Torsion members - The Behaviour and Design of Steel Structures to EC3 - page 464

10.4.4 Bimoment section resistance

The bimoment section resistance of Class 1 and Class 2 equal-flanged I-sections
B Rd ,12 is given by equation 10.61. The bimoment section resistance of a Class 3
equal-flanged I-section can be approximated by using

B Rd ,3 = B y + ( B p − B y )( 14 − λ)/ 4 (10.74)

in which B y is the first yield bimoment given by equation 10.59. The bimoment
section resistance of a Class 4 equal-flanged I-section can be approximated by
using

B Rd ,4 = B y ( 14 /λ) (10.75)

These equations may also be used for the bimoment section resistances of other
open sections by using their fully plastic bimoments B p [18] or first yield bimo-
ments B y .Alternatively,thebimomentsectionresistancesofotherClass1,Class2,
or Class 3 open sections can be conservatively approximated by using

B Rd ,123 = B y (10.76)

andthebimomentsectionresistancesofotherClass4opensectionscanbeapprox-
imated using equation 10.75.
Itisconservativetoignorebimomentsinhollowsectionmembers,andsothere
is no need to consider their bimoment section resistances.

10.4.5Plastic design

The use of plastic design should be limited to Class 1 members which have suf-
ficient ductility to reach the plastic collapse mechanism. Plastic design should be
carried out by using plastic analysis to determine the plastic collapse load factor
α x (see Section 10.3.3.3), and then checking that

1 ≤ α x (10.77)

10.4.6 First hinge design

The use of first hinge design should be limited to Class 1 and Class 2 members
in which the first hinge of a collapse mechanism can form. The design uniform
torques T t and bimoments B can be determined by using elastic analysis, and the
member is satisfactory if

2 2

T t
T t , Rd ,2

B
B Rd ,2

+ ≤ 1 (10.78)

issatisfiedatallpointsalongthemember.Thefirsthingemethodismoreconserva-
tivethanplasticdesign,andmoredifficulttousebecauseelasticmemberanalysis