This can't be right- an 80', W10x22 I-beam (W), holding a 100k truck? 7

[cdowling]

Hey Guys,

First time posting here because it's the first time I've ever had to solve this sort of problem. I have some land that we just bought that has a ravine blocking the entrance. I am designing/building a very simple beam bridge to cross the ravine that consists of 5 W-Type I-Beams, spaced about 3' apart, with both of their ends bolted to very large reinforced concrete abutments. The decking will be railroad ties (6"x9"x8.5' set end to end for a total deck width of 17 feet), with a simple butt joint over the center I-Beam and about a 2' overhang on either side of the outer most I-Beams.

One day, we will be building a house on the other side of the ravine, so I am trying to design the bridge so that a fully loaded concrete truck can cross-over. My reasearch indicates that this should be about 70,000lbs. I am using 100,000lbs just to give myself some margin.

So, in a furious pencil-whipping session tonight, I ran across this website:

http://www.engineersedge.com/beam_bending/calculators_protected/beam_deflection_2.htm

after entering in all the dimensions, I found that, suprisingly enough, a 80 foot long, 10" beam can single handedly support a fully-loaded concrete truck suspended by a cable at its midpoint, and only deflect a little over a quarter of an inch... Which was very exciting at first... but then pretty scary after I thought about it a little bit. Something doesn't seem right...

I used this website:

http://www.efunda.com/designstandards/beams/RolledSteelBeamsRltsW.cfm

for all of the dimensions and Moment of Inertia information. I also used 29,000,000 for the modulus of elasticity of structural steel.

So, does this seem right? I know I haven't accounted for the weight of the timbers, but I was just looking for a really rough estimate to get an idea on how much the materials are going to cost to build the bridge.

I've attached some very simple (cheesy) pictures to help show what I am trying to do. Please be advised that this is a 27" beam shown in the picture, but the calculations I ran were with the 10" as detailed above. This is just to give you a rough sketch of what I'm trying to do.

Thanks in advance for any advice,
Cody

 

[msquared48]

Your title says a W10, but the sketch a W27. Which is it?

I can believe the W27.

Mike McCann
MMC Engineering

 

[cdowling]

Hey Mike,

Thanks for responding. I was worried that this would cause some confusion. I actually did the calculations for both the 27" and the 10". When I saw how little the 27" deflected in intial calculations, I decided to go to something I thought was obviously too small as a sanity check.

Also, please be advised this is for a single beam... I haven't even added in the other beams in parrallel.

Thanks!
Cody

 

[msquared48]

You do not have to put the entire 70K load on only one beam. Check the AASHTO code for multiple beam design for bridge decks.

Mike McCann
MMC Engineering

 

[BAretired]

My advice to you is to retain a structural engineer to check it out. Your gut feeling is absolutely right. You cannot use a 10" beam on a span of 80' to carry anything, let alone a 100k truck.

BA

 

[cdowling]

Hey Mike,

Thanks again for the quick response. I really appreciate the input. My thoughts were just to see how much one beam could support, and then work towards doing the calculations for multiple beams. However, when I saw that, at least according to those preliminary calculations, just one beam could support the entire load, it made me put the breaks on and want to ask if I am on the right track or if the wheels have fallen off somewhere.

Thanks again,
Cody

 

[cdowling]

Thanks BA,

I may end up doing that, but I am really just sort of stumped about how the calculator that I posted could be that far off. While I'm not a structural engineer, that engineering side of me wants to know what went wrong when calculations are so far off...

Thanks,
Cody

 

[msquared48]

Just a side thought here. If you do enlist the aid of a structural engineer here, and you should, you should also check out what the local fire marshall will require for the structure - as what the axle loadings will be for the fully loaded vehicles the local fire district uses.

Mike McCann
MMC Engineering

 

[hokie66]

Probably something to do with the units of your input into the calculator. Anyway, it is grossly wrong.

A useful rule of thumb for steel beam depth in a floor is to start with the depth in inches as half the span in feet. For your 80' span, that would give a 40" deep beam. For a bridge where a bit of bounciness can be tolerated, you can probably go a bit shallower, but not much.

 

[GregLocock]

Hi Cody, do you work for NASA? They've had a few problems with units in the past. For a 112 lb/ft 10" section I get about 5 feet of deflection. If I enter the length as 80 inches then I get a result more like yours.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[cdowling]

BWAHAHAHA! I'm an idiot... I just caught the inches vs. feet thing.

Ok, lemme go try again. I knew something was awry. I guess it doesn't pay to be pencil-whipping in the middle of the night.

 

[cdowling]

Ok- first mystery solved... It looks like a W27x84 beam will deflect around 9".

Any good recommendations for references that would help me figure out how to properly distribute the load over multiple beams? I am assuming it is not just "divide by 5"....

The other thing that has become clear is that those railroad ties my also not work out so well. If I've just convinced myself that a 27" steel I-Beam will deflect 9 inches, those little creosote soked 9 inch timbers spanning those beams won't stand a chance...

 

[ishvaaag]

You can use the standing AASHTO Standards for distribution of loads. Anyway in your state there maybe another standard applicable for roads and maybe your application.

Also, be aware of that the current trend on structures involves quite small deflections, and almost any perceptible deflection in a structure is rejected. This means that if under a proper distribution of the loads of the truck your bridge shows say 4 inches of deflection, it may still not be correct. Some codes establish absolute limits to deflections for specific classes of structures.

 

[JAE]

Any good recommendations for references that would help me

Yes - get a good structural engineer to do this right. It isn't just the beam design you need to deal with. You have the connections at the beams at their ends, web buckling, lateral torsional buckling, deck design, impact considerations, side rails and their design, deck camber and drainage taking into account beam deflection...I could go on.

This is something you will most likely screw up and regret if you don't get some professional help. If this is the main/only entrance to you land and your future house, you really should do this right.

 

[BAretired]

Amen to that, JAE.

BA

 

[racookpe1978]

My congratulations on your recognition that "something wasn't right" !

Now, keep thinking: But you DO need a "real structural engineer" to check the bridge calc's.

The total load can be reduced several ways" to reduce the costs of your entry way bridge to your new house: First, by allowing two (or three) partially loaded concrete trucks to pour the foundation, you'll reduce the need for that 100,000 lb load. If you are the only resident "up the creek" YOU can reasonably control the people (fire trucks, loaded furniture moving vans, and possibly the construction trucks carrying plywood and roofing material and bricks) coming up the up and over the dell. (Obviously, put a sign up at the entrance warning of a weight restricted bridge ahead.

The max truck load will be distributed over the front and both rear axles - your pinpoint single load assumption isn't correct; and if you control the driver properly, you can reduce speeds and allowances for "one-of-a-time extreme loads" without eliminating the real need for a safe driveway for your family vehicles over the long term.

The load will be further distributed over the 5 (?) WF beams by the wood (railroad tie ?) cross members.

All of these reduce the margins, and bring you closer to yield point and give you a lower cost for the bridge - but if it is NOT a highway bride or a public road, then you can more reasonably plan for reduced margins and "crazy drivers at high speeds driving overloaded trucks."

 

[cessna98j]

As others have said there's a lot more to beam design than just the deflection criteria you're considering. For example, the spreadsheet you found gives you a deflection of 1/4 inch for an 80(INCH) span with 100K load. While the deflection is relatively small, the stresses in that single W10x22 are over 85KSI - way more than allowable. Shear stresses are also greater than allowable.

 

[cdowling]

Hey Guys,

Thanks for the replies- I appreciate you taking the time. Some of the replies were very helpful, and I am looking more into the suggestions offered there. I'll let you know how that turns out.

However, I am not sure that I agree with the "you must have a real structural engineer do this for you" or " you'll probably screw this up" replies though. Now, I fully intend to have a structural engineer look over the drawings, check them and give me feedback, but I think that I should be able to get to a good first order approximation, work up the drawings and submit the design package to the structural engineer- without just throwing my hands up and saying "it's too hard, please do it for me."

At the end of the day, I have to believe that I should be able to complete the design to the point where it is an accurate representation of a several run-of-the mill, well understood wide flange I-Beams placed in parallel and bolted to some engineered slabs buried into good ol' mother earth, with some wooden timbers bolted across them for decking. My intention is not to go off half-cocked and build it from this drawing, but instead to put the drawing in front of this gentlemen (or lady) and have them give me reasonable feedback and suggestions.

Thanks again for your help. I'll let you know how things progress.

Cody

 

[JStephen]

Some thoughts on that: First off, if you're starting off on design, and don't realize what you even need to check on it (lateral-torsional buckling, for example), there's not a very good chance of getting everything else right, in which case it doesn't benefit the structural engineer much. Secondly, if you have a structural engineer check it, at some point, you're also liable to need PE-sealed drawings, and in many cases, that engineer can't seal your drawings, regardless of how well he checks them. So he'll be starting from scratch to design it and draw it anyway. Thirdly, part of the motivation to hire somebody that does this kind of stuff regularly is they'll know what's normally done, what beam sizes to work with, whether to use beams or trusses or concrete beams, how to detail them so the steel fabrication people know what's going on, what the codes require, etc.

 

[rb1957]

i feel your pain cody, but i don't think there is any middle ground. i don't think you can design the bridge and then have a structural engineer look it over. His professional exposure is too great

i think it's either have the engineer do the job; i sense you feel he'll overdesign the job ... design a highway bridge whereas all you want is a "residential" bridge.

alternatively, do it all yourself; which i think we all agree is a bit risky.

but who's designing your house ? couldn't they also do the bridge ??