mo=.
fz = .
to =.
ma = mo - fz x
mb = to - fz x
ta = to
tb = mo - fz la

theta = Expand[Integrate[ma/eia D[ma,mo],{x,0,la}] +
               Integrate[mb/eib D[mb,mo],{x,0,lb}] +
               Integrate[ta/gja D[ta,mo],{x,0,la}] +
               Integrate[tb/gjb D[tb,mo],{x,0,lb}] ]

rule1 = Solve[theta==0, mo];
Expand[mo /. rule1[[1]][[1]]];
mo = %

phi = Expand[Integrate[ma/eia D[ma,to],{x,0,la}] +
             Integrate[mb/eib D[mb,to],{x,0,lb}] +
             Integrate[ta/gja D[ta,to],{x,0,la}] +
             Integrate[tb/gjb D[tb,to],{x,0,lb}] ]

rule2 = Solve[phi==0, to];
Expand[to /. rule2[[1]][[1]]];
to=%

dz = Expand[Integrate[ma/eia D[ma,fz],{x,0,la}] +
            Integrate[mb/eib D[mb,fz],{x,0,lb}] +
            Integrate[ta/gja D[ta,fz],{x,0,la}] +
            Integrate[tb/gjb D[tb,fz],{x,0,lb}]]

dz1 = Simplify[dz]

eia=1;
eib = eia;
gjb = gja;
gja = 0.85 eia / 1.3;
Simplify[Expand[dz1]]