Lateral Bracing for Pedestrian Bridge

[JonathanEngr]

I'm a civil/environmental engineer by trade, but had the typical structural and steel design courses during my course of study--just enough to be dangerous! I have a friend I'm trying to assist who is wanting to span an area roughly 50 feet in length with a pedestrian bridge capable of handling typical pedestrian loads an possibly a small riding mower from time-to-time (he's essentially joining two pieces of property via a bridge). I did a rough estimation on the steel required trying to keep it in the realm of "typical" sections. I came up with a max moment of 13.88 kipft, which will be supported by three 50-foot w8x13 steel sections (to keep my deflection less than L/360) braced every 12.5 feet allowing a max load of 17.84 kipft. My questions are... would I be better served to resize this for two steel beams? The bridge will only be 5-6 feet wide. Secondly, exactly how should I go about laterally bracing these pieces of steel? Do I need to utilize diagonal bracing, or will welding a steel piece perpendicular to each piece of steel every 12.5 feet suffice (does anyone have a good detail/sketch for lateral bracing?). Lastly, I told him to use treated wood as decking, but since I'm not familiar with bridge decking tie-ins I'm not sure what the best way would be to tie the boards to the steel sections (I'm also wondering what the best way to tie the railing to the steel would be, as well). Any suggestions? Thanks in advance!

 

[SperlingPE]

Lots of questions.
1. First your max moment. I back calculated the continuous load on that to be about 44 plf using a simple supported 50' span (w*l^2/8). That seems light.
2. What are you using for your deck? Have you included this as dead load?
3. Have you considered the wind load (or other lateral load) on the beams?

 

[FalsePrecision]

First of all, I agree w/ Sperling. When I see W8x13 spanning 50 feet, that immediately looks far too shallow and light to span 50 feet - deflection would be extremely large.
Have you considered GluLam beam in lieu of steel?
The long, lightly loaded pedestrian bridges are actually most efficiently designed using steel truss system...the load is relatively small, and span is long.

 

[JonathanEngr]

Ergh... you can see my point! Okay--I haven't considered the decking load (very good point) nor the wind load. What I did consider was the following:

A 900 pound point load at the center (lawnmower and passenger), the distributed load of 13 pounds per foot per beam (dead load beam weight) and 10 pounds per foot PER beam (30 pounds per foot across the span) distributed load for other pedestrians, etc. I came up with a max load PER beam of 13.88 kipft (I assume it's okay to divide the point and distributed load by 3 for three beams? Thus, 300 pounds/beam point load and 10 lbs/ft per beam distributed load). I had not considered a truss system for the crossing. Again--this area is (obviously) greek to me--I'm unfamiliar with what is "out" there, or available, for such systems. I'm assuming you mean the metal trusses that I see regularly for ceiling supports in commercial/industrial buildings?

 

[FalsePrecision]

Trusses for roof or floor support, yes. Google: SJI Steel Joist Institute.
For your span/load situation, a W-beam is inefficient. Also, an 8" deep beam would deflect huge amount.

 

[JonathanEngr]

Thanks, FalsePrecision. I'm looking into it now.

Regarding the loading, you mentioned it seemed light. Does this still seem to be the case with the numbers I posted?

 

[SperlingPE]

I would look at a live load distributed load of at least 50 psf. Parking garages are 50 psf. You could probably consider less, but I don't know the circumstances of the project (relationship with owner, legal jurisdictions, etc.). The other live load to check is the wheel load for a mower as it moves across the bridge.
If you have two beams @ 6' apart each beam would carry 3*50=150 plf. If you have 3 beams with 3' between them, teh middle beam would carry 3*50=150 plf and each side beam would carry 75 plf. Be sure to account for all dead load (deck, side rails, secondary structure, etc.)

 

[FalsePrecision]

Assume 3 beams, each W8x13. Width of bridge=6 ft.

Assume railing weight is small. Therefore center beam governs, trib=3'

Assume live load = 40 psf (ignore the riding tractor). Any less live load, you are taking a huge legal risk.

Dead load of decking, misc about 6 psf. Also include the beam self-load, 13 plf.

Total load = 153 plf

Bending moment = 47,800 ft-lbs.

Deflection of W8 x 13 = 18", about L/33

No way !

 

[FalsePrecision]

Sperling -
Live load for garages = 40 psf. per IBC codes

 

[SperlingPE]

I was looking in UBC (what I had open) for the garage load. Pedestrian bridges are listed at 100 psf.

 

[JonathanEngr]

Thanks so much for the input to my question. I know it's pretty mundane, and I appreciate the thought and effort everyone has put into the solution.

To clarify a bit more on the bridge, my friend lives out in the country, and he's wanting to cross a creek to reach his parent's property directly either to visit, take the lawn mower, etc. Thus, this *will not* be engineered, signed and sealed by me. He's looking for an inexpensive way to make the crossing, and my initial reaction was to saw down a couple of the huge oaks on the bank of the creek <grin>. I did call a steel truss mfr and went through some criteria, and he rec'd three 26k8 beams, and they're very inexpensive.

If you don't mind... for my own knowledge. Why would I need/want to use 50 psf when that is used for parking decks? I'm not planning to allow for the load of a car to cross the bridge. This is NOT my field (obviously!), but I'm curious why I'd want to use such a high load. Also, for 48 kip-ft I'm coming up with 9" deflection on the w8x13.... not 18". (Which would obviously be too much, too. But with the <too small> loading I had anticipated I had a 1.64" deflection).

 

[JonathanEngr]

I think this is where I've really dropped the ball. I'm not familair with standards of practice or code for Structural Engineering just like we have in the Gen Civil/Environmental world. I'm trying to make too many general assumptions and deriving my values from basic textbook formulas. I think these loading figures will go a long way towards what I need... can I find a table of loadings for various structures somewhere online?

 

[FalsePrecision]

Pedestrian bridge live load = 100 psf is fine if you are bound by that code. If you are not bound by code, 40 psf is more reasonable. If you want to prove it to yourself, get some really close friends (who have bathed recently) to stand next to each other, clustered close as possible. Get the perimeter marked off on the ground. Weight of people/area = 70 to 75 psf.

 

[SperlingPE]

False
I am with you on the live load. I wanted to point out the 100 psf in the case that the bridge is under someone's jurisdictional control. 40 psf sounds appropriate.

 

[FalsePrecision]

Jon...
If you are getting 9" deflection...are you using I=40 in^4 for the W8x13, and E=29e6 psi?

 

[swine]

JonathanEngr, you should have posted your question in the bridge engineering form.

This is what happens when "structurals" design bridges--this thread is amusing, chaotic, but amusing.

For one reason or another "structurals" don't feel daunted by a pedestrian bridges. Yesterday, I inspected a pedestrian bridge that was probably designed by a "structural" at least it looked like it was. One of the seats is showing signs of failure and I don't think the bridge was long enough to allow for the meandering of the channel, scour mitigation required. They would have saved saved money if they had hired a bridge engineer that is a "civil engineer." We don't get a lot of pedestrian bridges in our office because non bridge engineers feel more comfortable with them, so we receive less references.

I for one have never used IBC or UBC for bridge design. I consider myself a civil engineer not a structural engineer.

 

[FalsePrecision]

OK "swine", now that you have shown your immaturity, can you give any helpful hints to this issue?

 

[JonathanEngr]

Sure am... I = 39.6 in^4, E = 29 e6, and I'm using the formula Pl^3/48EI... Ugh--sorry. I was using Pl/2 instead of Pl/4 to determine the point load from the given moment. Man am I getting rusty with my old age. I'm now getting 18".

Regarding the loading, I absolutely agree that 100 psf makes sense if you ever anticipate the bridge being loaded with people. I was assuming (bad thing to do I'd say!) that the bridge might have 8 to 10 people on it at any given time along with the mower and its passenger. This is where I derived my loads, and compared to what was listed here they are veeeeeeeery light.

Okay--final question. Someone mentioned the lading for a parking deck to be 50 psf. Is this correct? That's all well and good assuming a car would take up, say, a 10x25 foot area (250 sf) plus no other car would be within 10 feet of that on any side (30x45, 1350 sf). Thus, you're talking about well over 60,000 lb capacity in that area at a 50 psf loading. However...... what about the immediate loading from a tire? Say the car weighs 5000 pounds with equal distribution on all 4 tires. The load would be 1250 in an area roughly 1 sf if that. Is there a separate calculation that is performed to prevent failure from this loading? I know I should know this... I did take structural in college, but geez--that was over 15 years ago...

Also--I quickly read the new messages to see if there was something else I needed to address. Also, I spoke with a guy from one of the truss companies, and he encouraged me to use 100 psf since the difference in truss cost would be exceptionally minimal. I'll admit--this has been a terrific question for me to have asked--I've learned quite a bit. Finally, I apologize if I posted to the wrong forum--I actually looked for a "bridges" section under structural. Thanks!

 

[swine]

Hi FalsePrecision,
I agree with SperlingPE's first post. I'd look into a premanufactured steel truss bridge or glulam. But if your going to get funky:

http://www.infrastructureconnect.info/pdf/outputs/R8133.pdf

The loads and nearly everything else should be per AASHTO Standard Spec's section 3. Deflection will control if your going with steel girders (this is an issue of comfort more than anything else). Use ASD, LRFD is typically more conservative, because the ratio of live to dead load is high. Be creative and just design it.

The layout, approach, and substructure are more difficult than sizing the beams.

And yes your welcome for the link above. Please get the AASHTO spec's before designing--after all your taking my work, so at least attempt to do it right.

 

[JonathanEngr]

Geez guys... this is a *forum* where we're supposed to help each other out, right? This forum wouldn't exist if people didn't have questions, so why all the back biting and snippy comments?