Help Calculating Bridge Beam Loads Over a Brook

Well I would ask the guy who designed it.....The beam shape, steel spec, support structure all come into play....

http://pada-structure.blogspot.com/2011/06/section-10-steel-beams.html

What is a 12x26 beam? Do you mean 12"x26"?

Are you driving battle tanks over this bridge?

Sorry Lyn, W12x26 beams.

Wide setion 12" tall 26 lbs/ft

Thanks greatly Lyn! So using the calculator on that page and plugging in what I can for a 10,000 lb point load at the middle of the bridge, I get: Total Load : 10000 (lb) Length of Beam - L : 192 (in) Moment of Inertia - I : 204 (in4) Modulus of Elasticity - E : 29000000 (psi) Distance of extreme point off neutral axis - y : 6.25 (in) Support Force - R1 : 5000 (lb) Support Force - R2 : 5000 (lb) Maximum Stress - : 14706 (psi) Maximum Deflection - : 0.25 (in) I'm guessing that .25" of deflection on 16' is not too much, but not sure on the stress of 14706 psi. The steel is A36, so from what I read in PADA link from Solar Eagle above, I use 36,000 PSI x 0.66 = 24,000 PSI for an allowable stress value since they say all W sections are what they call "compact". So if all that is true 14706 PSI is within 24,000 PSI calculated. And 10,000 lbs per beam times 4 beams would be 40,000 lbs. That seems pretty good..

Remember you have to deduct the weight of the materials from your total....The wood is not going to be light, and the beams themselves are quite heavy....pt 2x6 is 51 lbs per 16'....

https://www.engineeringtoolbox.com/green-kiln-dried-pressure-treated-lumber-weights-d_1860.html

That's what, "This may help, after you determine the static load." meant here #6.

Thanks very much for your help guys, Okay, so, to figure static load, the steel beams are 26 lbs/ft, so that's 16' (span) x 26 lbs/ft = 416 lbs. ea. Times 4 for four beams say 1700 lbs. Decking is 6" thick x 16' span, x 12' wide = 96 cubic feet. At 37 lbs/cuft for yellow pine that would be 3552 lbs, say 3600 lbs. Wood railing, paint fasteners, misc. say 1500 lbs. Grand total = 5800 lbs....well, let's say 6000 lbs dead weight on 4 beams. That's 1500lbs/beam. I should re-do the beam figures in the earlier post -- I only used a hypothetical point load at the center of 10,000 lbs. for a single beam. And that turned out, according to the engineeringbox calculator well under the max permissible stress. Correct me if I'm wrong, but I think I should do the stress for a uniform load (for the dead weight) and then a center point load for the full truck weight, and add them together -- which seems like a conservative way to do it, since the axles will never be a single point load together like that -- especially on a short 16' long span. Does that seem reasonable? Thanks for your help again. --Steve

So using engineering toolbox calculator for a single beam w/ 1500 lb uniform load (dead weight), it shows: Unit Load - q : 7.81 (lb/in) Total Load : 1500 (lb) Length of Beam - L : 192 (in) Moment of Inertia - I : 204 (in4) Modulus of Elasticity - E : 29000000 (psi) Distance of extreme point off neutral axis - y : 6.11 (in) Support Force - R1 : 750 (lb) Support Force - R2 : 750 (lb) Maximum Stress - : 1078 (psi) Maximum Deflection - : 0.0234 (in)

Sorry about the one paragraph format of my replies here -- they are broken up into separate lines and paragraphs when I post, but for some reason, they run together when it appears on the forum. Makes it hard to read. Apologies!

Good of you to do some math.

I did a quick check for that beam size (x4) and to me, the allowable load with a FOS of 2:1 would be 35,000 lbs+/-.

That does make the assumption that the beams would be tied together at the abutments and at least one point in between the span to reduce any torsional load.

I haven't done any bridge design/build in a very long time so, please consult a pro to ensure you have the necessary cross bracing.

Thanks Solar Eagle! The pic looks like a long suspension bridge, this is just a 16' long simple beam bridge over a brook. Sorry, I can't contact the original designer, as he's not alive. I do thank you for the link, I've tried to work out some of those formulas, but I still get lost with some of the terms. I would still appreciate some help from someone, if that's possible. Steve

I found some W12x26 beam specs in the table here: https://www.engineersedge.com/standard_material/Steel_ibeam_properties.htm Sorry I left the "W" out in my first post,,,,,

Yes I was guesstimating that the max load was the same ...remember safety margin...

https://www.aisc.org/globalassets/modern-steel/archives/1997/03/1997v03_bridgexings_no05.pdf

Thanks very much Solar. Safety margin -- where can I find that, exactly? Or better, what is an accepted safety margin factor for a simple small steel beam bridge like this one? .

I'm afraid the links you gave were for much more complex highway bridge design studies for lifetime reliability assessments involving unspecified parameters like "load factors" and "resistance factors".There's no way I can do that.

This creek bridge seems like it ought to be a more straightforward problem with simple parameters.

I also thought that the recommended 0.66 multiplier for the A-36 steel steel section's max stress was a safety margin. But willing to be orrected on that. Can someone tell me what safety factor is customary for this type of small simple steel beam bridge?

A new wrinkle....so to speak. The steel beams were delivered today, and they were stickered as ASTM-A992. Not A-36, which was what I had tried to calculate with earlier. A36 has a yield of 36 ksi. A992 is listed at 50 ksi. So that is a bonus.

From what I can calculate so far with what I've been given here (subject to any suggested changes) I get :

1.) about 1000 psi static deck load stress, per beam, and

2.) assuming a point load at center span of 22,000 lbs:

Total Load : 22000 (lb)

Length of Beam - L : 192 (in)

Moment of Inertia - I : 204 (in4)

Modulus of Elasticity - E : 29000000 (psi)

Distance of extreme point off neutral axis - y : 6.11 (in)

Support Force - R1 : 11000 (lb)

Support Force - R2 : 11000 (lb)

Maximum Stress - : 31628 (psi)

Maximum Deflection - : 0.55 (in)

If I add the static load stress of 1000 psi to the calculated point load of 31628 psi I get a little under 33000 psi, which just equals 66% of the 50,000 psi yield strength of A992.

For four beams, that gives a max center span point load of 88,000 lbs.

Now, the question for me is how much to reduce that for this margin of safety -- or does that actually even apply here? It's a driveway bridge, not a highway bridge, so the simple question for me is, can a fuel delivery truck, or a septic pump-out truck occasionally cross? If 88,000 lbs is 66% of calculated yield strength,

I think these trucks are less than that, and also, their weight isn't all concentrated at the center of the bridge -- they have at least 2 axles, and the bridge is only 16 feet. Even if they were only a single point load, I think of 80,000 lb trucks as things like full concrete mixers or 10 yard loaded dump trucks. Am I thinking about this correctly?

A typical 20 foot, 3 axle, loaded water tanker weighs in at about 50K# MAX.

Federal law limits axle loads to 20K# per axle.

Safety factors are somewhat ambiguous, but I think your bridge will support much more weight than it will ever see.

Build it and they will come.

Yes I think that is correct as a safety factor....I would be more concerned with the foundation support at this point, the bridge structure seems up to the task, and quite capable of the 20k load per axle as an occasional max load...Regular inspection and maintenance only remain after that...

https://extremehowto.com/build-your-own-bridge/

Lyn and Solar Eagle, thank you guys greatly. I'm feeling more comfortable knowing what the online calculators are showing with the specs I have, and the weights of trucks and axle loads.

The hard part is not knowing, and it really helps to be able to confirm what the original drawing was showing for a bridge. I like to understand things myself. It helps the confidence level, plus it's interesting to know what is happening.

Solar Eagle just to let you know about the abutments, they are three rows of stacked concrete blocks . Six blocks on the bottom, 5 blocks next row, and 4 blocks on the top. They are 4' x 4' x 2' thick 3000 psi concrete and weigh about 4800 lbs each. The beams lay on top of those, and then are backed up by another three 4800 lb blocks whose tops are at driveway level. There is riprap stone at the ends of the block. The blocks are plaed on a 2' thick bed of coarse crushed rock.

Solar Eagle, thanks for that bridge building link it's very interesting. The deck details look similar, but I notice that they have tied the deck wood down to the steel beams with bolt holes through the outer flanges along the length of the bridge.

I thought that drilling through flanges was a no-no. I was considering using toe clips to tie the wooden deck to the flanges.

Is the direct through-bolt hole method acceptable?

Thanks, Steve

I don't think the effect is significant....but I would defer to the bridge design engineer...

You may be confusing us with people who actually know something about bridge building.

I know that I, and I believe SE and others here as well, are just bystanders who are using mostly google as our references.

I am not a structural engineer, I can't speak for SE nor the others.

I "think" your bridge will be more that adequate, but that is only my opinion.

Thanks Lyn, easy mistake to make -- I appreciate the candor. It does reduce my confidence level in statements here to a small degree.

There was useful information provided nevertheless, and that allowed me to work my way through calculations which increased my level of confidence. So a net gain overall

Even if there had been structural engineers on-board, I wouldn't have expected them to calculate for me, or assure any particular performance. I mainly wanted help from those with experience in working out the calculations. And who could advise on conventional practice (eg. flange penetrations.)

I do thank you for your help -- I definitely understand more now than I had before.

Most typically, W12x26 are classified as I-beams...

Simplistically, you're mixing steel beams with (very shallow) wooden joists, without providing a clear diagram, or clear details, of how they are going to be held-in-place with each other...

Is the structure going to support the moving load(s) only in simple bending, or is there to be any sort ot truss-structure on top?...

What is the intended service life of this structure to be?...

Specifically, to what penetrations are you referring?...

What (organisational authority) is approving this structure's design as ''acceptible''?...

I, as a Registered Civil Engineer could sign-off on the this structure, in my State, but I would not because it clearly is not going throung an appropriate approval procedure...

All good points.

Even more to make - the design of the abutments/foundations at the ends of the structure, and any roadwork leading to and away from the bridge have to be properly designed and constructed. All the other what if's as well, such as,,, any water flow and what rate of flow especially at peak conditions. You can design and build a heavy enough structure to transport a tank, but,,, alas,,, if the foundations aren't done properly the whole thing can just be washed away or fail due to loading.

Seek professional help

Quite so.

Any structure with a faulty foundation is an ''accident trying to happen''...

''GA'' from me to you and to the providers of the good pictures, and links, for use as (informational reference by professionally responsible persons)...

FYI, even Registered Engineers can lose their license for practicing engineering in disciplines other than their own...

Yes indeed. I am very careful as to what I apply my seal to. If it's not within my license, I refer to others that are qualified. Where I have little or no option, I always increase the F.O.S. substantially.

Mr.LaPaire,

A survey was performed by a registered engineer of flow rates and terrain and base flood elevation was established, as well as the effect on water level downstream of the bridge abutments. These were submitted during the permit approval process. The results showed the design far exceeded requirements

The abutments were designed, as stated earlier by a licensed engineer and the plans submitted and permit approved by municipal and state authorities. The abutments are substantial, and utilized 175,000 lbs of concrete. They weren't the actual subject of my original question, which was how to calculate the acceptable load for the beams in question.

Very good. Your original post made no mention of these facts.

Glad to hear you did the appropriate things prior to construction. My concerns/comments were based on the many other posts in this forum that never give enough initial information, as I am sure your have read as well.

Mr. Guest. You've made a lot of unfortunate negative assumptions.

Actually this is a design by a registered engineer, stamped and permit approved by both the state and municipal authorities. I did not design it. It is a vehicular bridge, stated already. It is a simple beam supported bridge, stated already. It does not have "joists" per se but a solid wooden deck 6 inches thick, composed of pressure treated ground contact rated 2x6's on edge nailed together. Over that are 3" thick by 39" wide runners lengthwise.

I asked about bolt hole penetrations to the flange as a general question becuase it was brought up by someone else, under the personal assumption that they are NOT appropriate, but willing to be corrected in my basic understanding. Looking it up online I did find a Canadian engineering study that suggested that holes less then 15% of flange area were permitted, but there will be no holes at all in my own bridge. I just wanted clarification of conventional practice with regard to wide flange beams (which I understand is the more appropriate term these days vs "I-beams" which refers to a different earlier common steel section.

I'm not sure what you mean it is "not going through an appropriate approval procedure". It has, and I was asking questions to build my own understanding, not to get put down.