Timber bike trail - lateral stability 4

[pilesmakesmiles]

We've been asked to check the adequacy of a timber mountain bike trail currently under construction.

The 'typical' detail is a 100x100mm post either side with a 200x38mm horizontal beam nailed between the two.

Vertical loading we can probably demonstrate by calculation that it works, however we are struggling to show that the frames have sufficient sway stability. As it is a curving mountain bike track there will be some lateral load on the frames (around 1 kN based on the speeds/rider weights/geometry we have been given).

I have analysed it as a pinned base portal frame, but the moment connections in the timber don't work (no surprise there).

I have also analysed as a pin jointed frame with fixed bases, but there is insufficient passive pressure to resist the moments (I have been conservative with soil parameters but it fails by quite a long way).

On site however you can lean up against a tree and push the structure and it barely moves. The client has built similar structures elsewhere but the curves are not as tight and the speeds achieved are much less.

Putting in diagonal braces on the sections already built is possible (but difficult) which would make the structure stable, however calculations might show the whole structure still slides sideways.

We have been told that the top speed achievable will be 15 mph and we are designing for a rider+bike weight of 131 kg. Knowing the kind of people who will be riding on here we're reluctant to reduce these!

Has anyone got any hints for analysing or other things I should take into account?

We are struggling to find any design guidance/codes specific to such structures so any links to such things would be quite useful. At the moment we are designing/checking to BS5268 and BS EN 1995.

 

[ishvaaag]

Only seeing the trail makes clear that one can be green and nuts. Anyway, as an engineer, follow the principles of stability, and if required, make a proper statement. Otherwise a drunkard's fall may become a lateral stability issue to your headache.

 

[ToadJones]

I thought being Green and Nuts went hand in hand?

 

[cvg]

your rider and bike weight seems very high. most cyclists weigh about half that. why not assume a more reasonable weight and then apply a factor of safety?

are the posts set in concrete? if so, you should use the bearing area of the concrete, not the wood post. Additional posts or buttress could prevent sway on the curves or even a concrete thrust block

 

[CaptainO]

"I have analysed it as a pinned base portal frame, but the moment connections in the timber don't work (no surprise there)."

You are going to get some fixity (and moment transfer) in your bases there.

 

[dhengr]

I’d worry more about damage to the tree trunks caused by helmets, and to a lesser extent by bodies. There is little evidence that mountain bikers are green when you see the damage they do, but there is no doubt that they are nuts.

On corners use deeper cross members, maybe one each side of the posts, and timber screws, lag screws or through bolts. Bury the posts deeper and do a good job of compacting the backfill around them, so they start acting as partially fixed canti’s. at some distance below grade.

 

[msquared48]

Timber constructed trail in a wooded environment for bikes? Is this a wood corderoy road?

How will the slippery conditions caused by mold growth on the timbers be handled to avoid slips. Mountain bike tires don't do well with the mold growth, and spills due to this will de3finitely impact the rail design you are discussing.

Mike McCann
MMC Engineering

 

[BAretired]

I think the curvature of the trail tends to resist centrifugal force by developing tension in the curved structure and spreading the lateral force over several supports. You could improve this capability by tying the beams together with steel straps on the outside of the curve.

BA

 

[SkiisAndBikes]

pilesmakesmiles,

Although my day job is engineering, I ride mountain bikes, race mountain bikes and have designed the odd hiking and/or mountain biking bridge. (an I hope others don't consider me nuts).

First place to check for design guides is the IMBA International Mountain Biking Association. They have lots of info on trail and I believe structure building. I have a design guide somewhere and will post more info when/if I find it ... it has been missing in a pile of paper for a while. I could probably find it if I spent more time filing my papers and less time biking.

These are relatively small structures, i.e. not bridge code worthy, however future liability must be considered (there was a lawsuit in our area due to trail maintenance). I think combining engineering judgement with typical details used at many other locations for similar structures is a good starting place. Maybe finding a way to attach angled braces on the exterior might provide more lateral stability. More later.

Also ... why not get your bike out and 'field test' the bridge once it's complete ... I'd be more than happy to.

skiisandbikes

 

[dhengr]

Skis... Is, ‘they live dangerously’ more to your liking? My apologies! But then, we’re engineers too, so that might put us right back in the nuts category, since there must be better ways to make a better living. My son broke his wrist two weeks before the Air Force had him scheduled for his final test flight. He tried taking an almost perfect swan dive over the handle bars of his mountain bike. He said his form and all was just perfect, except for the rock that he hit and his landing. The general said that wasn’t the kind of landings they had spent all that money and training on, what the hell was he thinking, or was he thinking. Mostly, I’m glad he’s still alive with some of the stuff he’s done. All of the kids could out ski me after the first couple years.

 

[MikeHalloran]

Are you talking about a 'board track' on which the bike tires will travel,
OR
are you talking about timber fences to keep the wheels off the wildflowers?



Mike Halloran
Pembroke Pines, FL, USA

 

[a2mfk]

Skis- I'm just starting out in biking, but that thing looks pretty fun and for beginners, they make them much narrower and higher than that. Sometimes they are just logs or balance beam size.

Very few people who participate in what are considered "extreme" sports are very different than your average joe, ie, nuts or crazy. I like a lot of these sports but am not very good at them... We just accept a little more risk for the reward of the excitement and adrenaline. And you build your skills over many years, one obstacle or stunt at a time. You don't do black diamonds right off the bunny slope...

OP- I'm surprised they bothered you with this. They build these and similar structures all of the time, most of the time I assume without a structural engineer just a decent carpenter. They last until they rot away... Anyway, a mountain bike exerts very little lateral loading at tire level unless it is a very tight turn, its practically impossible actually. Therefore I'd only be concerned with banked turns, non-banked turns the rider will have to be going relatively slow. I'd imagine there is enough inherent lateral capacity in these for it to never be an issue.

Can you suggest that an aggressive rider(s) tests the structure while you observe for lateral movements, and then base the need for additional analysis on this evaluation? You can always go back behind the curved sections and run diagonals back to the ground. In fact, I'd be tempted to just tell them to do this, though it'd be fun to go watch and be involved they don't need an engineer. Have you seen what skateboard/bmx ramps look like these days?

 

[dhengr]

A2mfk:
Look back at the 150' dia. roof thread, @ 8JUL11, 17:22. The thread I should have posted in seemed to have dried up or melted away. And, thanks for the kind words, in that roof thread. Answer here and let’s let the 150' roof guy rest in peace.

 

[hokie66]

Pushbikes on that track? The riders are obviously expendable. I wouldn't worry about the stability of the track.

 

[a2mfk]

Here is a website I found after a quick search of some examples of what is out there. What is in this post is in all seriousness very mild mannered...

Hokie- you make me laugh all of the time, what's a push bike?

dhengr- I will check that thread, I was too annoyed at the time to look back again.

 

[hokie66]

That's what Australians call bikes without power other than feet, as opposed to motor bikes. Admittedly mostly used by annoyed automobile drivers.

 

[a2mfk]

hokie- That doesn't make a whole lotta sense, "push" bike... Bikes got pedals at least a century ago- thus no pushing, just using mechanical principles to your advantage

dhengr- I had to get that guy one last time, too easy, and annoying. I'm a cheesehead whose parents got sick of the weather and moved me south to FL. But spent a pretty good 9 years between Stevens Point and Appleton, and we got some pretty nasty lake-effect snow systems (74-84). Maybe being 9 had something to do with my memory of those snow drifts, they seemed huge!

 

[dhengr]

Put enough negative (reverse?) super elevation on the curves and the centrifugal force problem should go away. No need for “slow down’ signs at these corners, the blood on the trees will be able to be seen from enough distance to have that affect; and the knee, elbow and leg meat on the boards will reduce the tire/board coef. of friction to almost zero. When they trim the branches on the trees, they should leave them project a few inches that will improve the adrenaline rush factor and keep the bikers from pilling up on the ground.

 

[BAretired]

dhengr,
Please explain to an old codger who has little understanding of engineering principles...why the centrifugal force should go away when the track has enough negative super elevation.

BA

 

[hokie66]

a2mfk,
You push the pedals, you don't pull them.

http://en.wikipedia.org/wiki/Bicycle

BA,
Don't try to test the theory. You are too old for that sort of thing.