BUCKLING OF STRUTS

OBJECTIVE1. To examine how shear force varies with an increasing point load2. To examine how shear force varies at the cut position of the beam for

various loading condition

LEARNING OUTCOME1. The application the engineering knowledge in practical application2. To enhance technical competency in structural engineering through

aboratory application.3. To communicate effectively in group4. To identify problem, solving and finding out appropriate solution through

laboratory application

INTRODUCTION A compresive member can fail in two ways. The first is via rupture due to

o the direct stress and the second is by an elastic mode of failure called

o buckling. Short wide compressive member tends to fail by material

o crushing.

When buckling occurs the strut will no longer carry any more load and ito will simply continue to buckle i.e its stiffness then becomes zero and it is

o useless as a structural member

THEORY

To predict the buckling load Euler buckling formula is used. The crictical value in Euler Formula is the slenderness ratio, which is the ratio of the length of the strut to its radius of gyration (L/K).

The Euler formula become inaccurate for struts with L/K ratio of less than 1.125 and this should be taken into account in any design work.

Euler buckling formula for pin struts :

Where;Pe = Euler buckling load (N)E = Young’s Modulus (Nm-2)I = Second moment of area (m4)L = length of strut (m)

PROCEDURE

Part 1

1. Fit the bottom chuck to the machine and remove the top chuck (togive two pinned ends). Select the shortest strut, number 1, andmeasured the cross section using the vernier provided and

calculatedthe second moment of area, I,for the strut. (bd3/12)

2. Adjust the position of the sliding crosshead to accept the strut usingthe thumbnut to lock off the slider. Ensure that there is the

maximumamaount of travel available on the handwheel threat to compress

thestrut. Finally tighten the locking screw.

3. Carefully back- off the handwheel so that the strut is resting in thenotch but not tranmitting any load. Rezero the forcemeter using thefront panel control.

4. Carefully start to load the strut. If the strut begin to buckle to the left,

“flick” the strut to the right and vice versa (this reduces any errorassociated wih the straightness of strut). Turn the handwheel untilthere is no further increase in load (the load may peak and then

dropas it settles in the notches).

5. Record the final load in Table 1. Repeat with strut numbers 2, 3, 4and 5 adjusting the crosshead as required to fit the strut.

THE BUCKLING OF STRUTS APPARATUS

THE STRUT START TO BUCKLING

Part 2

1. To study the effect of end conditions, follow the same basicprocedure as in part 1, but this time remove the bottom chuck andclamp the specimen using the cap head screw and plate to make apinned-fixed end condition.

2. Record your result in Table 2 and calculate the values of 1/ L2 for thestruts.

3. Fit the top chuck with the two cap head screws and clamp both endsof the specimen to make a pinned –pinned end condition. Calculatethe new values of 1/L2.

4. Enter the result into Table 3

RESULT

STRUT NUMBER

LENGTH (mm)

Buckling Load Experiment (N)

Buckling Load Theory (N)

1/L^2 (m^-2)

12345

STRUT NUMBER

LENGTH (mm)

Buckling Load Experiment (N)

Buckling Load Theory (N)

1/L^2 (m^-2)

12345

TABLE 1

TABLE 2

STRUT NUMBER

LENGTH (mm)

Buckling Load Experiment (N)

Buckling Load Theory (N)

1/L^2 (m^-2)

12345

DISCUSSION AND CONCLUSIONS

Part 1:1. Examine the Euler buckling equation and select an appropriate

parameter to establish a linear relationship between the buckling load

and the length of the strut. Write the relationship below.2. Calculate the value and enter them in Table 1 with an appropriate

title.3. Plot a graph to prove the relationship is linear. Compare your

experimental value to those calculated from Euler formula by entering

a theoretical line onto the graph. Comment on the result.4. Explain that the Euler Formula can predict the buckling load or not.

TABLE 3

Part 2:1. Plot separate graphs of buckling load versus 1/ L2 and calculate the

gradient of each line.2. Fill the table below showing the comparison between experimental

and theoretical ratio by end condition

Notes:1. *Use the experimental gradient fom Part 12. Experimental ratio = Exp. Gradient / gradient of pinned-pinned.3. Theoretical ratio can be obtained from Euler Formula for pinned

fixed and fixed-fixed.4.Comment on the experimental and theoretical ratio.5. What conclusion can you made from the experiments.