’t end up with a bowl-shaped footing.

*Not a building engineer, just have some knowledge on the subject.

[quote]tedro wrote:
http://www.awc.org/calculators/span/calc/timbercalcstyle.asp

Go here to figure your spans. With a pergola what you are actually looking at is more of a ceiling joist where the dead load is only the weight of the joist itself. Don’t put in rafter or you will get a much greater span.

My guess would be that 2x6’s will probably start to sag in very short order, not to mention how bouncy they will be. Play around with the span chart and I bet you’ll find 2x8’s are sufficient though. However, if you are planning on notching them I would definitely stay with the 2x10’s as you are creating a real weak point here. If you go with something smaller you can always just put some hurricane ties on instead. You also might consider some cross-bracing.

A true (2)2x12 beam and notched posts will be much stronger than the example more pain posted, but 18’ is an awful long beam. Probably fine for a purgola, but you’d hate to see it sag. You may want to go for a 3-2x12 or add a post. Beam calculations are more difficult than joist span so you can’t really just look up a chart.

I think you are probably fine, the only thing about your plan that really worries me is your hole depth. First you need to check where the frost line is for NE Ohio. Second, you have to remember that you’re going to have 13’ sticking out of the ground and there will be a lot of leverage at this point. You get some soggy ground and streamline winds and those joists are just going to be windbreaks. I could easily see a purgola going over in such conditions. If you’re going to be that tall, I’d probably go a bit deeper. Also definitely make sure to use a good form so you don’t end up with a bowl-shaped footing.

*Not a building engineer, just have some knowledge on the subject.
[/quote]

Thanks for the reply.

I have used that chart before, thanks for the link.

I originally was thinking 2x10 for the header but sag was a possible issue. I was thinking the 2x12 for the 18’ span to eliminate any possible sag. I don’t want the third post, just want to keep it at the corners want a clear view.

The notching would be no more than an inch. Just something to lock it all in place add a little stability.

[quote]morepain wrote:
pic

6 x 6 posts
2 x 10 mains
2 x 6 rafters

[/quote]

what length are the 2x6 rafters? My corner posts will be 18’ on center, so all lumber will be 20’.

Thanks again.

I AM 18’ only in the longest span, then the joists run the short lenght so they are only 12’ and some of the is overhung so the unsupported span is only about 10’ … run your joists parellel to the short side…that is the majority of the boards and it will save significant money.

I will be an 18’ square, all 4 posts 18’ on center. So all my lumber will be 20’.

Did you cement the posts fully in the ground? I have read that the posts should sit on a concrete footer and then the hole is backfilled with sand/gravel/dirt. From what I have read, fully cementing the posts is less stable and the concrete shrinks leaving a pocket for water to fill and increase rotting.

In another life, before being a lawyer, I was a structural engineer, but I focused on blowing things up. I have, however, built a pergola and a deck that has survived 12 seasons on the Med shore.

The most important tip is #3.

  1. The tip re: span charts is very valuable. Note there is a typically correction to these for ambient humidity and temperature, or at least “zone.”

  2. You can do a lot more with spans by doubling up in close parrallel smaller planks. This is often a very good look. There are metal braces sold that can be slipped in to join planks in a very subtle manner.

  3. Many deck companies sell metal “corner” pieces that make the joint where the vertical meets the horizontal far more robust, by joining the corners with what we call a “lag bolt” in Israel. I do not know if this is the same thing in the USA. They can be concealed on the inside of the joining or the outside, depending on the astetic you want, and the type you pick. They also make the corner square and plum at the same time. When you have these installed the vertical stability of the uprights also becomes much greater. You can also push to the ends of the span charts a little more.

  4. The cement on the base of the uprights does provide some stability, but it is not as important as you think. A properly built pergola is essentially a table. It should be steady just placed on the ground. (Hence, why the corner joinings in #3 is so critical.) What the cement does do is: (A) keep the wood from rotting by being in direct contact with the ground and (B)provide some dead weight to keep the pergola in place if you get a strong up draft. On the topic of B, its a good idea to put some bolts or screws sticking out of the sides of the pole in the cement to “catch” the cement. (Think of a mace from medieval warfare — pour the cement around the “business end” of the mace.)

Also sold is metal sheeting that can go around a base to protect from rot. Looks like a long shoe box missing a side.

  1. The pergola I built was fairly large, probably spanning 15 meters. I built it in two sections (the long spans) and raised them together, leveled and plumed them — then poured the cement.

Opinions vary on this method. Most people probably put up one verticle, make sure it is plum, then assemble going in one direction (e.g., clockwise), making sure spans are level.

[quote]74 wrote:
I will be an 18’ square, all 4 posts 18’ on center. So all my lumber will be 20’.

Did you cement the posts fully in the ground? I have read that the posts should sit on a concrete footer and then the hole is backfilled with sand/gravel/dirt. From what I have read, fully cementing the posts is less stable and the concrete shrinks leaving a pocket for water to fill and increase rotting.[/quote]

This is true. A concrete footer also helps to make sure that, during instalation, you can more easily adjust the height of the post (by jamming crap under the footerm while cussing profusely).

[quote]Jewbacca wrote:
In another life, before being a lawyer, I was a structural engineer, but I focused on blowing things up. I have, however, built a pergola and a deck that has survived 12 seasons on the Med shore.

The most important tip is #3.

  1. The tip re: span charts is very valuable. Note there is a typically correction to these for ambient humidity and temperature, or at least “zone.”

  2. You can do a lot more with spans by doubling up in close parrallel smaller planks. This is often a very good look. There are metal braces sold that can be slipped in to join planks in a very subtle manner.

  3. Many deck companies sell metal “corner” pieces that make the joint where the vertical meets the horizontal far more robust, by joining the corners with what we call a “lag bolt” in Israel. I do not know if this is the same thing in the USA. They can be concealed on the inside of the joining or the outside, depending on the astetic you want, and the type you pick. They also make the corner square and plum at the same time. When you have these installed the vertical stability of the uprights also becomes much greater. You can also push to the ends of the span charts a little more.

  4. The cement on the base of the uprights does provide some stability, but it is not as important as you think. A properly built pergola is essentially a table. It should be steady just placed on the ground. (Hence, why the corner joinings in #3 is so critical.) What the cement does do is: (A) keep the wood from rotting by being in direct contact with the ground and (B)provide some dead weight to keep the pergola in place if you get a strong up draft. On the topic of B, its a good idea to put some bolts or screws sticking out of the sides of the pole in the cement to “catch” the cement. (Think of a mace from medieval warfare — pour the cement around the “business end” of the mace.)

Also sold is metal sheeting that can go around a base to protect from rot. Looks like a long shoe box missing a side.

  1. The pergola I built was fairly large, probably spanning 15 meters. I built it in two sections (the long spans) and raised them together, leveled and plumed them — then poured the cement.

Opinions vary on this method. Most people probably put up one verticle, make sure it is plum, then assemble going in one direction (e.g., clockwise), making sure spans are level. [/quote]

Much appreciated!!

What did you use for connecting the rafters to the beam? I have read several different options.

Thanks.

[quote]morepain wrote:
pic

6 x 6 posts
2 x 10 mains
2 x 6 rafters

[/quote]

The more I look at this the more it is exactly what I was thinking.

What is the spacing of your rafters?

What did you use to connect the rafters to the beam? I have read anything from toe nailing, toe screwing, hurricane ties.

Thanks again.

Most likely, code for your footers is 48" to prevent frost heave, as that is what it is in Michigan. Also, to prevent rot do not sink your posts in the concrete, there are many types of deck post anchors that keep the posts above grade.

[quote]74 wrote:

[quote]morepain wrote:
pic

6 x 6 posts
2 x 10 mains
2 x 6 rafters

[/quote]

The more I look at this the more it is exactly what I was thinking.

What is the spacing of your rafters?

What did you use to connect the rafters to the beam? I have read anything from toe nailing, toe screwing, hurricane ties.

Thanks again.[/quote]

i think they are about 6.5" on center…it comes out as adn odd number becasue the ones that “sandwich” the 6 x 6 posts are a given on location then you measire teh span between these and equally space the rest. One thing i will tell you is with the height you are talking about it will do nearly nothing for shade, you will need to skirt the top portioin and “roof” with lattice. you can easiily test this by putting a pole in teh ground of the height you are considering and see wher it casts its shadow during different times of day.

oh and i just used 3" zinc/yellow wood screws to toe screw the joists in…notching them leads to a whole mess of problems and extra work that is just not necesary…it is not a load bearing surface…although like i said i was fairly shocked that i could walk around on it fine with no sag at all.

[quote]74 wrote:

What did you use for connecting the rafters to the beam? I have read several different options.

Thanks.[/quote]

I used what are called “seismic ties” and construction adhesive ---- because we have earthquakes and the occassional grad missle, not hurricanes in Israel. Really anything would work. Deck screws at an angle and wood glue. They are not load bearing.

Look for something that you won’t be able to see and that will help you cheat into making the rafters square to the beam.

I think you would be able to see a hurricane tie. Maybe paint it first, then touch up after you install.

Look here, go through all the pages:

Hurricane ties have the advantage of being nailed, not screwed, so they go quicker. (Screws have a weaker sheer strength in this particular application.)

Don’t knotch stuff. What pain.

This is a slick post surface anchor system:

http://www.ottercreekforestproducts.com/titan-post-anchors.html