Design of cycle bridges

I have just spent an enthralling hour or so reading British Standard
5400-2:2006, which covers the load calculations for cycle & foot
bridges. Actually there's no such thing as a cycle bridge because you
can't legally keep the peds off it, so the ped calculations are the
worst case.

So, suppose you wanted a modest bridge to convey an NCN route over a
small river, say the River Cam for a wild, totally off-the-wall, nothing
to do with what I'm concerned with, example. The span of the bridge is
25 metres, and let's say that the width of the cycle path is 1.7m -- not
ideal, not to standard, but the most we^W anyone is going to get. What
load must it support?


You can think about it, but you won't get the right answer without
reading BS5400-2.


Still thinking?


How about 33 tonnes?


Yup, for a narrow bridge for cycling over a narrow river, today's
standards say you must be able to support 33 tonnes. Mike Burrows'
8-freight must be able to carry more than I'd ever imagined!



Mike

Mike Causer wrote:
I have just spent an enthralling hour or so reading British Standard
5400-2:2006, which covers the load calculations for cycle & foot
bridges. Actually there's no such thing as a cycle bridge because you
can't legally keep the peds off it, so the ped calculations are the
worst case.

So, suppose you wanted a modest bridge to convey an NCN route over a
small river, say the River Cam for a wild, totally off-the-wall, nothing
to do with what I'm concerned with, example. The span of the bridge is
25 metres, and let's say that the width of the cycle path is 1.7m -- not
ideal, not to standard, but the most we^W anyone is going to get. What
load must it support?


You can think about it, but you won't get the right answer without
reading BS5400-2.

4 pedestrians abreast, each weighing 75 kilos gives 300kg. Allow 2 per
metre along the bridge, that is 50x300=15,000 kg.

Double it for an acceptable margin for error.

That would give the 33 tons quite happily.

Now are you satisfied that the bridge will cope with a crowd
chock-a-block across it?

Seems like the BS are not BS.

...d

Still thinking?


How about 33 tonnes?


Yup, for a narrow bridge for cycling over a narrow river, today's
standards say you must be able to support 33 tonnes. Mike Burrows'
8-freight must be able to carry more than I'd ever imagined!



Mike

On Mon, 06 Nov, Mike Causer wrote:

How about 33 tonnes?

Plausible, though I haven't checked your figures.
Crowd loading, you see. Crowd loading has occasionally collapsed
bridges.

Btw, you probably don't actually want to use 5400, you probably want to
use BD 37/01 (being the 2001 revision of BD37). The BDs are what the
highways agency wants. Mostly they modify BSs. Most local
authorities go along with the BDs, on the grounds that the highways
agency wants is probably a good thing.

Most of the time, there's not much difference, to be honest. However,
if you actually want to get into a cut-and-thrust with a
professional, you'd be better referring to BD37.

regards, Ian SMith
--
|\ /| no .sig
|o o|
|/ \|

On Mon, 06 Nov 2006 14:32:19 -0800, David Martin wrote:

4 pedestrians abreast, each weighing 75 kilos gives 300kg. Allow 2 per
metre along the bridge, that is 50x300=15,000 kg.

Double it for an acceptable margin for error.

That would give the 33 tons quite happily.

I think they rate peds at 6/sq-metre. Times 1.75 for the overload
condition.


Now are you satisfied that the bridge will cope with a crowd chock-a-block
across it?

To BS 5400 it will, but what is the likelyhood? That load on the current
1939 road bridge would bring it down. (IMHO as an ex-structural steel
designer.)


Mike

in message pan.2006.11.06.22.59.19.713351@firstnamelastname. com.invalid,
Mike Causer ') wrote:

On Mon, 06 Nov 2006 14:32:19 -0800, David Martin wrote:

4 pedestrians abreast, each weighing 75 kilos gives 300kg. Allow 2 per
metre along the bridge, that is 50x300=15,000 kg.

Double it for an acceptable margin for error.

That would give the 33 tons quite happily.

I think they rate peds at 6/sq-metre. Times 1.75 for the overload
condition.

But if they march in step at its harmonic frequency they'll still bring it
down.

Now are you satisfied that the bridge will cope with a crowd
chock-a-block across it?

To BS 5400 it will, but what is the likelyhood? That load on the current
1939 road bridge would bring it down. (IMHO as an ex-structural steel
designer.)

This bridge:
http://pictures.mug-uk.co.uk/albums/...0243.sized.jpg

Now has a gross weight limit of six tons (/not/ six tons per vehicle). It
is about 200 metres long and carries a main A road. It does have automatic
weigh bridges and big red flashing lights at both ends, and big signs
saying HGVs verboten, but...

I mean, I'm not saying it's acceptable to have a main road bridge over a
dangerous tidal estuary that is this weak, but let's have some sense of
proportion. Is it better to have /a/ bridge that people need to exercise
reasonable care in using, or to have no bridge?

Sings at each end saying 'weak bridge: no more than twelve persons on the
bridge at any time' wouldn't be /too/ hard to organise, would they?

--
(Simon Brooke) http://www.jasmine.org.uk/~simon/

There are no messages. The above is just a random stream of
bytes. Any opinion or meaning you find in it is your own creation.

Following on from Ian Smith's message. . .
On Mon, 06 Nov, Mike Causer wrote:

How about 33 tonnes?

Plausible, though I haven't checked your figures.
Crowd loading, you see. Crowd loading has occasionally collapsed
bridges.

Btw, you probably don't actually want to use 5400, you probably want to
use BD 37/01 (being the 2001 revision of BD37). The BDs are what the
highways agency wants. Mostly they modify BSs. Most local
authorities go along with the BDs, on the grounds that the highways
agency wants is probably a good thing.

Most of the time, there's not much difference, to be honest. However,
if you actually want to get into a cut-and-thrust with a
professional, you'd be better referring to BD37.

regards, Ian SMith

Serious question:

In Witham there is a perfectly good and very useful footpath/cycle
crossing across the East Anglian main line. What an oooprtunity for
feasibility studies...
....which didn't bother to ask cyclists where they wanted to go...
....and made up (and eventually after two years of hassling I managed to
get the report) the idea that the line would be widened to 4 tracks from
the present two. The report admits they havent actually got any
evidence from this from Railtrack or whoever but clearly the objective
of the exercise is to make this into a mammoth task employing
consultants and options disappearing up their own slide rules until the
money runs out or the rate payer foots a massively inflated bill.

Here is the Q: If you increase the span by 33% what is the order of cost
increase? (I know the total cost included approaches - let's just
concentrate on the main span.)


--
PETER FOX Not the same since the bookshop idea was shelved

2 Tees Close, Witham, Essex.
Gravity beer in Essex http://www.eminent.demon.co.uk

Simon Brooke wrote on 06/11/2006 23:34 +0100:

Sings at each end saying 'weak bridge: no more than twelve persons on the
bridge at any time' wouldn't be /too/ hard to organise, would they?


I like the concept of choral advice ;-) But you mean like the signs at
the entry to towns and villages saying no more than 30mph?

--
Tony

"Anyone who conducts an argument by appealing to authority is not using
his intelligence; he is just using his memory."
- Leonardo da Vinci

On Mon, 06 Nov 2006, Mike Causer wrote:
On Mon, 06 Nov 2006 14:32:19 -0800, David Martin wrote:

4 pedestrians abreast, each weighing 75 kilos gives 300kg. Allow 2 per
metre along the bridge, that is 50x300=15,000 kg.

Double it for an acceptable margin for error.

That would give the 33 tons quite happily.

I think they rate peds at 6/sq-metre. Times 1.75 for the overload
condition.

5 kN/m2 - 500 kg per square metre. Longer spans (over 36m loaded
length) have a lower figure, as do bridges wider than 2m. Footways
alongside a roadway also have a lower figure.

Load factor is 1.5 for pedestrian loading, and effectively a further
1.1 (though there are subtle issues relating to that one, concerning
whether it's actually a factor on the loading) giving an effective
1.65.

regards, Ian SMith
--
|\ /| no .sig
|o o|
|/ \|

Ian Smith wrote:
On Mon, 06 Nov 2006, Mike Causer wrote:
On Mon, 06 Nov 2006 14:32:19 -0800, David Martin wrote:

4 pedestrians abreast, each weighing 75 kilos gives 300kg. Allow 2 per
metre along the bridge, that is 50x300=15,000 kg.

Double it for an acceptable margin for error.

That would give the 33 tons quite happily.

I think they rate peds at 6/sq-metre. Times 1.75 for the overload
condition.

5 kN/m2 - 500 kg per square metre. Longer spans (over 36m loaded
length) have a lower figure, as do bridges wider than 2m. Footways
alongside a roadway also have a lower figure.

Load factor is 1.5 for pedestrian loading, and effectively a further
1.1 (though there are subtle issues relating to that one, concerning
whether it's actually a factor on the loading) giving an effective
1.65.


Well, I think my point was well made that it is not hard for a
non-engineer to determine that the weight limit quoted was well within
a reasonable order of magnitude.

Intrigued by Simon's bridge - how do you limit it to 4 vehicles at a
time? or is that per span?
Maybe the main A road is actually one of these rural Scottish ones
where busy traffic rates are counted on the fingers (per hour).

...d

in message .com, David
Martin ') wrote:


Ian Smith wrote:
On Mon, 06 Nov 2006, Mike Causer wrote:
On Mon, 06 Nov 2006 14:32:19 -0800, David Martin wrote:

4 pedestrians abreast, each weighing 75 kilos gives 300kg. Allow 2
per metre along the bridge, that is 50x300=15,000 kg.

Double it for an acceptable margin for error.

That would give the 33 tons quite happily.

I think they rate peds at 6/sq-metre. Times 1.75 for the overload
condition.

5 kN/m2 - 500 kg per square metre. Longer spans (over 36m loaded
length) have a lower figure, as do bridges wider than 2m. Footways
alongside a roadway also have a lower figure.

Load factor is 1.5 for pedestrian loading, and effectively a further
1.1 (though there are subtle issues relating to that one, concerning
whether it's actually a factor on the loading) giving an effective
1.65.

Well, I think my point was well made that it is not hard for a
non-engineer to determine that the weight limit quoted was well within
a reasonable order of magnitude.

Intrigued by Simon's bridge - how do you limit it to 4 vehicles at a
time? or is that per span?
Maybe the main A road is actually one of these rural Scottish ones
where busy traffic rates are counted on the fingers (per hour).

It's this bridge he
http://www.streetmap.co.uk/newmap.sr...5&y=551235&z=3

It was built of reinforced concrete in the 1920s after the previous bridge
had been lifted off its foundations by ice(!) and destroyed; apparently
corrosion has got into the reinforcement in the main beams and this has
caused them to crack badly, and the foundations of some of the piers are
also sinking into the silt. But it's too expensive to replace so a
complicated system of lights limits the number of motor vehicles on the
bridge at one time. Tidal stream is up to 4 knots rising and up to eight
knots falling. Tidal range is about nine metres at springs.

--
(Simon Brooke) http://www.jasmine.org.uk/~simon/

;; single speed mountain bikes: for people who cycle on flat mountains.