Page 272 - Handbook of Structural Steel Connection Design and Details

Welded Joint Design and Production

Welded Joint Design and Production 257

connection detail used for SMRF systems, at least prior to the
Northridge earthquake. If it is assumed that the web is incapable of
transferring any moment (a simple assumption, but probably justified
by the lack of confidence in the ability of welds to share loads with
bolts), it is critical that the plastic section modulus of the flanges Z ,
f
times the tensile strength be greater than the entire plastic section
property Z times the yield strength in the beam. All five material
properties must be considered in order for the connection to behave
satisfactorily.
Existing ASTM specifications for most structural steels do not place
an upper limit on the yield strength but specify only a minimum
acceptable value. For example, for ASTM A36 steel, the minimum
acceptable yield strength is 36 ksi.
This requirement precludes a steel that has a yield strength of 35.5 ksi
as being acceptable but does nothing to prohibit the delivery of a
60-ksi steel. The tensile strength range is specified as 58 to 80 ksi.
While A36 is commonly specified for beams, columns are typically
specified to be of ASTM A572 grade 50. With a 50-ksi minimum yield
strength, and a minimum tensile strength of 65 ksi, many designers
were left with the false impression that the yield strength of the beam
could naturally be less than that of the column. Owing to the specifi-
cation requirements, it is possible to produce steel that meets the
requirements of both A36 and A572 grade 50. This material has been
commercially promoted as “dual certified” material. In actual fact,
regardless of what the material is called, it is critical for the connec-
tion illustrated in Fig. 3.36 to have controls on material properties
that are more rigorous than the current ASTM standards impose.
Much of the focus has related to the beam yield-to-tensile ratio,
commonly denoted as F /F . This is often compared to the ratio of
y u
Z /Z, with the desired relationship F /F being
f y u
Z F
f > y
Z F
u
This suggests that not only is F (yield strength) important, but the
y
ratio is important as well. For rolled W-shapes, Z /Z ranges from 0.6 to
f
0.9. Based on ASTM minimum specified properties, F F is as follows:
y u
A36 0.62
A572Gr50 0.77
However, when actual properties of the steel are used, this ratio may
increase. In the case of one Northridge damaged building, mill test
reports indicated the ratio to be 0.83.

Downloaded from Digital Engineering Library @ McGraw-Hill (www.accessengineeringlibrary.com)
Copyright © 2009 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.