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walkereng, Consultant
Category: Structural Engineering
Satisfied Customers: 2742
Experience:  Over 30 years of Structural Engineering experience.
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Building light duty (4000lbs) 55'x80" clear span bridge....

Customer Question

Building light duty (4000lbs) 55'x80" clear span bridge.... Need to know beam size or truss size and design..... proposed decking: PT underlay, white oak tire path... proposed material: trusses created from trailer home frames. Q: how tall need the truss be, what size and configuration need the truss verticals and diagonals be? All suggestions solicited and appreciated. Thanks Joe
Submitted: 2 years ago.
Category: Structural Engineering
Expert:  walkereng replied 2 years ago.
For a 55' span bridge you are going to have to use steel beams. It would be way too complicated to try and design trusses out of trailer home frames.
Are you OK with me sizing a steel beam option for you?
Customer: replied 2 years ago.
evidently we're on two different 'just answer' pages.....
Expert:  walkereng replied 2 years ago.
I am not sure I follow you comment? Did you ask a second question on the site?
Let me know if you would like to continue with this question.
Customer: replied 2 years ago.
emailed you earlier re truss bridge.. you suggest I use steel beams. I( am interested in your sizing - definitely. Right now is a bit inconvenient to supply you with the details however. I was out earlier sizing beams at the local steel yard.... but naturally, the good folks behind the counter responded to substantive questions by noting that they weren't engineers... OK: One bank is fully twenty foot higher than the low scalloped bank of a creek that is dry in the summer but that drains several thousand acres and rampages when it runs. No desire to build a mid creek pier. It's fifty feet from the high side abutment location to the location of the low side abutment. The low side towers would include diagonal braces running back to the beams thus reducing the absolute span on the order of six - ten fee. I intend to limit the weight load to two tons, not including decking materials.
Customer: replied 2 years ago.
that then is the general shape of the problem
Expert:  walkereng replied 2 years ago.
I will work on your question when I get to my office this morning.
Do you have any pictures of the site that you can share with me? You can attach them to the Chat Window with the paperclip in the toolbar.
Customer: replied 2 years ago.
I will send new photos sometime today... Thanks....
Expert:  walkereng replied 2 years ago.
I like to make sure I fully understand the site conditions for the proposed bridge.
Customer: replied 2 years ago.
Right.... I shall send photos with discriptions and fairly accurate distances and elevations. Thanks Joe
Expert:  walkereng replied 2 years ago.
Customer: replied 2 years ago.
I shot a short video that gives a better view of the problem but alas it's too large... So I could send it to you by email. The attached 1991 jpg shows the low side of the creek and 1993 the structure of the high side. The back fill defines the scalloping cause by winter creek erosion. My current plan is to build the low side abutment after I strip away the backfill. 1988 shows two markers, the first a plastic pole, the second a red ribbon at the edge of the bank. That plastic pole is 54' from the high side abutment. The base of the pin is 17'8" below the plumb string line that defines the bottom chord of the bridge. It's likely to be another 10 below the base of the pin to the foundation rock on the creek. The ribbon marker, at the edge of the scalloping is 61'6' from the high side abutment and 13'6" below the string line. A much better perspectiveof the project is available in the 10.8MB ten second video.... Let me know what you think;currently we're in the midst of a storm cell so I don't think there will be any more photo shoots today. Look forward to hearing from you..... Yes, it's a monumental task; no, it will be built. Thanks for your help.
Expert:  walkereng replied 2 years ago.
Let me look at your information.
Customer: replied 2 years ago.
Hey there mr Walker. I went out and bought four 18w76 x 40'. That will yield too not undersized 80 girders sufficient to carry clean span out of creek entirely. So here are my new set of questions: how far will the above beam carry itself without deflection? Presumably further than forty feet. How much further? And how many degrees of camber would be correct for an 80' 18w76?
Finally: any thoughts and considerations you might think I should be aware of would certainly be appreciated Joe
Expert:  walkereng replied 2 years ago.
Is there anyway you can upload your video to a site like Google Docs or to DropBox and then you can send me the link to it so I can watch it?
I have a few comments and questions:
1. What grade steel are your beams, 36 ksi or 50 ksi steel?
2. I am not going to be able to design a splice plate for connecting these beam, that is a lot more complex of a design that hit forum can handle, so you will need to get a local engineer to design that for you. I can still look at your beam sizing for you assuming that there will be an adequate splice plate designed and installed.
3. What spacing are you looking at for your two beams under your 80" wide deck? What size lumber will you be using to span between your two steel beams?
4. You abutments will need to be founded on solid soil or bedrock. From your description, I am not sure what your span needs to be, but maybe I will be able to tell when I get a link to your video.
5. Depending on the final span, you are going to be looking at a possible 2"+ beam deflection at your beam midspan. Not sure what your question of "how many degrees of camber would be correct" means? Cambering would be something that would have to be done at the fabricator shop through a Cold Bending process, it can't really be done with two members that will be spliced.
Customer: replied 2 years ago.
5-Yes out here in the provinces the local lingo calls the angle created by a butt joint that isn't vertical = 90degrees, but rather some degrees from 90 to accommodate deflection, as the camber. I take your answer to be: "such that a V joint would cause the top of the beam to be two inches above true horizontal" at forty feet or at eighty? I'm certainly thinking a 18w76 will carry its own weight without any deflection ar forty feet - is that correct? Those two inches would create a crown where the two forty foot beams meet but the total beam would be expected to be horizontal after the weld is competently done. Here abouts all I see on big carrying beams are butt welds very definitely reinforced by a diamond plate over the butt and welded to both beams.4- an 80 foot (more or less) span allows a concrete abutment safely out of rampaging creek on the low side. A sufficient footprint to spread one half of the beam weight 3tons plus the 'steel joists' salvages from trailer frames, plus PT two by decking is planned. Getting to 'solid soil will be our aspiration.The high side is a bit more problematical. I am somewhat worried that the weight upon the pad will tend to encouraging erosion of the 'cliff' face. That's probably not the case however as the weight should be directed downward and not towards cliff face. My current plan is a two coffin abutment, the lower pad receiving the steel bridge and the higher pad, connected by rebar and concrete to the lower, anchoring the abutment and reducing the possibility that the weight will encourage cliff erosion.3. I'm currently thinking of trailer frame joists at two foot centers with PT two by decking and perhaps white oak cross decking in the truck tire travel zone. The vastly increased beam size encourages me to consider a wider deck gor ease of driving.2- I understand and refer you to diamond plate reference above1 oh dear, here's a rub. Sounds like a critical question requires answering. So I shall seek to learn from the steel yard the answer: 36 or 50.Ok thanks. More tomorrow over and out joe
Customer: replied 2 years ago.
yes and here's the short video..
Expert:  walkereng replied 2 years ago.
I will look at the video this evening and get back to you then, if that is OK?
Probably around 10 p.m. Eastern Time if that works.
I have to drive to Dayton (2 hour drive each way) to pick something up this afternoon/evening.
Customer: replied 2 years ago.
No problem at all. Steel delivery scheduled for Tuesday. It comes to top of hollow. Will be days before actual work begins. No problem. Do you fly fish? Farm on Cumberland River. Excellent brown rainbows and not all that far from Ohio, even Dayton. Come on down. Thanks Look forward to hearing your thoughts etc Joe
Expert:  walkereng replied 2 years ago.
5. That was a rough number for an approximate 60' span. For a 40' span, the deflection of the beam under it's self weight alone would be 0.11", or approx. 1/8". I will assume that you have the butt weld and splice plate designed adequately, by an engineer or by your steel fabricator.
4. OK, I will use 80' as your span length. Not sure of the term "coffin abutment"? As a rule of thumb, bearing pressure exerted on the ground will distribute out at a 1 to 1, or 45 degree angle. So you will need to place your footing in a location with a depth that allows a 1 to 1 projection from the toe of the footing, to not extend out of the face of the adjacent slope.
3. This sounds like a good idea. I would place the beams approx. 6' apart to more or less line up directly with your two beams. What is the approx. size and shape of the trailer frame joists you want to space at 2' o.c.?
2. I will assume that you have the butt weld and splice plate designed adequately, by an engineer or by your steel fabricator.
1. I can look at both 36 ksi and 50 ksi steel for you.
Let me run some numbers, I will be back with you shortly.
Expert:  walkereng replied 2 years ago.
I ran some numbers with an 80' clear span and assumed a 2,000 pound vehicle load at midspan on each beam (each beam takes 1/2 of the 4,000 pound vehicle load).
With this configuration, the deflection is unacceptable at 4.86" at midspan.
With this same configuration, but a 70' span, the deflection is 2.96" at midspan.
With this same configuration, but a 60' span, the deflection is 1.64" at midspan.
Expert:  walkereng replied 2 years ago.
Let me know what actual span you can work with and I can finalize my recommendations
Customer: replied 2 years ago.
Ok I appreciate your continued interest. Likely I will be unable to answer the outstanding questions before Sunday evening or Monday.At 70' the midpoint deflection would be2.96". Is that acceptable? And does that consider the joists and decking? The joists, the trailer frame = 10" web, 2.25"flange, 3/16 thick at 2' centers.
A 70' span would be well above a raging creek but 12'4" below the proposed bridge elevation. I have steel to create the required supports.A sixty foot span would not be practical.So let us assume a 70' span and the calculated deflection. Hereabouts, those who consider the problem suggest that the beams be welded to accommodate the expected deflection. That is to say, rather than create a straight 80' beam, weld the two girders to create a crowned beam with a 'positive' deflection equal to the anticipated deflection. Your thoughts please.Re the high side pad. As I understand you, the force is dispersed at a 45 degree angle. But without a detailed knowledge of the soil etc, it would be impossible to calculate the extent to which the e would extend. Are there any rough approximations, any outer depth or length estimations for the proposed weight?A clarification re proposed pad: the slope of the high side abutment and the proximity to the embankment suggest to me that I might pour a pair of reinforced and connected and parallel rectangular piers separated by undisturbed ground. This would allow the uphill pier to carry its load at an acceptable removal from the bank. But that poses the question: if I have a monolithic slab on hard ground but the weight falls on let us say the forward side of that pad, is the force distributed across the whole of the pot or does the force transmit from the forward side in the one to one manner you described.Again my thanks. Hope you are enjoying the weekend Joe
Expert:  walkereng replied 2 years ago.
1. 3" of deflection can work for a private structure like this. This does consider the weight of joists and decking (I used a 20 psf Dead Load for that). If you place trailer frame steel members at 2' o.c., that should be fine to span the approx. 6' between the 2 main steel beams.
2. I really can not comment on your idea of butt welding your beam at midspan to create a 3" rise there above horizontal. That would require a complete analysis on my part. If your fabricator does this regularly for this type of structure, then you will have to rely on their experience for this issue.
3. The main issue is that you need to have a large enough footing at each abutment to make the soil pressure at the interface of the bottom of the footing and the top of the soil be maintained at an appropriate value. The Building Code usually allows a 1,200 psf soil pressure without a soil report.
Customer: replied 2 years ago.
Ok I suppose I have to accept your reluctance re welding to accommodation deflection... when you have the time, if you haven't previously read, consider "Zen and Motorcycle Acquaintance" . A good book to read for both BS and BA types...But returning to the pad..... I'm wondering what you figure the gross weight at... Let's say 18000lbs. Presumably we require only half that load per pad = 9000/1200=8 square feet, definitely manageable.
Expert:  walkereng replied 2 years ago.
Your loading will be approx. 15,000 pounds per abutment.
If you assume an allowable 1,200 psf soil pressure, you would need (15,000 lbs/ 1,200 psf) approximately 12.5 square feet of footing at each abutment.
I’d also like to point out that a Professional Engineer’s standard of care typically includes a site visit to assess field conditions and get an overall understanding of the structure. This can obviously not be accomplished through the internet. The information provided here is meant for planning purposes only (general sizing and budgeting) and is based on the information provided by you. All loading cases considered are for vertical loads only, no lateral analysis has been completed. The information should be verified by a professional engineer who can visit the site to ensure that potentially important information has not been overlooked or omitted.
Customer: replied 2 years ago.
I understand your concerns and recommendation re site engineer thanks. What is the gross weight that leads to the 15,000 per abutment, and am I wrong to divide the gross weight between the two pads? ThanksAlso: I'm hoping you'll answer my question regarding force applied to the front half of a pad: will the rear portion of the pad receive the same force? Again, my thanks Joe
Expert:  walkereng replied 2 years ago.
I took the weight of the two steel beams, the 4,000 lb vehicle and 22 psf Dead Load for the decking system. I took the total load and divided it by 2, so 1/2 goes to each abutment. You will also need to add the weight of the concrete abutment material to the 15,000 lb loading.
You can divide the load between two pads if you like, but you will need to tie them together in some manner, so they will act more or less as one.
If you place the load on your footing off center (say toward the front of the "toe"), there will a higher soil pressure under the toe than under the rest of the footing. The load will taper back toward the heal. So the pressure diagram could be trapezoidal or even triangular if the load is off center.
You will need to provide solid cross bracing/blocking at each support and at 10’ spacing across the 70’ span.

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