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Re: Beam sizing
Re: Beam sizing -- mikeweber3	Post Reply	Top of thread	Forum

Posted by: swearingen ® 04/09/2007, 08:09:22 Author Profile eMail author Edit

There are many things to consider to design a top chord for a bridge. Loads are one, and you have started to address them. With the loading given, the chord force would be just over 8,000lbs compression. The problem is, I think you are missing at least one load case - distributed load. ASCE recommends a 60psf load be placed on walkways, bringing the chord force up to just under 13,000lbs for a 3' wide bridge. Buckling under this load is the main concern. Buckling in the plane of the hand rail is usually not a controlling factor - it's pretty easy to ensure that the chord can span between your vertical supports. Buckling out of the plane of the hand rail is a difficult problem. Essentially, what you are constructing is a C shape laying on its back. If you bend a piece of plastic channel, you'll notice that the sides bow out. This is what will happen to your bridge (with catastrophic consequences) if this out of plane buckling isn't properly accounted for. You may have noticed small triangular outriggers on bridges held up by their hand rail. They are there to prevent the top chord from buckling out of plane. I hope I've given you enough information to show you that you need some further help. I recommend speaking with a qualified structural engineer on a 50' span bridge. The loads are quite high and failure modes such as out of plane buckling of the top chord are not intuitive and not straightforward calculations.

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Thanks but still need more
Re: Re: Beam sizing -- swearingen	Post Reply	Top of thread	Forum

Posted by: mikeweber3 ® 04/09/2007, 11:13:23 Author Profile eMail author Edit

Thank you for your reply. I beleive I have already adequatly accounted for the "out of plane buckling". My design includes a deck structure that is 14" tall consisting of a practical non racking system. From this deck will be 2" square tubing vertical risers connecting the deck to the hand rail. These risers will be welded at the bottom and top of the deck. This will essentially create a simple lever that will give the rail a 3-1 ratio advantage when it attempts to buckle out of plane. The force even at 3x will not overcome the anti-racking I have built into the deck. I am not a structural engineer and I very much appreciate your help with the 13000 lbs of compression force on the top chord. Can you tell me what material would meet this requirement. My desired choices are 2"x3" 1/4" wall square tubing. 2"x4" 1/4" wall square tubing. If neither of these is acceptable I will probably go with a built up assembly to achieve the needed section moduluous. Does my Idea for anti buckling of the top chord sound reasonable to you? XXX XXX l l l l l l l l l l l l l l l l l l l l l l l______________________________________l Deck boards l--------------------------------------l 2x2 square l0\ /\0 / 0 / /0l l \ / \ / \ / \ / l bar truss l \/ \/ \/ \/ l l--------------------------------------l 2x2 square 0 0 0= 6 50' long curved members 2x2x1/8 Thanks again Mike

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Drawing didn't work
Re: Thanks but still need more -- mikeweber3	Post Reply	Top of thread	Forum

Posted by: mikeweber3 ® 04/09/2007, 11:16:01 Author Profile eMail author Edit

Sorry my drawing attempt didn't work.

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Thanks, I am done
Re: Drawing didn't work -- mikeweber3	Post Reply	Top of thread	Forum

Posted by: mikeweber3 ® 04/13/2007, 11:53:01 Author Profile eMail author Edit

A member of this board contacted me via E-mail link and spent at least 4 hours of his time helping me with my project. I plan to proceed using 2x4 for my top chord. More importantly, I will be using 2x4x1/4 instead of 2x2x1/8 for my vertical supports joining my top rail to my deck. Sometimes I amazed by the goodness of some people. After about 2 hours on the phone and I don't kow how many hours he spent off line, I asked him what motivated him to help me. He didn't give me a clear answer and maybe doesn't even know himself. Still wondering I asked about his faith. He proclaims to be a Christian. I'm curious and yet also very appreciative, Why would religion motivate someone to an unselfish act? Thanks Mike

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Re: Thanks, I am done
Re: Thanks, I am done -- mikeweber3	Post Reply	Top of thread	Forum

Posted by: Administrator ® 04/14/2007, 15:08:47 Author Profile eMail author Edit

"A member of this board contacted me via E-mail link and spent at least 4 hours of his time helping me with my project." Engineers Edge has been around seven+ years. The regular contributors on this Engineering Forum are incredibly knowledgeable and generous. Engineers Edge is very fortunate and appreciative.

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