Prestressed beam weight for crane lifting - a practical or theoretical approach? 1

[iain98765]

Hi there,

I'm in a bit of a dilemma. Two arguments for the self weight of a beam.

1. to take a standard 2500kg/m^3 for the beam as (unit weight)which would include for the steel. for a beam with theoretical volume of 20m^3
so 2500*20/1000 = 50tons

or
2. take the cylinder tested density of 2430kg/m^3, the steel strands 1.1tons, the actual steel weight 8.9tons and miscl.of 2 tons for grout, collars etc.
2430*20/1000+1.1+8.9+2 = 60.6tons.

problem
1, in calculation 1, the density is underestimated as the steel is much denser, as is the concrete.
2. in calculation 2, the steel hasn't been taken out of the concrete.

Problem,

For a crane lift it is obvious you want to be as conservative as possible. Do you:
1. take calculation 1 as a blanket calculation and risk underestimating the load, or,
2. factor everything in and give yourself a bit of legroom?

What is the standard, or best approach?

I would really appreciate the thoughts and input on this!

 

[iain98765]

Anyone agree then?

 

[CANPRO]

I get the following:

Theoretical Volume of Beam = 20 m^3

Mass of Steel = 8900kg + 1100kg = 10000kg
Density of Steel = 7850 kg/m^3
Volume of Steel = 1.274 m^3

Volume of Concrete = 20 - 1.274 = 18.726 m^3

Theoretical Density of Concrete = 2500 kg/m^3
Theoretical Mass of Concrete = 18.726*2500 = 46815 kg
Theoretical Mass of Beam = 46815 + 10000 = 56815 kg

Actual Density of Concrete = 2430 kg/m^3
Actual Mass of Concrete = 18.726*2430 = 45504 kg
Actual Mass of Beam = 46815 + 10000 = 55504 kg

I may have interpreted it wrong, but when you said you had an extra 4 m^3 of concrete I took that to mean an extra 4 m^3 per beam. The only way that is possible is if your beams where significantly larger. If its an extra 4 m^3 for all 72 beams than that is only a difference of .3%

 

[rowingengineer]

has anyone done an institute measure of the beams? depending on forms used, sometimes you can get larger dimensions than expected.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[gwynn]

Pre-stressed bridge girders run close to 26kN/m^3. This is the number we use for lift evaluations with pre-stressed bridge girders. At 20m^3 this comes out to 58.4tons which is slightly higher than you have.

I agree with the method you have followed (though I haven't run the numbers on the calculations to see that they are correct), but from experience I would just take the density of the beam as 26kN/m^3 and call it a day.

 

[IDS]

52 kN is 53.0 tonne, not 58.4 tonne.

If the steel density is really as high as stated then the beam weight should definitely be adjusted to allow for that. It's not as though it's a difficult calculation.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[BAretired]

Doug, I agree with you that it is not a difficult calculation, but mixing units seems to be a bit of a pain in the tail. I think you meant to say 520kN = 53 tonne, right?

BA

 

[IDS]

Doug, I agree with you that it is not a difficult calculation, but mixing units seems to be a bit of a pain in the tail. I think you meant to say 520kN = 53 tonne, right?

Oops! Yes, I meant 520 kN.

I wish we could use force units for weight, but as the people who handle and transport heavy things think in tonnes I guess we are stuck with it.

How does it work in Canada? Are lifting loads given in tonnes, or tons, or what?

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[BAretired]

Canada supposedly adopted SI in 1970 but there are still a few holdouts. Loads are sometimes expressed in pounds or kilos but cranes are usually rated in tons or tonnes. Structural engineers use kiloNewtons for force and kPa for pressure, but there are still a few engineers who use Imperial units. When reviewing old drawings, it is necessary to know both systems. Adopting a new system of weights and measures is a long and tedious process.

BA

 

[concretemasonry]

lian -

This whole thread seems to be an exercise in assumed precising relating to the actual weight of the elements to be erected.

When designing, engineers make assumptions of the loads on the conservative side to cover possibilities.

For a contractor to be given precise weights seems to be a futile goal, since in the end, all of the 72 elements will probable be set using the same crane/cranes unless there are some radical differences in the assumed weights and the geometry /angles needed for erection. Normally a crane is selected as being adequate and AVAILABLE depending on the construction process and schedule. The crane capacity (minimum plus 20 10 or 20 tons)and to be selected will be determined by the contractor unless you want to give precise weight and accept the liability of future results. - It makes little sense to schedule, move in and set up different cranes for minor differences in element weights.

Usually the contractor is the one that has to be concerned with the end result ot the erection and placement, unless someone wants to give precise weights.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[BAretired]

Maybe the simplest solution is for the precast plant to simply weigh each beam before shipment and mark the weight on the delivery slip.

BA

 

[iain98765]

Thanks for replying guys!

Dick,

The calculation of the girder weight is simple and doesn't take long at all.

You already know;

1. The theoretical volume.
2. The weight of the steel, strands, ducts etc.from supplier notes.
3. The tested concrete density.
4. The steel density.

What if your actual weight is several tons over the assumed weight and your safety factor takes you over the capacity of some of the lifting ancillaries...wasn't is best to check the assumed against the actual then?

BA,
These are site fabricated beams otherwise that would certainly have been the preferred solution.

 

[iain98765]

Dick,

I agree you have to assume weights for design. My point is once the structure has been built you find that the weights are higher than assumed, can be a problem.

Checks on tested densities and actual steel weights need to be carried out in case the assumptions were wrong. Particularly for critical lifts, which is the case we found.

I read a few articles on this saying that in reality the actual weights were almost always lower due to assumed weights being conservative. Statements like that are crazy and misleading as you should always use common sense and be mindful of what your steel% and concrete spec. actually is.

In our case the loads were found to be higher after checking.

Anyone disagree?

 

[Ussuri]

I was wondering if it is really so critical to you being able to lift the object safely then can you not hire some load cells and weigh it?

When we do the design for heavy lifts we make a number of assumptions as part of the design process but before we lift anything we will have the item weighed. Especially if we are going to be close to the capacity of the crane.

 

[iain98765]

Ussuri,

Weighing will be the thing to do in this case.