steel bridge design 7

[greznik91]

It's a small steel bridge for people(walking only).
Bridge is 1,20 m wide and its span is around 7 m.

I'm wondering about my selection of steel cross sections and details.

Primary steel beam is 'C' section 85/240 mm. I like using 'open' sections since they are easy to install - bolt.
Secondary beams are bolted to the primary beams as shown below (pinned connections at both sides). I was thinking about using 'I' section since flange provides bearing seat for steel grids.
Steel fence in fixed to the flange of primary beams.
At the end of primary beam there is a steel plate that is welded to beam.
Plate is anchored to existing concrete wall bellow.

One side of beam has standard holes and other side has elongated holes because of steel temperature - expansion/shrinkage since bridge will be outside where temperature change.

What do you think about my design? Any better suggestion?
The only thing I don't like is that primary beam has pretty thin web (9,5 mm), but I think it shouldn't be a problem since forces are small. Buckling of compression flange (from bending moment) shouldn't be a problem since there are secondary beams between that provide lateral support.

 

[bridgebuster]

This may be of some help

 

[bridgebuster]

and this

 

[bridgebuster]

Here are some sample plans, scroll thru the set for the steel bridges

 

[greznik91]

tnx for suggestions WARose and KootK.

thank you bridgebuster for this...

 

[SlideRuleEra]

greznik91 - There is a straight forward and cost effective way to design this walking bridge and avoid all complex bracing.
You have a question about the span-to-depth ratio (L/D =29) being too high. This number is the span length (7000 mm) divided by the main beam depth (240 mm). That number is a very good and easy way to see if the main beams will be "stiff" enough to keep the bridge from moving too much when it is used. This is not how a bridge is designed but lets the Engineer know if the estimated beam may be good. A ratio of 29 is too high. A good ratio would be less than 24; probably about 20 is better for this type project.

For the 7000 mm span, I would consider main beams that are about 350 mm deep (7000 mm / ratio 20) = 350 mm (14 inches).
Note: I am not good with metric, so will have to write in customary US units.
Since 14" deep beams are a reasonable choice, look at selecting a W14 section that has a "wide" flange. A beam with a "wide" flange has a much longer allowable unbraced length than a channel or a beam with a "narrow" flange.

The bridge is very narrow (1.2 m). Each of the beams will carry half (0.6 m). We don't know the design live load, but say it is 100 lb/ft². The total load on each beam will be low for 7000+ mm long, W14 beams. Because of the grating, the dead load will be low, also. Therefore the design bending moment that each beam must resist will be low.

For that calculated bending moment it will be easy to select a (reasonable size) W14 that has an allowable unbraced length greater than the total length (7500 mm) of each beam. Use your original cross members (maybe a different size to better fit a W14) to tie the two main beams together and help anchor the grating.

The W14 (or similar sized beams) will not be much heavier (expensive) than the minimum size main beams would be. This approach avoids all the design time, bracing material and labor needed to make minimum size beams work.

For What It Is Worth, I don't know how much experience you have, but your original design is a pretty good first effort.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[greznik91]

SlideRuleEra
thank you for your insight, I appreciate it and yes this has been my 1st bridge.
So guidence like this is very helpful.
Thank you again.