Structual Repair with metal components

[biggjb]

This is more a theoretical question as I have not seen posts pertaining to using steel 2x4s to replace rotten wooden ones. My camper has some significant repairs and I thought about trying to incorporate metal frame components in order to reduce weight. I can see a problem if the wooden part only needs to be partially replaced, however, if the whole component requires replacement could one use a metal component in its place? Any thoughts would be welcome.

[Jack_Spratt]

Kayteg1 wrote:
In deck construction pressure treated wood require "hot dip galvanized" brackets and fasteners.
Beside harmful fumes, most of PT wood is secondary quality lumber, so its strength is not the best.
For job like that I go to Home Depot and run via pallet of lumber to find 4 nice and dense pieces.
Still see it old houses where lumber has density that will bend the nail, or shear a screw.

Ceramic coated screws are the preferred choice for pt lumber

[Kayteg1]

I don't think I would sleep well having wood joint under main holding bracket. New wood suppose to go to the corner where you have metal reinforcement.
Than stapled frame calls for well glued siding for shearing strength.
One more way to repair partially rotted wood is laminating.
Some epoxy will soak rotten wood, giving it new strength and couple layers of fiberglass will make structural strength. It is messy job, so takes some practice, but with excelent results.

[jefe_4x4]

When I had a leak under the propane compartment door that dry rotted the wood framing underneath, I asked my RV guru dude the same question. The former owner turned the tie downs too tigh pulling down the bottom frame. My guru discouraged use of multi materials and simply peeled back the sheathing, and cut out the offending wooden framing and (industrial) STAPLED the 1x2's back together, just the way the factory made it.
As fasteners, screws, NBW's, and glue do not work and will not last. I've gotten another decade out of his repair.



jefe

[free_radical]

biggjb wrote:
These are all very good comments. As the newer TC have a metal(aluminum) frame I thought I could use some while making repairs to my Fleetwood. As it turns out it now makes sense for me to just stick with wood. I have measured some of the framing and I have some places where it is 1-1/2 X 2-1/4. However, most of the frame is 1x1, 2x2 or 1x3. Thanks everyone for your comments and I think I'll stick wood. My next question now is what type of wood. I read in one beech was used. I was thinking of pine, pressure treat of course.

Id use lightest wood available and laminate/glue few thin strips together to make any thickness you want,need,that would be stronger then single piece of wood

Gluing aluminum to wood can be done also,however requiers very expensive epoxy..plus very expensive special caulking gun..

[Cannon_George]

Interesting question, I agree wood as you indicate is probably best in your case. Structurally, what the framing member is doing stress-wise is important. Section modulus and moment of inertia properties vary widely between wood and aluminum box or C-channels, and for a camper application I expect a lot of off-axis loads - not to mention transient and sub-transient dynamic shock loads - which defy a static analysis performed when sitting level while parked.

Retro-fit framing repairs introduce possibility of inadequate load transfer capacity as compared to original framing design (whether documented or not) (and potential liability issues)

I've previously asked/looked for actual stress analysis/design figures used for framing design from manufacturers; naturally most are hesitant to provide this.

I've seen and repaired buckled square Al tubing on campers before, when failure is tracable to a known cause (ie, accident) that is preferable to failures with no obvious reason.

[Kayteg1]

In deck construction pressure treated wood require "hot dip galvanized" brackets and fasteners.
Beside harmful fumes, most of PT wood is secondary quality lumber, so its strength is not the best.
For job like that I go to Home Depot and run via pallet of lumber to find 4 nice and dense pieces.
Still see it old houses where lumber has density that will bend the nail, or shear a screw.

[joerg68]

Do not use PT wood in the camper. It can be highly corrosive due to the chemicals used. So the wood may hold up, but may cause issues with any metal parts around it, such as aluminum siding. See https://galvanizeit.org/hot-dip-galvanizing/how-long-does-hdg-last/contact-with-treated-wood for more info, or just google "pt wood corrosion".

It also used to contain arsenic, but this has been banned for about 15 years now, so any PT wood currently on the shelves should no longer contain any.
See https://en.wikipedia.org/wiki/Chromated_copper_arsenate for more info if you like. However... the chemicals now used for the purpose may or may not be safer, I do not know.

ETA: this document has some impressive images of steel vs. PT lumber:http://www.ggashi.com/wordpress/wp-content/uploads/PT-Lumber-Causes-Steel-Corrosion.pdf

[Grit_dog]

Depends what you're replacing and what it's supporting.
I'd use hardwood in structural locations. Use whatever in low stress areas.

[Kayteg1]

You should not use pressure treated wood in sleeping quarters.

[biggjb]

These are all very good comments. As the newer TC have a metal(aluminum) frame I thought I could use some while making repairs to my Fleetwood. As it turns out it now makes sense for me to just stick with wood. I have measured some of the framing and I have some places where it is 1-1/2 X 2-1/4. However, most of the frame is 1x1, 2x2 or 1x3. Thanks everyone for your comments and I think I'll stick wood. My next question now is what type of wood. I read in one beech was used. I was thinking of pine, pressure treat of course.

[ticki2]

biggjb wrote:
This is more a theoretical question as I have not seen posts pertaining to using steel 2x4s to replace rotten wooden ones. My camper has some significant repairs and I thought about trying to incorporate metal frame components in order to reduce weight. I can see a problem if the wooden part only needs to be partially replaced, however, if the whole component requires replacement could one use a metal component in its place? Any thoughts would be welcome.

There will be almost no weight savings between metal suds and wood studs . There will be some difficulty using both in the same wall . Metal studs will transfer more heat and cold . Metal strapping and cleats at joints will greatly strengthen the wood framing and repairs . That's about 2 cents worth .

[gbopp]

I've read that there is a concern in Tiny Houses, screws in metal studs vibrate loose while being towed. And they are towed less than a RV.
Welding or nuts and bolts would be an option.

I've done a few small projects with metal studs, they're really easy to work with as opposed to wood.
But, for what you plan, wood may be a better choice.

[ppine]

Welded aluminum. Find someo0ne that is good at it. If it is a small job you could probably screw and bolt it together.

[Kayteg1]

2x4 aluminium angle is way lighter than 2x4 fir and much stronger, water resistant and such.
That said where do you see 2x4 in the camper?
I did extensive front end repairs on my Fleetwood and the thickest wood member was 3/4".
Than the slide support on my Lance sag and broke off on 1 end. The wood construction used .75 x 2.5 wood, but it is beech, so much stronger than fir.
Reinforced it with aluminium angle as well, who hid conveniently under siding.
That said such repairs are not easy as going from metal to wood frame require some engineering.
It is not for just anyone.