Frequently Asked Questions

Structural Steel

I am presently checking the roof joists of a building that was constructed in 1967. If I backtrack using the snow loads of that period, it would appear that the joists were built with steel having an F_y of 380 MPa (55 ksi), which is rather surprising. Would you, by any chance, have a list of historical steels that were used during that time, either plates or rolled angles?
-- B.C.

6/1/2005

Backtracking can give you a rough ballpark figure, but I would apply caution when using the 'detective' method for ascertaining F_y, especially when you can find a lot of useful information about historical steels on our site, page 6-5 of the CISC Handbook, and in a book published by AISC. You can locate the table, Historical Steels, on our web site. There you will find that the standard in effect in 1967 was the CSA Standard G40.12, 1964, with F_y = 300 MPa (44 ksi) and F_u = 450 MPa (66 ksi).

That was the conservative answer. We have information that Truscon joists went to an F_y of 380 MPa (55 ksi) for chords in 1968. With joists, you must be very careful as individual producers used higher strength chords before these steels were in general use for other structural elements. DOSCO made the chords in its own mill for Truscon, which was then a division of DOSCO (now ISPAT). Joist manufacturers have used steels of different strength over time, although the exact years are not known precisely as their catalogues were not always dated. In fact, few people used to put dates on their catalogues. For your information, open-web steel joists date back to 1930's in Canada. In the United States, the Steel Joist Institute’s specification of 1962 introduced steel of 350 MPa (50 ksi) for chords.

If in doubt, Clause 5.2.2 of CSA Standard S16.1-94 provides “default” values for unidentified steels.

For older steel shapes, what could you do? I was shocked when I couldn't find a book I have relied on for years, which included Iron and Steel Beams from 1873 to 1952. In fact, AISC replaced it by a new guide. As well, this publication describes how existing structural systems can be enhanced for increased strength and stiffness. It's called:
Design Guide 15: AISC Rehabilitation and Retrofit Guide:
A Reference for Historic Shapes and Specifications

What about welding to existing structures? The older steels usually have a higher carbon content, which decreases the weldability of a member. What might be useful is an article in Engineering Journal authored by David Ricker, entitled Field Welding to Existing Steel Structures, 1^st Quarter 1988, 16 pages.

This question appeared in the "Ask Dr. Sylvie" column of Advantage Steel no. 17, Spring 2003.