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.

 

[Ingenuity]

It's a small steel bridge for people(walking only)

Your span-to-depth of L/D = 29 is "up there" and consider that many a pedestrian bridge often has to accommodate more than 'walking' excitation.

 

[greznik91]

can you please elaborate what do you mean by this (English not my language)?

Your span-to-depth of L/D = 29 is "up there"

consider that many a pedestrian bridge often has to accommodate more than 'walking' excitation.

Yes I know. Tnx.

BTW - I'm frome Europe so I'm using Eurocodes.

 

[WARose]

A couple of points:

1. You say that: "Buckling of compression flange (from bending moment) shouldn't be a problem since there are secondary beams between that provide lateral support." If you are referring to the wide flanges in Section B-B.....they do NOT provide lateral support unless you are counting on them (in some way) being tied into the grating and the grating being capable of transferring lateral load to the support. (I.e. like a diaphragm. Something I never do with grating.) The connection doesn't look right for torsional support either. (See AISC (Appendix 6; 13th ed.) for further guidance.)

2. With that kind of span (and channel support), I'd worry about it being too flexible. Check the deflection/Frequency requirements for pedestrian bridges (as per AASHTO).

3. I don't know what length of slot you have on that one end.....but be sure it can accommodate a high differential drift for those walls. You get up high enough and the seismic drift can get outrageous.

 

[KootK]

Inside or outside? I'm okay with assuming that the secondary beams torsionally brace the primary. Unless you're willing to claim diaphragm capability for the grating, you'll probably want some horizontal bracing there.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[greznik91]

Its outside.
Would you choose a different primary beam section? I would gp with a 'C' section or 'H' section.
I dont think there are lots of options here regarding that...

@WARose - why do you think that connection of secondary beam to primary beam is not ok for torsion?

 

[WARose]

@WARose - why do you think that connection of secondary beam to primary beam is not ok for torsion?

Primarily because the web is typically not stiff (nor strong) enough for it. Not saying it can't be done......but I've tried it before myself and couldn't make it happen. You have a stiffener and that helps.....but it's something to carefully check.

It would probably be easier to just put in horizontal bracing. You'd probably need that anyway to transfer the horizontal loads.

 

[jreit]

@WARose/@Kootk could you explain why the I-shapes will not provide lateral bracing to the C-section primary beams?
I don't see how the grating is needed - seems similar to the concrete deck on a bridge not being needed for the lateral bracing to work.

 

[JoshPlumSE]

I'm with Jreit, I think the connection is capable of bracing the compression flange for LTB. Though adding in some horizontal bracing would eliminate any doubt.

 

[mats12]

Where would you put/fix bracing?
At upper flange of 'I' beams?

WARose looks like you have done something like that before. What sections did you use?

 

[greznik91]

I would fix bracing on upper flange of 'I' beams.
Would that be allright?
I rarely do something like this...
TNX for help.

 

[mats12]

What do you guys think about this detail?

Primary beam as "H" section
Secondary beam as "I" section
Wherever secondary beam is connected to primary beam I would add a steel plate that is welded to flanges of primary beam + stiffener welded on both sides of "H" section in the middle of a plate.
Secondary beam bolted to this plate.
Bracing fixed on top flanges of secondary beam.

Do you guys think this is overkill?
Since "I" beams are away from center of "H" beam, can torsion of primary beam be an issue?

 

[Klitor]

Here is something similar. For your span, I would go full welded,one piece.

Be sure to dimension for 5kN/m2 load, and check that your vertical natural frequency is more than 3Hz.

 

[greznik91]

Tnx for replies.

Klitor, I like your design but I have a problem - I cant deliver large elements to a field (big welded parts are not OK) so I have to use smaller elements that are bolted together at the site.

what do you think about details that mats12 provided?

 

[gmd255]

Can someone please tell me how do you design bracing?
I mean on what loads?
Wind load is pretty damn small on horizontal construction like this.
People on a bridge contribute to some horizontal forces but I have no idea how to consider that load.
Im also from EU so Im wondering do Eurocodes deal with this kind of loading?

 

[greznik91]

Klitor, can you please post a connection between primary beams (bolted connection in the middle of span - bolts 6 x M20).
Is this a moment connection with all bolts inside IPE 360 flanges?

tnx again.

 

[KootK]

I like your original scheme with the channels. As I mentioned previously, I feel that the cross beams brace the channels sufficiently on their own. I didn't suggest the horizontal bracing for that purpose. I suggested it for overall lateral capacity of the bridge for wind, earthquake, and incidental lateral due to live loaded. Maybe design the bracing for 5% of the gravity load on the bridge applied laterally. If the two primary beams are strong and stiff enough laterally, they could even do the job without the bracing. For my purposes, the bracing does not need to be at the top flange and would probably be easier to detail centered on the cross beams. That way it wouldn't interfere with the deck attachment. I'd think that some light angle bracing arranged to take compression like Kiltors Schon would be efficient.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[greznik91]

I added bracing and chose larger channels. Here are some details. What do you think?
I designed it on 5 kN/m2. max deflecetion in the middle is 7 mm.

I dont know how to make a detail at supports since channels are only 100 mm wide. Something like this perhaps?

 

[WARose]

I'd leave the beam & brace connection design to your friendly neighborhood fabricator. (Unless your contract makes you the fabricator. You can indicate the forces on the design drawings and he'll take it from there.) You would still detail those end connections (with the slots).....and the grating hold down call outs (and anything else he doesn't have the info to do). But the rest I'd leave to him. You would likely pour a lot of time into something he is going to change anyway.

 

[WARose]

[blue](mats12)[/blue]

WARose looks like you have done something like that before. What sections did you use?

I don't recall (at this point). I just remember trying all kinds of web thicknesses and it not working. Not saying it couldn't work with some combination of web thicknesses and (required) torsional bracing forces.......but it winds up being a time consuming endeavor (when you could just add horizontal bracing (which no one would question) and be done with it).