fb=MyI
MI=fby
At the extreme wood fiber
(MI)wood=1200ˉy
At the extreme steel fiber
(MI)steel=18000/2010.5−ˉy=90010.5−ˉy
(MI)wood=(MI)steel
1200ˉy=90010.5−ˉy
1200(10.5−ˉy)=900ˉy
12600−1200ˉy=900ˉy
2100ˉy=12600
ˉy=6 in
You can also find ˉy with the aid of stress diagram as follows:
Aw=6(10)=60 in2
As=0.5(20b)=10b
A=Aw+As=60+10b
Aˉy=Awyw+Asys
(60+10b)(6)=60(5)+10b(10.25)
360+60b=300+102.5b
42.5b=60
b=2417 in=1.41 in answer