Please Critique My Porches Construction

[mgramann]

The rough in has been "completed" and inspected for my porch in the Michigan U.P.  Other than need deck ties in the corners, and larger screws holding the posts in place, he passed it.  He said that he himself would have put posts larger than 4x4s in place, but that I should be fine.  They are 8ft O.C., just under 10ft from the house.

I wanted to run it by my friends here.  I plan to install the ties and larger screws per his suggesting, and probably will install at least one 2x4 next to each 4x4, increasing post size by 50%-possibly two if you all think that would be best.

The only other thing I thought might be necessary is "lateral bracing" between the posts under the deck.  Alternating angles to prevent any "tilt"  The inspector said nothing about this though, and I wonder if I am just being too critical.

Other pertinent information:12" concrete piers with 28" big foot STYLE (apparently I need to specify) footings, 60 PSF snow load(geographically close to 70)  You can see in the pictures that the roof will be steel.

Pictures are below, what do you all think?  The guys that did the building haven't been paid yet, so if changes are needed, I am still the boss.

[Don_P]

The posts look to have about 5200 lbs on them at design load, if the longest are about 6' the ultimate capacity is about 11,000 lbs. They do have insufficient bearing area under the rim to safely prevent crushing of the rim. a 6x would have helped there, I'm not a fan of 4x4 posts either. There appears to be a nissing corner post under the hip

The ceiling joists should be running perpendicular to the house. with the house forming a rigid base to which the deck diaphragm is attached I dont see that kneebraces are neccessary. Some of the piers appear to be eccentrically loaded so bracing wouldn't hurt

[mgramann]

Thanks Don!

By 6ft you mean the height under the "deck" correct?  They are all around 5ft, so we should be set there.  Do you think putting a 2x4 next to each post would solve the bearing problem?  1 or 2?  The hip doesnt have a sonotube under it, but the other 2 tubes are within 2ft of that corner.  I don't think this is a problem, but may be wrong.

I thought the ceiling was odd too.  The joists run both ways.  it will only support soffit material and a few can lights, so I dont think its an issue.  Correct me if i'm wrong.

Tha's good to hear about the other bracing.  I'm hoping that adding the 2x4 will center the loads on the sonotubes.

Thanks again.

[Don_P]

The bearing issue, or whether to add more 2x4's, I'd keep an eye on the deck rims over the posts especially when there is a full snow load, if you see crushing then add something... remember we design to "allowable stress" but it may be the wood can take higher load without a problem. At the design stage it should have been checked but now that the deed is done I'd practice watchful waiting.

The corner post, you may well be fine, again not how I would have done it but it'll probably work, look for distress under load again.

The correct way to have done the ceiling if you wanted the joists running the length of the porch would have been to run header joists perpendicular to the house, from house to carry beam and then to run the parallell joists hung from those "girder" joists. Right now it is not very strong. Strong enough? your call, I'm not really sure what is holding the outer one up  Some more hangers from the rafters might be worth considering. Please give the contractor a heads up, he needs to quit doing that.

Eccentric loading of a pier is a no-no but is pretty common. Doesn't guarantee a failure but is often what you can point to in a failure. When the load comes down off center it tends to make the pier want to kick out to that side, that was what was causing the piers to tip on the stage I described a few weeks ago. Bracing wouldn't hurt but to do it's job it needs to be well attached.

Hope that all helps.

[mgramann]

The problem with waiting is I would have to remove the railings to make adjustments, which I really don't want to do.  I guess I was curious not so much if I had to, but if adding it would solve any potential issue and bring it into spec.  Do you think one 2x4 would eliminate any potential for problems?

I'm not too concerned about the cantilevered corner.  It was tough, we wanted to center the post openings around the doors and have them even, so we spaced everything accordingly.  There wasn't a really great solution.

I'm really not sure why they did the ceiling that way.  I assumed I was getting as you described, perpendicular to the house, and didn't request anything else.  Right now, I wonder if they are hanging on the edge of the 4x4.  If I beef up the posts by putting a 2x4 on one side, and a 1x6 on either side (cosmetic purposes), the 1x6 would support it.  Maybe I can install some hangers.

Regarding the eccentric loading, I think installing a 2x4 under the porch will bring the load more to the middle.  That should help alleviate the issue there.

Thanks again for weighting in Don-it's really appreciated.

[Don_P]

Ahh, I think I was looking at the pic wrong. are the ceiling joists that are perpendicular to the house one solid joist from house to post and then the parallel joists run between those? If so he is fine. I was thinking the perp members were 2' pieces of blocking earlier.

I don't think we're worried about the posts that run from deck to carry beam, let's do the math.
Each post is supporting 4' each side of the post in the carry beam (8') x (half the distance to the house (5') + the overhang (2'))=56 square feet X (70psf snow + 10 psf dead load)=4480 lbs. Divide that by the bearing area of post to carry beam 3.5" x 3" = 10.5 square inches = 426 pounds per square inch. The carry beam looks to be SPF lumber, the allowable compression perp to grain is 425 lbs... close enough.

I just found an error from my calcs last nite, I was really worried about crushing in the rim joist below by the bottom 5' post. The deck is carrying the same tributary width, 8' X half the 10' depth (5') so trib area is 40 sf x (40 psf live load + 10 psf DL)=2000 lbs. Add the roof load to this and the total load on the rim at the post top is 6480 lbs (I forgot the roof overhang load last night). The allowable compression perp to grain for #2 SYP which I'm guessing the treated (double) rim is would be 565 psi. This is the joint I was concerned with and it doesn't seem to be too far off if it is a double rim and SYP, you're at 617 psi under full load. There isn't a rail down below is there? That is the area I as talking about watching. Now you see how I was doing the math so I'll leave the choice of whether to beef it up to you 

[mgramann]

I'll have to look at the ceiling closer.  So even without hanger, and just screws, the ceiling should be adequately supported?

Thank you for talking me through the post calculations.  You had me up until asking if there was a rail below.  I wouldn't mind if you elaborated there some.

After looking at the math, I think I will put a 2x4 on the lower posts, as it reduces the PSI at the top to 432, and centers it more on the concrete.  The question is, do I need to put a bracket under that 2x4, or is a tight fit against the concrete adequate-I would think so, since it will be nailed to the 4x4 which is properly mounted via post brackets.

The only reason I was so concerned about the upper posts was that you could really feel the tension in them from the roof load.  I know it can probably take it, but still, the inspector pointed it out which caused me a bit of worry.  It sounds like I'm within spec for the upper deck posts.  It doesn't matter that they are positioned on top of the "decking" material, and not directly on the double rim, does it?

Thanks for answering all of my questions so patiently.  I want to do this right before "undoing" is any more tedious.

[Don_P]

Tight to the concrete and rim with the 2x4 and very well nailed should help it share the load.
Ignore my rail comment, I'm confusing things.
You should have hangers on the ceiling joists... regular "deck" screws are never to be trusted as structural connectors, they are brittle and unreliable. That's why joist hangers specify nails (they are ductile) or structural screws ($!)
The posts can rest on the decking, the same type of compression math applies that we just did. The posts should be connected down to the foundation below through a load path that takes care of not only the downward gravity load but also adequately resists the upward wind uplift load, straps connecting post to rim to post, lags, or...., more than one way to skin that cat, the idea is to chain the roof down to the footing with something adequate against the wind if it gets underneath and tries to lift the roof.  The posts to carry beam should continue that load path through some type of uplift connection and then the rafters should have hurricane ties connecting them to the carry beam. We tend to think about only downward forces but wind uplift can be a concern especially on something like a porch roof.

[mgramann]

You have given me a lot to look for this weekend.  Thanks Don, I really appreciate it!