Steel Designers Manual - TheBestFriend.org

This material is copyright - all rights reserved. Reproduced under licence from The Steel Construction Institute on 12/2/2007
To buy a hardcopy version of this document call 01344 872775 or go to http://shop.steelbiz.org/
Steel Designers' Manual - 6th Edition (2003)
692 Welds and design for welding
24.2.8 Standards – inspection and weld quality
All structural materials contain imperfections, and a series of standards defines the
methods of inspection and the extent to which the imperfections can be accepted.
The significance of an imperfection depends mainly on its size, shape and position,
and the local applied stresses and temperature. Imperfections that cause failure
should be rejected and repaired, but many common imperfections, such as minor
porosity, are acceptable. A useful guide to the scope of inspection and the weld
quality acceptance criteria and corrective action is shown in Tables 1 and 2 of the
National Structural Steelwork Specification for Building Construction. 21
24.3 Recommendations for cost reduction
Small changes in design details can have significant effects on welding productivity
and costs, without adversely affecting analysed stresses. This section is directed at
design improvements that are qualitative in nature, but which reduce costs by paying
attention to a series of simple principles which are summarized below in 24.3.5.
24.3.1 Overall principles
Modern computer aided design is very effective in the analysis of structures to gain
‘maximum efficiency’ in the use of materials, but the associated fabrication costs are
not generally included in the software. Improved efficiency in the use of steel, especially
for stiffening, usually leads to increased complication, the need for many short
welds with difficult access, and a general increase in fabrication costs that will exceed
the cost savings associated with the use of less steel. The use of standard rolled sections
can save significant cutting and welding costs. Design priorities vary from one
structure to another, but unless weight saving is crucial and is given highest priority,
designers should aim for the minimum total cost which will, among other things,
be a function of the number of welds to be made. When comparing design alternatives,
the fabrication costs will be reduced if the number of individual pieces is
minimized. For many structures the use of ‘standard’ 6 mm leg length fillet welds is
convenient, but the associated heat input may be insufficient to prevent cold cracking
of thick sections. BS EN 1011-2: 2001 4 indicates that welds are liable to crack
adjacent to 6 mm fillet welds when the combined thickness of the elements being
joined exceeds about 40 mm. In such cases the higher heat input from larger fillet
sizes will avoid the need for preheating. Welding engineers should be consulted for
advice when in doubt.
The sizes of welds should be no larger than required to transmit design stresses.
The effective throat size a of a fillet weld should be taken as the perpendicular distance
from the root of the weld to a straight line joining the fusion faces that lies