load bearing help

[Howard Callahan]

I'm almost done with a project that I'd like some help determining it's load bearing capacity. It's almost done so it's too late to change much, so I'd just like to figure out how much it will hold. The reason for this project is to support a 2400 pound boat and tandem axil trailer. I have a rv pad next to my garage that is elevated from the sloping yard with a rock retaining wall. It's been in place for over 20 years, but because of how it was built, I lost approximately 6 feet off the back of the pad so I only have 14 feet and I'm building a structure trying to recapture 6 feet. I'd like to ask my questions in little pieces. First, I've spand 12 feet with two 12 foot 2 by 12s prime doug fir boards that i screwed and nailed together and placed in joist hangers then for good measure I added another 2 by 12, 12 footer pressure treated construction select board screwed and nailed to the first two boards. What is the load bearing capability of this of this beam?

[Brian Tymchak]

Sagulater (https://woodbin.com/calcs/sagulator/) says that a 4x12 doug fir beam spanning 12' will have acceptable sag bearing a uniform load of 10,000 lbs over the span. Not sure how to calculate the benefit of the 3rd 2x board if is just attached to the 4 x12 vs actually bearing on the supports of the 12' span.

And welcome to the Creek!

[Bill Howatt]

Not a structural engineer! If the 3rd 2x12 is well fastened so the 3 boards act as one, and it's endpoints are near the ends of the 2x12, it should make the beam look like a 3x2x12 beam. I think the non-bearing area would be a very small loss assuming the joist hanger is within ratings.
Just a thought: What you might find interesting is to run the Sagulator calculation again but put 2 -2x4s on edge under the existing 2 2x12s and see what you get. The depth of a beam has a much larger effect on sag than sistering them.

[Jerome Stanek]

I think that the hangers are what will limit you

[Bill Dufour]

What about rot reducing load ability. Note that load tables are for stiff structures that do not bounce. For trailer parking who cares if it bends down 1/4" under load? Are you sure all this wood will be cheaper then getting in more fill.
No mention of what holds up the ends of this beam. Those need to hold up the entire weight as well. How tall will this be. What prevents racking and going down like a house of cards.
Bill D

[Bill Dufour]

see page three. A double 2x12 hanger is rated at just under 3,000 pounds each. Of course this is unrusted in non rotted wood.
Bill D

https://www.fastenersplus.com/cdn/shop/files/HUTF_HUSTF_Heavy_Top-Flange_Joist_Hangers_spec_2021.pdf?v=1260386797582 4274238

[Brian Tymchak]

Sagulater (https://woodbin.com/calcs/sagulator/) says that a 4x12 doug fir beam spanning 12' will have acceptable sag bearing a uniform load of 10,000 lbs over the span. Not sure how to calculate the benefit of the 3rd 2x board if is just attached to the 4 x12 vs actually bearing on the supports of the 12' span.

And welcome to the Creek!


Oops. Should have been 3x12. Guess I needed another cup of tea this morning. Sagulator still calls the sag of 0.10 in over the span acceptable.

Also not an engineer.

[Howard Callahan]

Thanks for all the replies and suggestions. I’ll be checking out sagulater to be sure, but it seems like my beam will work. I said I would ask my questions in little pieces, so my next question deals with the load bearing of a suspended concrete slab. Without getting into too much detail I’m going to have four 3 foot by 2.5 foot squares that I plan to build using 2 by 8s, nine inches on center, with 2 by 6s as the decking to pour 2.5 inches of concrete on. Each square will use about 3 bags of 80 pound concrete, so 240 pounds per square. I feel building it this way should hold the concrete initially, but what I don’t know is after it’s done how much will each square be able to hold. Your help again will be appreciated.

[Bill Dufour]

Add iron. This was figured out in 1889 San Francisco. The bridge is still in use.&nbsp;<br> A single slab will spread the load out more.&nbsp; With&nbsp; 12 foot long beams why only span 3 feet at a time tripling or quadrupling the cost of supporting posts.<br>
Bill D

[Brian Tymchak]

Thanks for all the replies and suggestions. I’ll be checking out sagulater to be sure, but it seems like my beam will work. I said I would ask my questions in little pieces, so my next question deals with the load bearing of a suspended concrete slab. Without getting into too much detail I’m going to have four 3 foot by 2.5 foot squares that I plan to build using 2 by 8s, nine inches on center, with 2 by 6s as the decking to pour 2.5 inches of concrete on. Each square will use about 3 bags of 80 pound concrete, so 240 pounds per square. I feel building it this way should hold the concrete initially, but what I don’t know is after it’s done how much will each square be able to hold. Your help again will be appreciated.

I'm having a hard time envisioning what you are building. I'm not in construction but I know enough to do some DIY concrete. Working backwards given your dimensions of the "squares" and 3 bags of concrete, your concrete pads are 4" thick. Concrete provides 3000-5000 lbs compressive strength per square inch, depending on the mix you buy. I think those ratings assumes 5" thick. Could not find strength ratings for 4" thick.

[Howard Callahan]

Without pictures it’s hard to understand, but I have two 12 foot long beams, 30 inches apart. They are elevated above the ground. I thought I would divide the 12 feet into four sections using a 2 by 12 in a joint hanger every three feet. In each section I would use 2 by 8s nine inches on center in hangers with 2 by 6s as the deck boards to pour the concrete on. When I use a concrete calculator for 3 feet by 2.5 feet by 2.5 inches, it comes up with 3 bags. I don’t have to do it this way and am just trying to come up with something that will hold the most weight. Any better ideas would be great.

[Bill George]

First without the actual weight your just guessing, weigh the trailer with the boat and your going to find it close to 3,000 lbs perhaps 4,000. I do not Get the concrete do your beams need to support that also? You can buy engineered floor joists that tell you the span and what they will support. I make beams out of 2x12s with 3/4 ply or OSB between with glue and then nailed.

[Howard Callahan]

Specs on the 22’ pontoon boat and trailer say 2200 lbs off their website. Rounded up for gas and equipment, probably pretty close at 2400lbs.

[Bill Dufour]

Put pan decking on top of the beams and pour a single slab with rebar.
Bill D

https://odonnellmetaldeck.com/resources/faq/what-is-pan-decking/

[Bill George]

Specs on the 22’ pontoon boat and trailer say 2200 lbs off their website. Rounded up for gas and equipment, probably pretty close at 2400lbs.

With outboard motor, battery, gas, anchor. canvas top and ?, My trailer has Load range E tires. Mine is the heaviest boat I have ever towed. Trailer alone is nearly 1500 lbs. My pontoon is a 23 foot.

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