WELDED JOINTS  home
welding

Notes: Mechanical Design Data Manual Chapter 13 welded_joints.pdf

You need to be able to design welded joints subject to the following loads:
Direct shear , Bending, Torsion and combinations of these. Welded joints are very common in engineering for the manufacture of fabricated assemblies. This is often done rather than casting or forging for production of machine components (particularly if only small quantities are required, or the parts are suited to design from standard sections and plates). So a mechanical engineer needs to have a good understanding of the fabrication using welded joints and how to perform stress analysis on these fabrications.

You should already understand the basic strength of materials theory involved with welded joints. If you need to revise this consult any basic strength of materials text such as Ivanoff - ­Engineering Mechanics.

This module extends the basic theory to more complex loading applications.

Basic Terminology:
weldment terminolgy


Study Guide

CIGWeld literature   
Thermadyne: Literature

References are to Mechanical Design Data Manual Chapter 13. 

1.         Read the Preamble on pages 311 and 312 that describes the two basic types of welded joint namely, butt and fillet welds, and outlines the basic method of stress analysis using the conventional design method (that you should be familiar with) and also the weld as a line method. The weld as a line method is used in this module because it is a more useful method when the welded joint is subject to bending or torsional loads.

2.         Read through the theory on bending and torsion loads on pages 312 to 313 and study the Table on page 314. This Table shows some common weld configurations and gives the formulas for calculating the section modulus Z and the polar second moment of area J.

3.         Study the theory on locating the centroid on welded joint as given on page 315. This theory is no different to the theory used for locating the centroid of any compound section except that the weld can be considered as a narrow area of thickness of 1mm with no area moment about the narrow axis. You should be able to follow the example on page 5. Note that as is the case with any area, if the welds are symmetrical about an axis then the centroid of the welds is located on this axis.

4.         Study Example 1 for direct load on page 316 and work through Exercises 1 and 5 in this section.

5.         Study Example 2 for bending + direct load on pages 316 to 317 and work through Exercises 2, 3 and 6 in this section.

6.         Study Example 3 for torsion + direct load on page 317 to 319 and work through Exercises 4 and 7 in this section.

7.         Work through Exercise 8 in this section. Note that part (c) is more difficult and you may require help from your class teacher.


Example exercises

These questions will help you practice as you work through Section 2. As directed in the study guide, attempt each of the exercises. 

1.  For the welded assembly shown below, determine the weld size if E41** electrodes are used. A design factor of 1.5 is to be applied.


2.   Repeat Exercise 1 with the load acting vertically down instead of horizontally.
3.   Repeat Exercise 1 with the load acting down at an angle of 45° to the horizontal.
4.   The arm shown below is welded on both sides to the shaft. The shaft is supported by close bearings on each side of the arm so bending stress in the shaft is negligible. The steel used has a yield of 280 MPa. The load fluctuates with some shock so a design factor of 4 is to be applied. Specify the shaft diameter and weld size.
 
5.         The grade 250 equal angle 200 x 200 x 20 is welded to the support with 2 runs of 12 mm fillet weld. The allowable stress in the angle and welds is 0.67 x yield stress. The cross-sectional area of the angle is 7660 mm2. Determine the maximum force F that can be applied at the centroid of the angle and the lengths L1 and L2 of the welds (so the welds are stressed equally).


6.         The parallel flange channel as shown below supports a load of 1 t and is welded inside and out to the support. The allowable stress in the channel and welds is one third of the yield stress. Select a suitable rolled steel channel (see Chapter 10) and specify the weld size.


7.         The steel bar shown below is welded on three sides to the support. Grade 250 steel is used and a design factor of 1.67 is required. Specify the plate thickness and weld size.

8.         The 50 mm diameter steel rod as shown belowis welded to the support plate. Maximum allowable tensile stress for the design is 100 MPa.

(a)            Determine the maximum load F that can be safely applied for tensile stress in the rod.
(b)            Determine the maximum load F that can be safely applied for shear stress in the weld if the rod is welded all round with 5 mm fillet weld.
(c)        It is necessary that the weld has the same strength as the rod, that is that the weld is able to support the load calculated in (a). In order to do this, two gussets will be welded (both sides) to the top and bottom of the rod as shown in sketch (2). Determine the height h of gusset required.



Assignment
6.         The parallel flange channel as shown below supports a load of 1 t and is welded inside and out to the support. The allowable stress in the channel and welds is one third of the yield stress. Select a suitable rolled steel channel (see Chapter 10) and specify the weld size.


7.         The steel bar shown below is welded on three sides to the support. Grade 250 steel is used and a design factor of 1.67 is required. Specify the plate thickness and weld size.