posts on spot footings

[Unregistered]

Typically here, portals (porches) are constructed with joists ledgered to the house on one end, and carried by posts/beam on the other. The posts are set and leveled with two nuts/washers on threaded rods set in spot footings, appx 12" dia. x 24" deep.
I am planning to build a freestanding carport, appx 20' x 20' that will sit on 8x8 posts. I've been told by two people more knowledgeable than I that setting the posts on threaded rods is the way to go, but it seems that because the structure is freestanding, without lateral support, that even with diagonal bracing at the corners the posts would need to be set in concrete. Setting posts below grade (even treated) seems less than ideal.
Advice?

[Wgoodrich]

What you are discribing is done everyday throughout the country. It is called pole building construction style. Two things you must ensure one being either a cement pad placed under the bottom of the posts in the bottom of the trench or cement the post in the ground creating a wad of concrete adhering to the bottom of the post at least 1/2 filling the post hole. Both methods are to address rot in future of the post explained below.

The second thing to ensure is to discover frost depth rating of your area then make sure the bottom of the post holes are below that frost level.

The poles must be treated or cedar or other wood impervious to rot. Even treated post as you suspect will rot. However over the years it was discovered the rot occurs in these treated posts from the bottom center of the post up due to being in contact with earth. When the concrete donut is installed it allows moisture when draining away to allow that post center to dry not being in contact with earth. This trick greatly extends the life of the post. The concrete poured in the post hole adhering to the post creates a wad around that post making like a webbed foot print greatly increasing the foot print supporting the down force. This design makes it so that it does not matter if the bottom of the post rots away totally the wad of concrete keeps the post nailed in place to eliminate settling of that post that would cause damage to your building structure.

Post are fine if the Code is followed.

Be careful placement of those posts. Remember your purlins are serving as rafters carrying the same snow load as your normal rafter does. This purlin is a spanning lumber and unless you have an engineer supporting your design they must be on edge not laying flat.

Be careful the placement of those posts. Remember the boards installed at the top if supporting rafters or trusses are serving as headers carrying both a ceiling load and a snow load whether a ceiling is installed now or not when using trusses with a bottom runner. This header must be sized for the span carrying the load applied per IRC span charts. This header concern does not apply to gable ends or to those buildings where the truss is connected to the post not sitting on the headers. If the trusses are on the post not on the horizontal boards installed then those horizontal boards are top plate only tieing the building together for horizontal pull concern not verticle support concern.

Hope this helps

Wg

[Unregistered]

Is the other method acceptable also? Is it true that the shape of the structure (far wider and longer than high--22 x 22 x 9 (avg ht)) ensures that the lateral force is not an issue? This is NM, where there are no earthquake issues.

Thank you for the fast reply (and full answer) on the original question.

[Unregistered]

In the threaded rod instance, the posts are set an inch or two above the footing and the space is concealed/finished-out with grout or stone.

[suemarkp]

What I can't tell from your question is how far will the poles extend into the ground? A key design factor of a pole building is how far the pole goes into the ground irregardless of the frost line. If the pole depth is too short, they can lift out or fall over in high winds. I don't think I've seen a pole building table that allowed poles to be less than 3' embedded.

You can take care of the fall over aspect by having an anti-racking solution such as plywood sheathing or substantial diagonal bracing between the poles (notice though that having solid walls greatly increases the force of the wind so the sheathing needs to be well nailed to the horizontal girts and the posts). The lift out aspect depends on how these threaded rods attach to the poles. Are there heavy metal baseplates that screw to these rods which in turn have substantially long "ears" that nail onto the 8x8?

Perhaps I don't understand why the threaded rods are there or how they're used. It seems easier to just cut off any excessive pole length if you didn't embed them all the same. Why mess with a leveling system? Putting poles into the ground as Wg described sounds more typical.

[Wgoodrich]

Size and hieght of wall only becomes a concern when you get about 120 mile per hour wind factor or have a siezmic rating in your area. Frost creates heave requiring any post hole or footer to be below that frost line in that area to prevent heave.

Post buildings are sized in posts and depth of holes depending on frost line and wind speed factor compared to size of wall being hit by that wind. The higher the wind factor the bigger to post and deeper it must go but always below the frost line unless less than 120 square feet as exempted as an accessory building without heat.

The building codes require all buildings larger than 120 square feet to be attached to the earth and below frost line per the new Code versions.

While there is a post building society that post building design is not expempt from building codes or engineering support backing an alternative design.

Wind bracing such as plywood or straps and cables is one form of wind bracing depending on wind factor of that area. However posts installed in teh ground sized to meet the wind factor also is a manor of wind bracing of walls depending on the area you are in.

New Mexico to my knowledge is basically 90 mile wind and little or no siezmic concern factors per Building Codes.

Good Luck

Wg