Draft

10.3CompactSections 177

Figure10.6: Stress-straindiagramformoststructuralsteels

13 Whentheyieldstressis reachedat theextreme ber,thenominalmoment strengthMn, is

referredto as theyieldmomentMyandis computedas

Mn=My=SxFy (10.3)

(assumingthatbendingis occurringwithrespectto thexxaxis).

14 Whenacrosstheentiresection,thestrainis equalor largerthantheyieldstrain(""y=

Fy=Es) thenthesectionis fullyplasti ed,andthenominalmoment strengthMnis therefore

referredto as theplasticmomentMpandis determinedfrom

Mp=Fy

Z

A

ydA=FyZ (10.4)

where

Z

def

=

Z

ydA (10.5)

is thePlasticSectionModulus.

15 TheplasticsectionmodulusZshouldnotbe confusedwiththeelasticsectionmodulusS

de ned,Eq.5.23as

S =

I

d= 2

(10.6-a)

I

def

=

Z

A

y

2

dA (10.6-b)

16 ThesectionmodulusSxof a W sectioncanbe roughlyapproximatedby thefollowingformula

Sxwd= 10 or IxSx

d

2

wd

2

= 20 (10.7)

andtheplasticmoduluscanbe approximatedby

Zxwd= 9 (10.8)