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Timber bike trail - lateral stability
4

Timber bike trail - lateral stability

Timber bike trail - lateral stability

(OP)
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.

RE: Timber bike trail - lateral stability

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.
 

RE: Timber bike trail - lateral stability

I thought being Green and Nuts went hand in hand?  

RE: Timber bike trail - lateral stability

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

RE: Timber bike trail - lateral stability

"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.  

RE: Timber bike trail - lateral stability

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.

RE: Timber bike trail - lateral stability

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

 

RE: Timber bike trail - lateral stability

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

RE: Timber bike trail - lateral stability

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

 

RE: Timber bike trail - lateral stability

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.

RE: Timber bike trail - lateral stability

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

RE: Timber bike trail - lateral stability

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?
 

RE: Timber bike trail - lateral stability

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.
 

RE: Timber bike trail - lateral stability

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

RE: Timber bike trail - lateral stability

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.

RE: Timber bike trail - lateral stability

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

RE: Timber bike trail - lateral stability

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!

RE: Timber bike trail - lateral stability

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.   smile
 

RE: Timber bike trail - lateral stability

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

RE: Timber bike trail - lateral stability

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.

RE: Timber bike trail - lateral stability

Too soon oldt undt too late schmart.

BA

RE: Timber bike trail - lateral stability

I really dont understand why you would bother when there is perfectly good ground only a foot lower.

One problem with the load test theory is that the timber will shrink and connections that are tight at the time of installation will be much looser in 3 or 4 months.

RE: Timber bike trail - lateral stability

csd72, they may be trying to prevent damage to the tree roots.

Piles, will you have to deal with frost heave? I revisited a boardwalk I had helped build years before, and found many of the posts had heaved, making it bumpier than a traveling carnival's roller coaster.

 

     Maybe the tyranny of Murphy is the penalty for hubris.  - http://xkcd.com/319/
 

RE: Timber bike trail - lateral stability

(OP)
Thanks for all the replies so far.  It turns out that 12mm bolts were specified for the connections rather than nails so this might give enough capacity for it to work as a portal if retrofitted, however something will still need to be done at the foundations to stop the whole structure sliding sideways.

In answer to some of the specific queries:

cvg:  The rider+bike weight comes from a design standard.  17 stone + 30lb bike isn't unreasonable for the location and type of trail.  I know most riders will be below this but there might be one or two who are over (someone I ride with is over 16 stone).  The posts are not set in concrete but this is an option.

dhengr:  This is an option for the remainder which hasn't been built yet.  

msquared48:  The client appreciates it will be slippery as it is now. On other timber structures they have applied various types of wire mesh.  Metal lath is preferred (but expensive compared to chicken wire) and they are also looking at resin/grit paint.  In winter it will have an inch or two of ice on it and people will still be trying ride along it...

SkiisAndBikes:  IMBA used to have some technical guidance on their website (mentioned bracing every other or every three bays etc.) but it seems to have been taken offline.  Do the books contain technical information that would be useful for a structural engineer? (e.g. typical details, fixing sizes etc.)

MikeHalloran:  The raised boards are for people to ride bikes on top of.  The ground is OK for walking on at the moment (middle of summer) but after a few hundred bikes go over it it will turn to bog.  We get a lot of rain here in the UK, even in the summer!

a2mfk:  The client is a commercial organisation and quite risk averse.  They don't mind people falling off due to their own stupidity/lack of skills but they don't want the structure to collapse.  The carpenter building it has already ignored the standard detail they were given which might have sparked off their concern.
I'd offer to test ride it for them but they would need to have the whole length completed, then if there is a problem it's more difficult to fix.  They're also under pressure to get it completed pretty quickly.
I've estimated the radius of the curve as 5m and it is pretty tight hence the superelevation (my boss said it looked like a wall of death).  On the straight sections I am assuming that the lateral force is insignificant for now so the problem is only at the curves.

csd72:  The ground will become boggy when people start riding over it, plus elevated wooden sections are liked by the sort of people who visit the forest.

ACtrafficengr:  Hadn't thought about frost heave, it definitely will need considering.  Some vertical movement will be acceptable but the posts coming loose probably won't.
 

RE: Timber bike trail - lateral stability

pilesmakesmiles,

The boggy ground issue could more easily be countered with some lightly compacted roadbase. Maybe the people who prefer to ride on timber platforms should not be coming to a forest to do it - but that is a personal bugbear of mine that people want to change the natural environment to make it more town like!

Have you considered timber shear rings between the mating surfaces of the bolts? I have not seen any published moment capacities but they must have some.

RE: Timber bike trail - lateral stability

BA:
Old codgers we both may be..., but that you misunderstand engineering principles I seriously doubt, so I'll try  splainin  again.  You are right, the centrifugal force won't go away, but the problems it causes on the deck might be reduced.   Without super elevation, but with reverse super elevation (the opposite of normal super elevation which allows increased speed) on the curves the riders will be forced to, or tend to, slow down after they lose a couple buddies, or they will get the trees all bloody.  With the slower velocities on the curves, the centrifugal force will decrease (but not go away) and maybe less blood will be spilt.  Alternatively, with enough people hitting the deck and leaving enough meat and blood on it, it will become slippery and the coef. of friction btwn. bike tires and deck will go to near zero, again reducing the lateral forces on the deck, but maybe not the centrifugal force on the bikers.  In either case the OP'ers. lateral force problem on the deck will be greatly reduced.  His idea of wire mesh on the deck is my idea of a liability issue waiting to happen, and gritty paint won't last either.  If the bikers act stupidly with their speed, etc. let them learn the hard way.  I think the whole concept of a boardwalk for bikers is crazy, but it will reduce the damage and erosion that they would cause otherwise, if they are allowed to ride there.
 

RE: Timber bike trail - lateral stability

You could try this: cut a deeper 2x compression member to fit snugly, horizontally, btwn. the posts; above and below this strut drill the posts for tension rods from out to out of the posts; with washers and nuts for clamping the whole system, and allow for periodic tightening.  Much like a reinf'ed. conc. beam this will develop a moment; the comp. block is a function of compression perpendicular to the post grain, and the rods being the rebars.  The end bearing area of the struts could be increased by end nailing bearing plates to the strut wider than its 1.5" width, and they may also have holes punched in them for the tension rods, helping hold it all together.

Without much more grading than your photo shows, I've seen some very nice nature and bike trails built, wide enough so they can be dragged and maintained by a small tractor.  And, sloped and graded to minimize erosion.  Proper attention to sub-base, drainage, geo-textiles, erosion control, and a riding/wearing surface (not paved), pressure treated edging or some such where needed, etc. etc.  This would probably be very cost competitive with your deck and its maintenance too.  The objective should be to minimize the damage they can do by running wild, by keeping them on a trail you can maintain.
 

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