Farm Bridge

I don't think so.

How could welding on a brace weaken the original strength of the I beam?

The worst it could do is nothing, I would think.

Well now, Grasshopper. welding does weaken the base metals. But only locally, which may not be significant. PWHT may not be necessary. Milo, and a few others, will be able to give an informed answer.

I was trying my best not to sound like I knew what I was talking about. But it does seem to me like the strength of the I beam is derived primarily through the vertical center section of the beam, therefore I would think that welding on the flat bottom of the "I" would have little impact on the strength of the beam itself.

Please tell me it's true master.

But it does seem to me like the strength of the I beam is derived primarily through the vertical center section of the beam,

Wrong. If you look at moment of inertia stuff, the parts farthest from the neutral axis contribute most to the strength--the beam flanges in this case. They reach yield stress quicker than the stem.

I agree with the X bracing mentioned later by Old Salt.

"I do not wish to fight you".

In this instance, I believe the weakening would be insignificant as you say.

My question to you is, what does "W6x16 I-beams 24" oc" mean?

W6x16 is a wide flange I-beam 6.28 inches high by 4.03 wide, spaced at 24 inches, on center.

Sorry ba/ael.

Structural Steel W Flange Section Properties Moment of Inertia ...

Wow. How far we can get off the mark! How do you get 6.28" x 4.03" from W6x16?

Is W6x16 a standard? That's why I hate standards; they don't make sense.

OK, talkin' sh*t now, maybe.

in × lb/ft Area (in²) d (in) bf (in) tf (in) tw (in) Ixx (in⁴) Iyy (in⁴)

W6x16 4.74 6.28 4.03 0.405 0.26 32.1 4.43

Sorry, I can't get everything lined up here.

With wide flange sections, the inner rolls are set for a range of members, in this case, all nominally 6"x4". The different weights are achieved by moving the rolls further apart for the heavier end of the range.

It happens.

It's a little risky telling someone that says they have "I" beams, they don't.

All's good over here, while I still was not able to snatch the pebble from your hand, at least I know I was right.

Well, at least I wasn't the only one here that was a little off.

http://en.wikipedia.org/wiki/I-beam

flyinghigh,

• W6x16 are not I-beams, they are wideflange (W) shapes. There is a 6" I-beam at 17.25#/'. It is designated S6x17.25.
  
• Welding an A36 angle brace to the bottom does not change the beam's yield strength.
• What is the purpose of the midspan brace on the bottom? It is not customary to brace the tension flange.
• It is also a little unusual to space beams at 24" centers.
  

I agree with ba/ael, on it being unusual to place the angle on the bottom flange. It is the weld metal that mixes with the steel of the member so if you use a "matching" weld rod it should make no difference.

I have been involved with constructing several bridges/structures such as you are planning. Steel beams were primary H/I's that were readily available for cheap. Wooden were primarily glulams. We always placed the braces from the top of each beam to the bottom of the adjacent beams. This formed sort of an "X" configuration between each adjacent set of beams. If the braces were placed along the bottoms of the beams as you propose there would be a tendencey of one beam causing the adjacent beams and others to follow its movement should it start to twist. With the "X' configuration of the braces each beam will resist the twisting of adjacent beams.

This is also how braces are installed between joists when building a house or structure. Each joist resists the twisting of its adjacent joist because of the "X" configuration of the braces. By only installing along the bottoms of the joists you would be at risk of parallel failure of all the joists.

Good Luck- old salt

What are you trying to determine? The load capacity? I would think the capacity would be with the heaviest loaded wheel at center span with the whole load on one of the beams. Don't forget the dead load of deck, beams, etc.

Thx to all who responded.

Good morning TrevorM,

Your note gives me comfort. The client has requested the design be based on a 60,000# GVW. This would equate to 30,000#/axle, or 15,000#/I-beam at mid-span. This would induce a moment of 247,500 in#, and a corresponding stress of 24,265 psi in the selected W6x16 Fy 50 ksi beam, providing a factor of safety of over 2:1. The AISC manual typically uses 0.66 Fy where I used 0.5 Fy. The section modulus of the beam is 10.2 in^3, exceeding the calculated required modulus of 9.9 in^3.

The manual emphasizes top bracing, but not bottom bracing. On top the I-beams will be placed steel plate, which I have called to be welded to both sides of the top flange at 1/4" x 2" fillet welds 48" oc.

flyinghigh alert!!!

"This would equate to 30,000#/axle, or 15,000#/I-beam at mid-span. This would induce a moment of 247,500 in#, and a corresponding stress of 24,265 psi in the selected W6x16 Fy 50 ksi beam"

M = PL/4 = 15*11/4 = 41.25'k = 495,000'#, but you should be using an impact factor of 1.25, so your design moment should be 618,750'#.

Beams at 2' centers does not appear to be a practical solution to the problem. You should consider other framing systems.

Good morning ba/ael,

Since the beam ends will be fixed, I used the corresponding formula.

flyinghigh,

What is going to fix the ends? A 28" x 16" x 30' long footing resting on soil? Give us a break! You should be considering the beams as simple spans.

In my humble opinion, you are going to find yourself in serious trouble if the design load ever gets on this bridge. I suggest you think it over.

The client has requested the design be based on a 60,000# GVW. This would equate to 30,000#/axle, or 15,000#/I-beam at mid-span.

What is the truck? Single or dual rear axle? If a single rear axle, your load distribution won't be 50/50; if a dual axle, the front axle still won't have 50% of the load. With dual back axles, is the moment worse with one of them at the center and the other toward an end; or is it worse with the centerline between the axles at the center of the bridge? You probably will have to calculate both ways.

Calculating for fire apparatus weight is an absolute; contact the local fire chief for their weights, then add some for future increases.

If the wheel(s) happen to be midway between the beams, the beams will be OK, but how about the deck?

Good morning Lehman57,

The design deck's 1" steel plate thickness checked out ok.

A one inch steel deck? Wow! At 490 pounds per cubic foot, that will weigh 30*11*490/12 = 13,500# all by itself. An additional 2800# of beams brings the dead weight to 16,300# which will create a substantial bending moment without any wheel loads.

This design really needs to be re-considered.

After solidifying, the weld cools and tries to contract which creates local tensile stress.

As trevorM points out, it is good practice not to weld on the tension flange of a beam.

In this instance I would suggest that the plate be continuously (or nearly so) welded to the top flanges of each beam on both sides. It's very important to provide full lateral support of the top flanges (ie, compression flanges) to prevent twisting of the beams (torsional buckling) and keep the Allowable Bending Stress (Fb) in the upper limits and not a lower value. You're going to have to compute Fb in accordance with the AISC Steel Manual.

Also, provide lateral bracing in the form of X-Braces between the beams, as old salt suggested....I'd place these at the mid-span and at the quarter-span locations.

Welding of the angles to the beam flanges will have no overall effect on the Yield Stress of the beams, only a slight localized disturbance.

One thing we don't know is what the maximum anticipated vehicle axle load will be travelling across this farm bridge, therefore we can't even hazard a guesstimate what the design Bending Stress could be and compare it to Fb to see if the beams are structurally adequate.

Also of concern would be the design bearing stresses at the foundation abutment walls........which we also know nothing about. So, what exactly is supporting this bridge???? I'd be very worried about the beam bearing locations too! You'll need adequately designed beam bearing plates and properly designed anchor bolts.

That's just for starters IMO.

===signed,

CaptMoosie, PE / PhD

Civil, Structural & Environmental Engineer

Good morning CaptMoosie,

Several of your ponderings I have addressed in my previous note. For the supports at each end of the mentioned I-beams, will be 28" horizontal x 16" v 3000 psi concrete footings, 30 ft long, with 5 ea #4 gr 60 continuous rebars, with 3" clearance. These were based on a soil strength of 1980 psf, and the associated dead and live loads.

As a firefighter, I feel obligated to ask a question concerning your design of this bridge. Can it safely and confortably withstand the load of a fully loaded pumper (up to 60,000#), tanker (up to 70-80,000#) or a ladder/platform truck (up to 70,000) should there be a fire and full size fire apparatus need to cross it?

Within the response areas that my dept. responds to there have been several incidents where structures have been lost (with the potential for lives to be lost) because only light weight equipment, brush trucks and mini-pumpers, could cross bridges. A year ago I was the Incident Commander for a barn fire which started to char the residence. Only because we carry compressed-air foam on our mini-pumper were we able to save the residence. We finally were able to pump out of the swimming pool to extinguish the main fire at the barn. Serveral out-buildings were also lost. The driveway was about a 1/4 of mile long with a lightweight bridge on it about 100 yards in from the rural road. No situation to try to hand lay about 1,500' of 5" LDH for a tanker task force from the road.

Good Luck- old salt

You raise a good point. Altho it's been my experience its impossible to get the fire trucks to your place if its outside the city, I should at least post a sign showing the maximum bridge GVW is 60,000#.

I have to add a brick to the load...good comment from Old Salt. I too am involved with firefighting, and we have several of "farm bridges' that area made from recycled flatbed train cars. While these are usually in decent shape, it's always a concern when we approach them with a fully loaded engine or tender. We have one that is constantly abused by loaded and unloaded tandum axle dump trucks that haul pit run up to and including some decent boulders used for riprap against sloughing hillsides. So far the only thing that seems to have suffered is the wood plank decking that most likely came with the original flatcar bed! Please do post the weight limit...you will put a few of us firefighters at ease as they approach the smoke! Take care!

The bridge is 30' wide.

This design is highly suspect, 1" plates over W6x16s is a mismatch. The foundation size stated cannot provide fixity.

Is there a constraint on depth that drives this design?

Good morning passingtongreen,

I was originally given a GVW of 80,000#, and designed for W10x19, S = 18.8 in^3, 24"oc. The client then decided a GVW of 60,000# would be adequate, and requested the lowest possible beam height. The revised design was based on a W6x16, 10.2 in^3, 24" oc.

Next he requested a consideration of beams closer together, and a 1/2" steel plate. This iteration was based on W6x8.5, 5.08 in^3, 12" oc, with 1/2" plate.

Next he proposed a steel reinforced concrete pipe, to which I made a rough draft utilizing a 5' od, 6" wt, concrete pipe of unknown crush strength, with steel reinforced 8" concrete over the top, going to reinforced footings on either side.

To this he advised he would investigate on his own the utilization of PE pipe(s) the mines use, and just cover it/them with dirt! I think he wants a bridge for zero dollars!

He probably wants an engineering fee of zero dollars too.

I hope he is thinking about fire protection!

How about a corrugated/galvanized steel culvert with earth fill on either side?

Sounds like a practical idea to me.

Some of the corrugated plastic pipes are pretty tough. There is one on the oil well access road beside me. It was put in the creek bed then covered with coarse fill. Size needed would depend on the largest creek flow in a huge storm.