Specify ‘w’ to be real, and then request 'ReturnConditions' on the evaluation to understand under what conditions ‘w’ will be real:
syms w real
eqn = Mc*D==(4*(w.^3));
V = solve(eqn, 'ReturnConditions',1)
That is likely the best you can do.
L=15.7;
l=11.6;
Wab=20;
Wbd=40;
Mab = Wab/32.2;
Mbd = Wbd/32.2;
Wbd2=(Wbd/(L-1));
d=2.58;
OB=L-l-d;
BD=d+OB;
Wbo=Wbd2*OB;
Wod=Wbd2*d;
delta=d*0.95;
syms Wc real
eqn = (((11.6/2)+1.52)*20)+((((15.6-11.6)/2)-1.52)*Wbo)-((d/2)*Wod)-(delta*Wc)==0;
S = solve(eqn)
% Wc = eval(S)
Mc = Wc/32.2;
D=490*(1/32.2);
syms w real
eqn = Mc*D==(4*(w.^3));
V = solve(eqn, 'ReturnConditions',1)
V.w
V.conditions
% w1 = eval(V)
% w = real(w1)
h=2*w
I leave you to explore those conditions at your leisure.
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