Resulting forces in the Z axis. 1

[Bert2]

Please can someone help me in the solution for calculating the resulting force in the Z axis. attached is the situation (rigging diagram) specifiaclly the top slings are in question. the end elevation shows them to have an angle 65-69deg.interior angle and in the side elevation 72-74deg. interior angle. Basically what is the resulting force acting on the top slings given the two angles their at?

Using a force off 75Te for the object itself.

thanks.

 

[desertfox]

Hi IDS

If the vertical deflections at each corner are not the same then the component being lifted cannot remain horizontal, which Bert2 stated it did remain horizontal during lift.
Good spreadsheet though.

desertfox

 

[IDS]

If the vertical deflections at each corner are not the same then the component being lifted cannot remain horizontal, which Bert2 stated it did remain horizontal during lift.

The deflections are small, so the load will stay very close to horizontal.

The only way the deflections would be the same is if the sling stiffness was proportional to the vertical load, but that wouldn't change the loads in each sling, because it is statically determinate.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[rb1957]

FWIW, here's my calcs ...

 

[rb1957]

desertfox, how do you calc displacements when we don't know the sling areas ? (assume each sling the same ?)

 

[desertfox]

Hi rb1957

Yes the idea is that all the slings have the same area and modulus.

desertfox

 

[zekeman]

Here is a comparison of the 2 methods for the simple picture frame problem.
It is clear that for some problems, like this, the difference is huge.

 

[zekeman]

OOps, problem w/ uploading. Try this

 

[rb1957]

this here is one flogged dead horse !

the problem is statically determinate, so you don't need energy methods to solve it.

please, done and dusted, no?

 

[Bert2]

@zekeman / rb1957

huge thanks for the time looking into this and the two different solutions.

If i was looking at it in a general case i would assume zero deflection in the slings.

to get a guide on the loads applied.

you could take it to the next level as zekeman has shown but in my case i dont need such an in depth approach-good even though it is.

 

[rb1957]

no, i wouldn't make any assumption about deflections ... the problem as posed is statically determinate.

notice though if you didn't have the parallel slings the problem would be indeterminate and you'd have to include deflections as a criteria for developing your loads.

the important thing to take from this is drawing a free body diagram will take you a long way towards solving the problem.

 

[gruntguru]

My 2c worth. Posts 2 and 3 got it right. The problem is simple and can be solved from the bottom up summing forces only and nothing harder than solving 2 simultaneous equations in 2 unknowns.

The problem is definitely statically determinate even if the object being lifted is rigid and the slings are inelastic. If any of the sling lengths was to be changed slightly the geometry of the problem would change ie the sling lengths have been adjusted to produce the geometry shown (lower slings vertical in end view.)

Engineering is the art of creating things you need, from things you can get.

 

[zekeman]

"My 2c worth. Posts 2 and 3 got it right. The problem is simple and can be solved from the bottom up summing forces only and nothing harder than solving 2 simultaneous equations in 2 unknowns."

In hindsight you are probably right, but, save for the special geometry of the bottom slings, you would be looking at a messy problem. So what happened, on first blush the problem looks easy. On second blush, maybe indeterminate, and finally got it right on the 3rd look.

Personally, my performance was pretty shoddy, especially after everyone was screaming that the problem was determinate. I was hung up on the upper sling arrangement and disregarded the link connections to the lower half. After doing problems for over 50 years, I should know better.

In any event the thread was probably useful as an exercise in statics and determinate and indeterminate structures.