Plastic lumber design guide

[Paperandpencil]

I have designed the structure of several elevated timber board walks over the years, but a client now wants to start using plastic wood (recycled HDPE) for these structures in the UK. I've not designed using this material previously. A reference document that appears in several sources is 'Structural Grade Plastic Lumber Design Guide' produced by McLaren. It seems that it good be a good starting point, but I can't find a copy of it anywhere. Any suggestions as to where it can be obtained or other guidance would be most welcome.

Thank you.

 

[phamENG]

I'm in the US - to my knowledge there's not really a design guide or spec out there on our side of the Atlantic. I've used this product on marine structures - docks/piers, bulkhead caps, etc. The manufacturers provided span data backed by testing and a warranty.

One thing to keep in mind, MOST of these products are not suitable for anything but flatwise use as decking over relatively closely spaced supports (16 to 24 inches). I've seen where people have tried to use them for guard rails, and it's usually a bit of a disaster. Unless you want the rail to look like a festoon after a couple years in the sun, don't do it. (Yes, even when bent about the strong axis, the creep can be very significant and very fast.)

 

[Agent666]

I have some of this fake wood on a deck at home, it gets hot! By hot I mean stand on it for more than a few seconds after a day of strong sun and you might as well plunge your feet into boiling water to cool them down.

As I understand it (and based on personal experience), the sun doesn't always shine in the UK so maybe it's not an issue? But something to be mindful of if it's a public structure, little kids falling on their face and going up in flames, etc.....

 

[phamENG]

Agent666 - do you have a dark surface color, or does it matter? After pressure washing and sealing my wood deck last week I'm seriously considering putting in the "plastic wood" next year...

 

[STrctPono]

Paperandpencil, I assume that you are looking to design the framing members and not just the decking out of recycled plastic lumber... I once designed a maintenance platform under a bridge out of recycled plastic lumber. I would suggest you look into Bedford Technologies. I have no affiliation with the company but they have some useful design guides on their website, the staff was really helpful to me and they have some unique products that utilize glass fiber reinforced polymer rebar embedded in their members to help increase the elastic modulus of the shapes. As previously mentioned, serviceability deflection issues will usually govern so any extra stiffness you can get from the product is a plus.

Now for decking material, that is much more standard. As Agent666 mentioned, they do get hot. My veranda decking that I installed on my lanai gets hot in the summer but I wouldn't trade it in even for IPE. It's maintenance free and I sure don't have time to strip and reseal every 3 years. Most decking manufacturers seem to have gone with a plastic lumber decking wrapped in plastic. I remember reading this was due to all the class action lawsuits that Trex was having with the old style product that wasn't as resilient.

 

[Agent666]

phamENG, light gray colour. It's definitely not maintenance free though. Gets quite grubby over winter and needs washing and scrubbing. Few areas that seem permanently stained from having planters on deck as well. It's probably about 12 years old, so maybe the tech have improved, but I'd take the stuff the manufacturers say with a grain of salt.

 

[Paperandpencil]

Thanks for all your comments to date. It is structural design information for the boardwalk framing members that I am looking for. The joists, cross beams and the posts. Also the bolts, whats the long term bearing strength of plastic when using an M12 bolt? I prefer not to place reliance on what the manufacturers are telling me, and I am trying to find something independent, ideally a standard. The HDPE manufacturer that my client wants to use does have load span tables for their decking. I will approach the manufacturers to discuss the design of the framing, but I am hoping to have some information from other sources.

The colour of the boardwalk will be dark brown, for appearance and maintenance reasons. I am personally not convinced about maintenance free, time will tell. Part of the boardwalk is beneath trees and they will be dropping stuff all over it. The sun does occasionally shine in the UK and I can see that the structure will easily reach 50 C in the sun. Winter temperatures possibly -20 C. My garden waste wheelie bin is made from HDPE and its behaviour is noticeably different from winter to summer. The last one that I had seemed to suffer a brittle fracture last winter when it was emptied by the Council. I think that I need to spend more time with people who arent engineers.....

I have looked at the Bedford Technologies site and their glass fibre reinforced sections look to be a good product.

The boardwalk will be a publicly accessible structure alongside a river that flows through a city centre. The end client wants to use recycled plastic, it ticks boxes from an environmental perspective. They also want 50 plus years design life, possibly 120 years, just as if it was being constructed from materials with proven track records of performance. There is probably a species of bacteria that has been waiting for several billion years for recycled HDPE to appear in its life.

 

[1503-44]

50-100 yrs. Plastics? Yikes! I'll give it 10. "Mind the gaps". Temperature expansion and contraction of plastics can be extreme. Possibly 10X that of steel. Maybe = 0.00015m/m/C Long bridge will need lots of expansion gaps, plate gap covers and sliding bearings. Short independent sections of handrails to eliminate snakes in summer and stretching in winter? Brittle handrails in winter? Don't want to crash into those in low temperatures.

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[XR250]

Mind the gaps". Temperature expansion and contraction of plastics can be extreme

I was called to look at a racked screened porch. Could not figure out why it was racked until I noticed the 50 ft. wide deck next to it covered in recycled plastic decking (installed without joints). It expanded a good 2" and was abutting the house at the other end. The porch lost the battle.