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Indeterminate Structures
12

Indeterminate Structures

Indeterminate Structures

(OP)
Why different methods for analysing them ?

RE: Indeterminate Structures

'cause there are many ways to skin cats ?

different methods exploit different characteristics. eg the unit load method says the deflection at the redundancy is zero. if you remove the redundancy you can calculate a deflection; a deflection that the redundancy prevents. apply a unit load (at the redundancy location) you can determine the redundant load.

Quando Omni Flunkus Moritati

RE: Indeterminate Structures

There are many roads, all leading to the same location...

No one way is understood by all...

Confucius say, use what work for you.

Mike McCann, PE, SE


RE: Indeterminate Structures

Quote (Realfedos)

(
Why different methods for analysing them ?

Why not?

--
JHG

RE: Indeterminate Structures

Some methods are better for hand calculation. Others are more suitable for a computer. It would be almost impossible to solve a 50 x 50 matrix by hand but it's a breeze with a computer.

The slope deflection method was common in engineering offices prior to the Hardy Cross moment distribution method which became the preferred method by many engineers prior to the advent of computers. Today, many engineers use structural software to solve determinate as well as indeterminate structures.

An engineer who knows the principles involved in the various methods of solving indeterminate structures has a much better understanding of structural theory than the guy who plunks data into a computer and awaits the output.

BA

RE: Indeterminate Structures

The vast majority of computer applications utilize some permutation of the stiffness matrix method. Since most indeterminate work is done with the aid of a computer, that's an important method to know.

As others have mentioned, when doing hand calculations, the best method often depends on the information available and the parameter of interest. For developing intuition via hand calculations, I like the moment distribution method the best. While virtual work seems very elegant, the energy stuff still feels like voodoo to me at times.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Indeterminate Structures

The simple answer is:
1. Statically indeterminate structures can't be analyzed with statics.
2. Statics is used for hand calculations of statically determinate structure because it is much easier for hand calculation.

RE: Indeterminate Structures

There are really only two general methods for analysing indeterminate structures.
1. Flexiblity, force, compatibility method.
2. Stiffness, displacement, equilibrium method.

RE: Indeterminate Structures

(OP)
Great Posts from you all; So what are the basic steps(tips and tricks) in analyzing IS using MDM

RE: Indeterminate Structures

Well, I would say the most important point is MSYEYDC.

BA

RE: Indeterminate Structures

read up on MDM (or Hardy Cross)

Quando Omni Flunkus Moritati

RE: Indeterminate Structures

(OP)
@BAretired ............ MSYEYDC would mean ?

@rb1957................ Thanks, but i need the very salient points from professional.

BR

RE: Indeterminate Structures

"i need the very salient points" ... MDM is not something you can write on the back of a "fag packet". if you research (google, wiki) MDM (or Hardy Cross).

i guess MDM distributes moment along a beam with many supports.

is this a student post ??

Quando Omni Flunkus Moritati

RE: Indeterminate Structures

My goodness..., if you are going to call yourself a Structural Engineer, why not invest in a couple of good textbooks on the basics of Structural Analysis, both Statically Determinate and Statically Indeterminate? A couple of computer programs, does not an engineer make. Understanding these basic, longhand, fundamental methods of analyzing various structural problems will help you understand all of the structural problems you face, and how and why they act the way they do. You need to have a gut feel for this structural action if you are going to do a good job of designing the structure. And, if you are too lazy to do a little of your own digging on the subject, you probably shouldn’t be doing this type of work. On the fundamentals, you learn by doing, over and over again, until you understand, without a second thought. You won’t learn by a few guys giving you some offhand comments, although a number of the answers above are very instructive.

RE: Indeterminate Structures

@BAretired, MSYEYDC = Make Sure You Exercise Your Damn Cat?

RE: Indeterminate Structures

This will not get you up to speed on moment distribution but it will give you an idea of what’s involved.

1. Pretend all spans are fixed and calculate their end moments (wL^2/12 for uniform load, PL/8 for point load center span, other end moments per loading conditions).
2. Calculate the span stiffnesses (EI/L).
3. Combine the stiffnesses of adjoining members and proportion each member’s stiffness at each joint.
4. Balance the unbalanced moment.
5. Carry over half of the balancing moment to the adjacent joints.
6. Do not balance an actual fixed end moment; it simply “absorbs” the moment carried over to it.
7. Lather, rinse, repeat to the degree of accuracy you want.

A trick that will simplify the process considerably: if an end span has a pinned end the stiffness of that span is ¾ what it would otherwise be. By doing that you can balance that moment only once and not carry over any moment to it. That cuts the amount of steps.

This should all be clear as mud right now but if you study up on the method it will make more sense.

RE: Indeterminate Structures

>>>MSYEYDC = Make Sure You Exercise Your Damn Cat? <<<king

RE: Indeterminate Structures

The word, really, for cats, is "exorcise". After all, he did say the cat was damned.

Mike McCann, PE, SE


RE: Indeterminate Structures

(OP)

Thanks all, for your advice.

I like Structures, and i want to design a superb structure; but my area of competence is Geotechnics;

I like the thrills here, it sure going to make me run on time.

Cheers


RE: Indeterminate Structures

@Realfedos,
MSYEYDC = Make Sure You Enter Your Data Correctly
but I like "Make Sure You Exercise Your Damn Cat" too.

BA

RE: Indeterminate Structures

Mike Says Yes Exorcise Your Damn Cat

Mike McCann, PE, SE


RE: Indeterminate Structures

2
I may have said this before, but I think computer analysis vs. hand calcs is a false dichotomy. You can plug numbers into a formula from a book without understanding what is going on, and you can use computers to gain an understanding of behaviour that would be impossible with purely hand calculations.

From another perspective, you don't need numbers anyway:
http://www.colincaprani.com/wordpress/2013/04/qual...

Quote:

Analysing structures without recourse to numbers is vital for engineers to have a basic appreciation of structural behaviour, and to help identify errors in computer input from the output. Dr David Brohn (New Paradigm Solutions and author of Understanding Structural Analysis ) has been an important figure in this field since the late 70s and his important benchmark tests of structural understanding. As part of an e-Training course for the Institution of Structural Engineers, Dr Brohn has developed a video series to teach qualitative analysis. The first three of these are freely available and are very useful:

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

RE: Indeterminate Structures

I would say that the biggest difference between statically determinate and statically indeterminate structures is not the mathematical/graphical method used but in the parameters that need to be, or could be, considered. Statically determinate structures behave independently of any changes in stiffness. Statically indeterminate structures are dependent on the variation in stiffness. This variation can occur along the structure and with time. So before you get to the analysis you'll need to know how the stiffness varies and what to account for. Do you rely on just an uncracked analysis, if concrete? Do you account for creep? Composite or staged construction? I sometimes perform a multi-level analysis of concrete beams, uncracked and cracked at a particular stage of loading for deflections. With statically indeterminate structures you also have additional loads to account for such as differential settlement, differential strain (i.e. thermal, shrinkage), secondary prestress. A lot of decisions and calculations may be required before choosing and performing the analysis.

Dave

RE: Indeterminate Structures

Quick clarification. By multi-level analysis I mean a series of different analyses with different stiffness assumptions. An uncracked analysis and one or more cracked analysis at certain stages of loading/construction.

Dave

RE: Indeterminate Structures

IDSs point is insightful, a plug'n'chugger who picks the wrong formula to start with is just as dangerous as an FEA enthusiast with a 10 million dof model, dubious boundary conditions, and no correlation data.

Cheers

Greg Locock


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RE: Indeterminate Structures

For me it was important to realise these 2 different approaches to solve SIS.

Start off by using pin jointed trusses and you will see the differnce in the 2 approachs and where they apply.

Then work your way to rigid jointed frames where equation such as slope deflection equations become a bit bigger due to momemts and deflections.

Computer methods normally use the stiffness method.

You will soon realise how stiffness matrices work and how computer analysis works.

RE: Indeterminate Structures

(OP)
@ Civeng et al, I am getting better.................working on my first design at work today, under the supervision of a Senior Engineer.

Looking at sitting for ISTructE CM .

Thanks, everybody..............I love you all

RE: Indeterminate Structures

But in real life a lot of people just be conservative and over design it (make it determinate structure by removing a constraint.

RE: Indeterminate Structures

"But in real life a lot of people just be conservative and over design it (make it determinate structure by removing a constraint." ...

I hope not, because ...
1) indeterminate reactions change the internal stresses, increasing them in places
2) not all redundancies are "constructive", they can be "destructive" by increasing the "other" reactions.

but I take your point, a typical redundancy in a fixed end; if this yields you approach a pin end. Fine if you are happy with the plasticity.

Quando Omni Flunkus Moritati

RE: Indeterminate Structures

Precast

Not really (although I sought of think I know what your getting at).

Could be many dangers in doing what you suggest.

RE: Indeterminate Structures

I worked four days and attended the Tech on one day per week until I did my National Service. When we started indeterminate structures I switched to pipe supports because there were always a variety of structures needed to support the supports. One week, we learned Moment Distribution so I used that; the next week, we learned Slope Deflection so I used that for the next structure. Then we did Column Analogy, and of course, I used it, followed by one of the least work methods, but before I could use it, the boss brought my column analogy calculation back and told me to limit myself to Moment Distribution or Slope Deflection because those were the ones the checkers remembered.

For me, the major difference between these methods and the computer programs is that these use only the primary load paths unless something special is done; the computer programs load up all available load paths, removing the built in redundancy of the manual methods.

One of the fascinations for me was finding how these were equivalent, not MD of course but the others. This interest first arose with trusses, I needed to work out why the methods of joints and the method of sections and the graphical method were equivalent.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

RE: Indeterminate Structures

for me most methods (apart from MD) are limited to 1 or 2 redundancies before the math gets hairy (or prone to error); but computer based solutions (particularly FEA) aren't limited

Quando Omni Flunkus Moritati

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