Still laterally restrained question

[SayGoodDay]

Hi Fellows

Refer to thread (

http://www.eng-tips.com/viewthread.cfm?qid=191669&page=2)

Now I have a situation, a footbridge (max span 24m) with steel beam, deck 3m wide.

--------------------- <-- timber deck
--- --- ---
l l l <-- mid 250UB sit on SHS
l --- l
l-----------------l <-- 200 x 9 SHS @ 6mctr
l l
l l <-- Edge beam 800 WB
--- ---

there are 12mm plate stiffeners at outside of edge beam at SHS location (6m c-c).

My question is, can I consider the edge beam laterally restrained at SHS location. I don't want to under estimate or over estimate the situation.

Also, does anyone know how to calculate the natural frequency and vibration for the pedestrian bridge. Any example refer to Australian standard.

Your comment is very valuable. Thanks.

Marco

 

[apsix]

I would add angle diagonals to create a horizontal truss; this should provide lateral restraint in my opinion.

Spacegass can be used to determine the natural, alternatively use the formula found here;

http://www.vibrationdata.com/tutorials2/beam.pdf

There is a simple formula based on deflection, a google revealed this;
fn = 18/(deflection^0.5); -deflection in millimetres

Just a question, why the use of 200 shs for the transverse beams? I would tend to use open section UB or UC.

 

[SayGoodDay]

Thanks apsix

I did not design this footbridge, but I need to verify the design and prepare a report for council open discuss. (So that is why I don't want to over/under estimate any problem).

In your point of view, can the stiffener and transverse beam system provide lateral torsional restrain?

Also, is fn = 18/(deflection^0.5) only for concrete floor?

Thanks.

 

[apsix]

Now that I'm in the office;
From AS1418.1-1986; fn = 16/(deflection^0.5) but this may be for point rather than UDLs, and it is applicable to steel only.

Without a stiff deck, or horizontal truss, I would assume the bridge beams to be unrestrained. As explained in the link you provided, any effect due to twist restraint can not be quantified by AS4100.