Black Locust 4x4x8

[Djs5017]

Hey y'all,

I've got 4 concrete piers that I've got to put posts on to support a beam that's estimated to carry as much as 15k lbs. If I round up to 20k for ease, that puts me at 5k per post. For the sake of this discussion, let's assume the posts are NOT braced or a part of a shear wall, and the posts need to be 8 feet tall.

I know, from digging around, I'm probably on the very outside limits for four pine, nominal 4x4 posts I'd get at a hardware store. Fortunately, I have a saw.

I'm considering oak or locust for my posts, and was curious what kind of support values I would be looking at. I've seen a couple calculators around here, and have attempted to use them with data input from MatWeb, but I'm not an engineer, and the numbers make my head spin a bit.

I know the compressive strength of the hardwoods are not the limiting factor, as in the case of black locust, the compression parallel to end grain is upwards of 10kpsi (which is insane). It's the other values that get my brain in a tangle.

Can anyone lend me a basic value for a 3.5x3.5 locust post (unbraced) at 96inches? Let's also assume they are of good grade.

Please and Thank you.

Dustin 

[scsmith42]

There is no way that I would want to stand under a 7.5 ton load only supported by 8' tall unbraced 4x4's!

Unless your load is perfectly centered across the 4 posts, odds are that one post may carry more loading than the others.

I think that I'd want to understand the engineering pretty closely on this.

[beenthere]

Likely the 4x4 is limited by the cross-section of a column at 8' length. DonP may have the right table to look at. 

But sawing your own, are you limited to 3½ by 3½ ?

[Djs5017]

There is no way that I would want to stand under a 7.5 ton load only supported by 8' tall unbraced 4x4's!

Unless your load is perfectly centered across the 4 posts, odds are that one post may carry more loading than the others.

I think that I'd want to understand the engineering pretty closely on this.

Thanks for the feedback. I share your thoughts. I'm working on a workaround that I'll write about below.

[Djs5017]

Likely the 4x4 is limited by the cross-section of a column at 8' length. DonP may have the right table to look at.

But sawing your own, are you limited to 3½ by 3½ ?

So.... It's it's a bit of a strange situation. I'm building a cabin to replace our current one. It's going to be much larger when it's finished. I'm working on the first part of the new structure alongside our current cabin. Then I'll be demolishing the old cabin and building onto its foundation. The foundation has the PBS ZMAX 4x4 standoff post based already installed into the concrete. 
Forgetting my new floor system (which is it's own issue), I have options:

Work with the connective hardware I have already installed in the concrete by using a strong enough/large enough post to make do. I could hypothetically just cut relief where it meets the hardware and leave the rest of the post considerably larger...or

Trim the flaps of the post base down half way, place a new form on top of the current piers, and pour a new cap (so to speak) on the existing hardware. 

Honestly, I'm leaning towards the latter option since I actually have some extra elevation to work with. It would be a strange job, but I feel confident that with a little rebar through the old hardware holes, I can tie the new caps onto the old piers well enough to hold a lot more than I'm asking of them. The other route would be to just dig out a bit around the current piers and fit a larger form around them completely... then pour.

Either way, it's a bit of a crap show. My calcs are based off a foot of wet snow on the 4/12 roof. Not normal in my neck of the woods (central PA), but you never know. 

[Don P]

"Not DanG likely"

Is the day you build for  . We've been hit by one hurricane here in the mountains in the 35+ years we've lived here, we didn't even have the windows and doors in yet.  Check the hazard tool for your design loads, it's in one of the sticky posts at the top of the timberframe section here.

I'm not understanding your build but can probably help a little. First I'd conservatively figure out the entire load per post, by conservative... err heavy. Then check to see if you have enough footprint on the ground. I assume 2,000 lbs capacity per square foot in good hard soils if I don't know anything more. If its plastic or gumbo that number is too high. Divide your load by the presumed soil bearing capacity and that is your minimum footing size. I suspect you'll be removing and replacing the footings and the connector can be easily changed.

If the existing is a go, you can kerf slots to sit over the existing connector, drill thru wood and steel and pin it. or scarf it off and weld to the existing pin.

A short column dies by crushing (although piers can overturn, uplift or slide, I'm using the term column advisedly). L/12 is max length for a short column. So a 4x4 at 4' tall only needs to check for crushing, maximum allowable compression parallel to grain. That 10k psi is an ultimate strength from the Wood Handbook. There are no published "allowable" strength numbers for locust but here I would use Beech/Birch/Hickory numbers as the closest. In #2 that is 750 psi. A nominal 4x4 is 12.25 square inches on end x 750=9187 lbs allowable in a short column.

Maximum column length in service is L/50, for temp construction I can go to L/75 and they can get spooky.
As you leave the short column slenderness and stiffness begin to come into play. Buckling is the failure. Stiffness is a "true" number so I would feel comfortable using the modulus of elasticity numbers in the Handbook and the "old" (pre turn of this century) method of designing a column. Go here;
Column Capacity Calculator (forestryforum.com)

Use 750 psi for compression, E=1.85 (which is Very stiff, but you knew that). I used your 5,000 lb load and passed with a 68% column capacity, or another way to look at it is slenderness reduced the capacity of the post by about 1/3.

[Den-Den]

So Don's calculations shows the column can support 5000 lb of compressive load and he mentioned checking for other loads.  Please follow that advice and consider side loads that may or may not be applied to these columns.  Usually side loads are from wind, which may blow off any snow accumulation.  Wind load and snow load are usually calculated as one or the other, not both at the same time.

[Prizl tha Chizl]

Hopefully not muddying the waters here, but why saw the posts at all? if maximum strength is what you're  after you only make them weaker by squaring them up, and for something like posts for a deck you've added work to a member that has no real need for four flat sides.
Don't use this as an engineering tool, but my understanding is that round timbers  can be about 50% stronger per cross sectional area than comparable milled Timbers.

[Djs5017]

Thank you all for the help!

There are some great considerations in here, and I've taken them to formulate a suitable plan.

Cheers everyone. Y'all rock.