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.

 

[BAretired]

Too soon oldt undt too late schmart.

BA

 

[csd72]

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.

 

[ACtrafficengr]

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/

 

[pilesmakesmiles]

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.

 

[csd72]

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.

 

[dhengr]

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.

 

[dhengr]

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.