Structural engineers needed for a beam calculation.

[Diggin It]

The cheapest thing to do is buy an old flat bed tractor trailer, remove the bogies and landing gear, and use it for your bridge.

Been suggested before. No go. Have no way of getting it to the property, removing the axles or getting it near the creek it has to cross.

Without knowing the layout of the property or why you can't get one TO the place ....

Leave the rolling gear on and use one of your machines to pull it to the creek.
Use your Ex to dig holes on both sides to bury the undergear so the bed is at ground level.
Pull the tires and rims off and drop the rest into place. It shouldn't ever move.

Use the aluminum I beams for hand rails.


But I have no idea what these things cost or what your budget is.

 

[aczlan]

Been suggested before. No go. Have no way of getting it to the property, removing the axles or getting it near the creek it has to cross.

Call your local heavy wrecker and ask if they know of any that are available, they may know of some with a mangled undercarriage, or that are otherwise not good enough to put back on the road and they will have the ability to deliver them for you.

Aaron Z

 

[Woody65]

I will give you a hint - You need to calculate the deflection to determine the safe working load. You are asking an engineering question with implied liabilities so not many people are going to put their neck on the chopping block for you on an anonymous user group. Hire a professional please and let them do the calcs. Safety first. If you dont know the answer then you probably shouldnt be doing the work.

I suggested the trailer because someone has already done the engineering for you.

Thank you for your advise.

I already have an engineered bridge, for the span in question, done by a professional using red oak logs. All I was looking for was an answer or advise on a specific beam that happened to be for sale near me. I am not familiar with aluminum beams. I have been in the building trade for 30 yrs and know how to calculate loads (live, dead, deflection) for dimensional lumber, laminate beams fitch plates etc.

 

[EddieWalker]

Curious how much do they want for the beams? I agree with the others that they are not big enough for anything of significant load, but wonder if they would make a fun project to build a bridge to walk across the creek, or even a cantilevered deck/sitting area that only goes half way across the creek.

 

[GSVette]

Thank you for your advise.

I already have an engineered bridge, for the span in question, done by a professional using red oak logs. All I was looking for was an answer or advise on a specific beam that happened to be for sale near me. I am not familiar with aluminum beams. I have been in the building trade for 30 yrs and know how to calculate loads (live, dead, deflection) for dimensional lumber, laminate beams fitch plates etc.

Since it doesn’t seem that anyone has given you the info you requested here is the formulae for calculating deflection at midspan, max bending moment and extreme fiber stress for a simply supported beam with a point load (w) at the center of the span (Roark, Formulas for Stress and Strain, 3rd edition, Chapter 8, Table 3):

Deflection=-1/48(wl^3/EI) at midspan (l/2)

Max M=1/4(wl) at midspan

Stress=Mc/I

You’ll need to know E (Young’s Modulus) for the aluminum alloy in question as well as ultimate tension capability (Ftu) and yield compression capability (Fcy). You’ll also have to calculate I (moment of inertia) for the beam cross section.

Formula for other loadings and support conditions can be found in Roark ( which is probably available on line somewhere).

You’ll have to determine applicability of formulae and suitability of material yourself:

Hope this helps

 

[LD1]

Above formulas are correct. E - is 10,000,000 for aluminum.

You can calculate the I4 if you want, I just like using charts online that already spec it. Closest I can find is for a 8x5 beam....and that is 59.xx so yours is gonna be a little weaker.

So "assuming" a moment of inertia of 50....and a 25' long beam (300 inches) and 5000 pounds (half of your excavator)

(300 x 300 x 300 x 5000) / (48 x 10,000,000 x 50)
135,000,000,000 / 24,000,000,000
Deflection = 5-5/8" of deflection. Which is WAAAYYYYY too much.

IF you are familiar with building trades and construction, I assume you know what L/360 deflection is? Over a 25' span.....0.833 would be the MAX you want. And to not exceed that.....midspan point load of 740# is the absolute max. And all that assuming properly braced and all. Because a 4" wide beam is NARROW given the span.

No liability here, all I did was math and answered a question based on variables.

 

[GLyford]

For lumber house construction L/360 is used for finished ceilings to keep from cracking the plaster. L/240 is allowed for lightly loaded attics, L/180 for sloped rafters with no finished ceiling.

None of those really sounded to me like a bridge, really, so I went looking...

Apparently, AASHTO likes to use L/800 for simply supported highway bridges...mostly to keep people from feeling queasy as the bridge moves under their vehicle. The limits for cable stayed and suspension bridges are little more generous.

ResearchGate

 

[CobyRupert]

Apparently, AASHTO likes to use L/800 for simply supported highway bridges...mostly to keep people from feeling queasy as the bridge moves under their vehicle. The limits for cable stayed and suspension bridges are little more generous.

There's a few bridges where L/800 wasn't used. Ever sit in line on the Peace Bridge over the Niagara River crossing from Canada into Buffalo? It's often backed up because....well, that's the U.S. for you...anyways......when you're sitting there and the big trucks in the opposite lanes are rolling into Canada that bridge feels like it's going up and down a few feet. Definitely a queasy feeling.

...maybe that's the design, or maybe it's just another one of N.Y.'s 1837 bridges that are deemed structurally deficient? .

 

[GLyford]

I suspect any place where you are sitting still instead of rolling, you notice it more.

I was always surprised by the motion when stuck going nowhere on the elevated expressway in Boston before the Big Dig opened. Even a couple of inches (L/800 for 120' or so span) feels like a lot when you feel the whole vehicle doing it.

 

[LD1]

Large bridges and large spans arent really a good comparison. This much shorter span can do fine with L/360. But thats only one piece of the puzzle.

Gotta account for stresses, and THIS is where the type of alloy needs to be known. And you do NOT want to exceed its yield strength. And you want to build in a good safety margin. The deflection isnt gonna be measurably different between different alloys.

Given the size of the beam.....and the L/360 limiting loading to under 750# for the beam, I dont see stress as an issue...

Just a quick calculation....750# being the load at midspan (limited by L/360)....and the 8" x 4" beam.....stress would be ~4500psi. Most aluminums are between 20 and 40ksi......some stronger, some weaker. But something shaped like an i-beam......I doubt 4500psi would be a problem. Deflection is your holdup.

L/800 would limit the aluminum beam loading to 333#