What is the Maximum Allowable Load for a Restrained Hollow Steel Tube Strut?

In summary, the conversation is about revising for a past exam paper question on design and calculation of a rectangular hollow steel tube used as a strut. The question involves determining the safety of the design and calculating the maximum allowable load for different end conditions. The basic equation for buckling of a column is provided, and the conversation also discusses the concept of safety factor and effective length. The person seeking help is unsure about how to calculate for safety and maximum allowable load, but is advised to use the wall thickness and cross sectional moment to determine the critical load and compare it to the applied load. The conversation also suggests using a table of different end conditions and associated effective lengths for the calculation.
  • #1
Tino
26
0
Revision on Past Exam Paper !

Hey there, i am stuck on this question in my past exam paper, and i have tried many ways of doing it but just simply not getting any where !
So please would u mind helping me out a little here !??

A rectangular cross section hollow steel tube (outer diameter 50 mm and 65 mm) is to be used as a strut. In initial designs the ends of the strut are held in position but not restrained against rotation. The distance between the restraints is 2m, and the load it has to carry is 180kN.

(a) The wallthickness is 4mm. Is the design safe? (you may assume Euler's buckling theory is valid) (v = 0.3, E=205GPa for steel; sigmaY = 250MPa)

(b) The tube is now restrained against rotation at one end while being pinned at the other. Find the maxiumum allowable load using both Euler and AISC formula (given below). Fully explain design process.

NOTE: The AISC rules state for L/K (effective length / radius of gyration) values less than (L/K)c, where (L/K)c = squareroot {(2*pi^2*E)/(SigmaY)} plot the parabolic curve: sigma = sigmaY*{1-((L/K)^2)/(2*(L/K)c)}
Smoothly join this parabola into the Euler curve for L/k values greater than (L/K)c.

Here is the question, sorry it is a bit long, but i just don't know where to start lol !

Thanks !
 
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  • #2
Buckling at it's finest. What is the basic equation for buckling of a column? That's the place to start.

[tex]P_{cr} = \frac{\pi^2 E I}{L_e^2}[/tex]

The end connections dictate what the effective length of the column is.
 
  • #3


So just get the numbers for this formula and only to get the LOAD !

What does the question mean by is the design safe !?? i don't know what to caluclate for that or wrtie about??


how do i caulate the maximum allowable load when the strut is restrainted is there a different formula ?

Thanks lot !
 
  • #4
You can decide the "safeness" by calculating the safety factor. The 4mm wall thickness dictates your cross sectional moment. Use that to calculate your allowable critical load and then compare that to the 180 kN applied load. Safe is a relative term. At the bare minimum, you want a safety factor of at least 1.0 which means your applied load is exactly the same as the critical load.

Usually on questions like this, you need to justify with your own judgment what is considered safe.

BTW, your text should have a table of some kind showing different end conditions and the associated effective lengths that is used in the calculation like this:

http://en.wikipedia.org/wiki/Image:ColumnEffectiveLength.png
 

1. How can I effectively use past exam papers for revision?

Past exam papers can be useful resources for revision as they give you an idea of the format and types of questions that may be asked on the actual exam. To use them effectively, start by familiarizing yourself with the exam paper's structure and marking scheme. Then, attempt to answer the questions under timed conditions to simulate the real exam experience. Finally, review your answers and identify areas where you need to improve.

2. How far back should I go when using past exam papers for revision?

This depends on the subject and the specific exam you are preparing for. Generally, it is recommended to go back at least a few years to get a good range of questions. However, if the course content has not changed significantly, you can go back further. It's also a good idea to prioritize more recent papers as they are likely to be more relevant to the current exam.

3. Is it necessary to complete the entire past exam paper for revision?

No, it is not necessary to complete the entire past exam paper. Instead, focus on the areas that you struggle with or the topics that are likely to be covered on the upcoming exam. However, it's a good idea to attempt a few questions from each section to ensure you have a well-rounded understanding of the subject.

4. How can I use past exam papers to identify my weaknesses?

Past exam papers can be helpful in identifying your weaknesses as they allow you to see which types of questions you struggle with the most. Take note of the questions you got wrong or had difficulty with and use that information to guide your revision. You can also compare your answers with the marking scheme to see where you went wrong and how you can improve.

5. Can I use past exam papers as my only form of revision?

No, it is not recommended to rely solely on past exam papers for revision. While they can be a valuable tool, it's important to also review your class notes, textbooks, and other study materials. This will give you a more comprehensive understanding of the subject and ensure that you are well-prepared for the exam.

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