Falling tree load case

bugbus · Jun 14, 2023

We are occasionally asked to design bridges in forested areas where there is a possibility for a tree or large branch to fall onto the bridge (e.g. nature walks, golf courses, etc.).

It is not a load case that is spelled out in any of the bridge design codes (as far as I'm aware).

For bridges on the slender side, e.g. a steel pedestrian bridge, I would imagine that a falling tree could easily cause it to collapse.

But as for estimating the collision load from a tree? I'm not really sure at all.

In AS5100, there is a 'minimum restraint load' case, consisting of a 500 kN load (sometimes 200 kN for pedestrian bridges), which is applied horizontally at the bridge supports and is mainly intended to provide a minimum level of robustness to bridge and avoid the superstructure becoming dislodged. Possibly it would be a case of taking a similar nominal collision force and applying it at any point along the bridge, at any angle between horizontal and downward? For a pedestrian bridge with steel railings, I doubt that it would be feasible to design the railings to take such a load, so possibly it would just need to be applied to the main structural members?

Curious to hear people's thoughts/experience.

LittleInch · Jun 14, 2023

This sounds to me like a v low probability / high consequence event which you can't realistically design for.

So then you need to mitigate against it and implement things like annual check on trees with overhanging branches or within a fall radius. Or closing it in period of v high winds / storms when in reality few people will be out so also reducing the risk.

Life is not without risk. The issue is whether the risk is sufficiently low to be acceptable.

This is often a 1 in a million risk per year of dying because of this unlikely event. Your difficulty will probably be finding reliable data on how many bridges a year are damaged by falling branches/trees especially in a forested area. Then how often would that event result in imbue or death of an individual.

A you say it is not a recognised load case and therfore not an engineering issue, but a risk issue. IMHO.

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3DDave · Jun 14, 2023

I recall a video of a 2 lane bridge on the receiving end of a rock-slide. The first, small rocks, it withstood, but when one about the weight of a VW Bug removed the truss rail from one side it went down like a, well, sack of rocks.

https://www.youtube.com/watch?v=v64tsuhg2C0

You cannot design for the potential load, you might design for absorbing the amount of energy without collapse.

Per

https://thetimberlandinvestor.com/how-much-does-a-tree-weigh/

the weight of a tree might be 1-3 tons. Assuming that the CG is roughly 1/2 way up with summer foliage, then that represents, upper middle, 40 feet * 6000 pounds, but if there is a hill, that might add additional height. Figure 240,000 foot-lbf of energy that might be focused on a bridge element 18 inch wide or less (trunk diameter.)

I did see a bridge hit by a truck carrying a 40 foot long, 12 foot diameter, roughly 0.500 to 0.75 inch wall tube at 60 mph. It stopped the tube, the trailer and the tractor in under 24 inches, folding the bottom of the 2 foot deep, 10 foot wide, section of the I-Beam about 45 degrees. The bridge survived absorbing that amount of energy. I think the DOT hired some guys with torches to bend it back.

LittleInch · Jun 14, 2023

This is a bit like designing those security barriers for truck impacts. deciding how much deformation and yielding you are going to see is half the answer.

Still think its a fairly pointless exercise as it it will not be economic to design for such an extreme event, especially for passenger bridges.

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Also: If you get a response it's polite to respond to it.

bugbus · Jun 15, 2023

Thanks all, I think you made some good points. This is probably more of an 'engineering judgement' or risk tolerance scenario rather than delving into the numbers for what is realistically a pretty rare event.

3Fan · Jun 15, 2023

The USACE published a document called "Maximum Impact Force of Woody Debris on Floodplain Structures" back in 2002. Though not the same situation as what you are describing, there may be some relevant information for you from their research.

BridgeSmith · Jun 15, 2023

Way too many wide-ranging variables to attempt to make any generalized estimate of the impact loading. Not only would the size, height, distance from pivot point, etc. vary from location to location, but probably the most critical parameter - the velocity at impact, would be impossible to narrow down enough to be useful. Trees felled by the wind are typically pulled up by the roots. I've seen ones that flopped over like they were cut off, and some that rotated 60 degrees and stopped. Most often, they go fairly slowly past 45 degrees, before they start to really 'fall'.

AS_PEng · Jun 15, 2023

What we've done on some of our park infrastructure projects is to remove or top the trees that may damage the structures. These would be large-diameter trees that lean toward the structures or trees that show signs of decay or disease that make them weak and prone to a fall during a wind event. We've also hired professionals that complete "Danger Tree Assessments". Parks folks are aware of the danger of falling trees not only on infrastructure but also visitors and complete tree assessments periodically. So there are resources out there that can gauge the risk and identify it.

SlideRuleEra · Jun 15, 2023

... or large scale shearing of tree trunks above ground by hurricane winds:

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