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Old 04-16-2015, 05:28 PM   #1
kYLEMtnCRUZr OP
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Attn building engineers: I beam retaining wall question...

I beam retaining wall with wood spans

My question is would 3 8' 4x4"s with 1" gaps be stronger than a single 6' 2x12"?

Original idea was to do 6' I beam spacing with 2x12" spans (typical styled wall)

I now want to do 8' I beam spacing with 4x4" spans with 1" gaps. I think it would look cooler, save money by spacing I beams ($100 each) further apart, and not having to cut 12 footers into 6 footers THEN having to dip cut ends with preservative.

Thanks!
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Old 04-16-2015, 07:01 PM   #2
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There really isn't enough info. to make an informed decision.

But, to answer your question as asked, the 4x4's spanning 8 feet are about the same as the 2x12 spanning 6 feet, all other things being equal. So, theoretically, you could do what you suggest. Theoretically.

By the way, your i-beams will most likely need to be embedded nearly twice as deep as their above ground height.

The best thing you can do is to have a professional structural engineer look at what you are planning, and advise. A few hundred bucks spent on an engineer could save your life. Screw up a real retaining wall and you could kill someone.

A professional engineer will also have some ideas that could save you a ton of money.

Sorry to be so blunt.
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Old 04-17-2015, 04:15 AM   #3
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I recently built a wall and wanted some professional advice. I could not find any engineers out there willing to look at the project for less than a couple grand. It is not worth their time to look at a homeowners retaining wall project. On top of the price, they wanted to take core samples of the earth in order to make a informed decision. Makes sense to me, but not in the budget here. In the end, I did lots of reading about different techniques and materials. Decided on versalok because after excavation, no big machines were needed. I kept it at 4' so I did not need a permit. I put a 50,000 pounds of gravel behind the wall and lots of drainage pipe. It could have easily been 100,000 pounds of gravel if I had not kept the excavation 12" back from the wall. It was a fun project. Next up, put a garage below the wall...
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Old 04-17-2015, 12:56 PM   #4
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Quote:
Originally Posted by Anvil Block View Post
There really isn't enough info. to make an informed decision.

But, to answer your question as asked, the 4x4's spanning 8 feet are about the same as the 2x12 spanning 6 feet, all other things being equal. So, theoretically, you could do what you suggest. Theoretically.
.
Agree with the not enough info. Disagree with the 4x4s being the same as a 2x12. Remember, you aren't loading the 2x12 on the axis that is strong in compression. You are going to get a LOT more deflection out of the 2x12 than the 4x4. As far as soil samples, a decent engineer could assume a nominal pcf and not take any, but the wall may be a bit overbuilt.

How tall is the wall,and what will it be holding back? Half in half out is a good general rule like the poster above states, but often on small walls you don't need that much, especially if the lower end is going to be restrained by a driveway slab or other structure.
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Old 04-17-2015, 02:25 PM   #5
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The stress in a uniformly loaded beam is proportional to the length and inversely proportional to the section modulus. The section modulus varies with the square of the material thickness. If you assume 2" lumber is 1.5" and 4" lumber is 3.5" then the 4" lumber section modulus is 5.4 times bigger. So the stress in 6 foot 2" material will be about 4 times higher than the stress in 8 feet long 4" material. So to answer your original question, a wall made from 8 foot 4x4s will be much stronger than 6 foot long 2x12s (assuming you stack more of the 4x4s to get the same height as the 2x12).
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Old 04-18-2015, 12:53 PM   #6
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While soaking in the spa last night I realized there was a mistake in my analysis. With the longer span the load increases so the stress actually increases with the span squared. Therefore, the 2x12 case will have a stress of 3 times the 4x4 case not 4 times as I stated above.
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Old 04-18-2015, 01:25 PM   #7
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Unless he means to stack the 2x12s flat, but that seems unlikely.

Would there be any benefit to doweling the 4x4s? 1" dowel 2" long on 2' centers would tie each 4x4 to its neighbors above and below, effectively making them into solid panels, limited by the shear strength of the dowels.

I wonder about the sense of using wood in the first place, because it seems like rot would necessitate rebuilding the wall every 20 years or so.
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Old 04-18-2015, 01:26 PM   #8
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I think for there to be rot, it might have to rain there first.


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Old 04-21-2015, 04:08 PM   #9
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wide flange beam
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Old 04-21-2015, 07:43 PM   #10
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Thanks guys. The only other info I have is that theres nothing of value above or below the wall, (was a) pretty mellow slope, tons of roots so hopefully all it ever will retain will be what i put behind it.

Where there are redwoods, there is rot! The nice versions of this wall used pour custom concrete header sized "beams" and slide them in. Usually found supporting million dollar homes.

GSWayne your pondering in the spa of the section modulus is what i believed to be true but in much smaller words :) otherwise i will go into The lumberyard, set up a 6' gap and an 8' gap, lay said options over it, and stand on each one t see which one feels more sturdy. Without even thinking about it, i know the 2x12 would flex on its weak axis and the 4x4 would hardly budge. Its just the "cuts" or gaps in the final 4x4 product that makes me question. Compared to the solid piece of 2x12. But assuming weight will be evenly distributed across the wod then that shouldnt be a problem. Otherwise that dowel idea would sound good.

Like when you sit on a patio chair with the 2" vinyl straps, 2 or 3 straps are holding most of the load and stretch a good amount, compared to a solid vinyl chair which would distribute the load better and not stretch as far.

But the better example with wod being the compared material would be plywood, or skateboards to be specific, they utilize the plys and grain to make a stronger product than a comparable single layered piece of equal size

Like some headers are 2-2xs and some are 1-4x. For sake of simplicity say they are true 2" and 4", which one is stronger? I would think the variety of grain pattern could be used to the advantage of the 2-2x_ example. Perhaps stronger over time as rot or disease may be hindered by the tiny gap between the two boards?

For quick clarification, it is 2x12x6 vs 4x4(3)x8 with load against the wider surface, and for simplicity 2x and 4x equals a true 2" and 4", so 12" is the end result in both products. I initially said 1" gaps to take up the slack that multiple 3.5" products would produce. Even though i will need at least a .25" gap in my finished product if i choose to use the 4x4's

Thanks everyone!
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Old 04-21-2015, 07:47 PM   #11
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"wod" is ipad for "wood"

And "i" is ipad for i am too farking lazy to use the shift "key"
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Old 04-22-2015, 01:20 PM   #12
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You could make some forms and pour your own concrete beams and then not have to worry about rot, and people would think you had a $1,000,000 home.
Then you would have to figure out the rebar schedule, but rebar is pretty cheap (only $0.20/ft for 3/8" in Santa Barbara) so you could probably do a conservative design (like 2" spacing of bars located 1" from outer face of wall in 3" thick concrete) without spending too much money. From local Home Depot prices, a 3" x 6" x 8' concrete beam with 3 bars in it would weight about 150 pounds and cost about $12 in materials compared to about $18 for pressure treated wood (3.5" thick 6" tall).
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Old 04-22-2015, 01:47 PM   #13
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Rather than forming and casting beams and then having to lift and slide each one in place, could he slip two plywood sheets and his rebar mesh into the opening and cast his panels in place, then slip the plywood sheets out to do the next panel? He could then lever his new wall panels forward in the gap before backfilling. He'd want to vibrate the concrete to fill any voids, but that's doable. Maybe put pins through the plywood to help support the rebar, then withdraw the pins before attempting to slide the plywood out.
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Old 04-22-2015, 08:45 PM   #14
kYLEMtnCRUZr OP
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Boy you sure love your pins and dowels. Then again you are, statistically speaking, a man and we like to fill things with caulking.

Anywho you have a great idea there. The rebar could be weaved into the mesh, otherwise i dont know if the mesh would help with anything else.

And yes, casting my own concrete beams is an option, with manpower and time being the only variables.

Thanks guys!
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Old 04-22-2015, 10:12 PM   #15
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Oh, I didn't mean mesh as in screen or hardware cloth, I meant putting the rebar together in a way that it resembled mesh.
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