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Section 5
5-71
Notes
All values f
1-9
must be supplied in current units.
Some important points to note in the case of the composite section
are:
1. The 'CM' parameter can be assigned to I-shaped sections only. A
'CM' (composite) section is one obtained by considering a portion
of a concrete slab to act in unison with the I shaped steel section.
FC is the strength or grade of concrete used in the slab. In the
USA, FC is called the specified compressive strength of concrete.
Typical values of FC range between 2.0 and 5.0 ksi, and 20 to 50
Mpa.
2. The width of the concrete slab (CW) (if not entered) is assumed to
be the width of the top flange of the steel section + 16 times the
thickness of the slab.
3. In order to calculate the section properties of the cross-section, the
modular ratio is calculated assuming that:
Es= Modulus of elasticity of steel = 29000 Ksi.
Ec= Modulus of elasticity of concrete = 1802.5
FC
Ksi
where FC (in Ksi) is defined earlier.
Some other general notes on this subject of member property
designations are :
4. The 'T' parameter stands for a T-shaped section obtained by cutting
an I-shaped section at exactly its mid height level along the web.
Hence, the area of a T shape is exactly half the area of the
corresponding I shape. The depth of a T shape is half the depth of
the I shape it was cut from.
What we refer to as I shaped sections are sections which look like
the english alphabet I. The American Wide Flange, the British UB
and UC sections, Japanese H sections, etc., all fall under this
category. Consequently, the 'T' shape cut from a Japanese H shape
is one obtained by cutting the H shape at exactly its mid-height
level of the web.