hyperref needs only text to include in the PDF bookmarks. \textsuperscript expands to something that's not simple text — it contains executable TeX commands and as such can't be simply turned into a sequence of letters. You'll have to replace it with a solution that only contains regular symbols.
First approach: use ASCII characters to approximate the needed effect:
\subsection{Wind load \texorpdfstring{$\mathsf{Z^{STAAD}}$ -- Direction}%
{Z\^STAAD -- Direction}}
Another approach would be to use Unicode superscript symbols at the risk that some PDF viewers won't have the needed font support to display this. But there doesn't seem to be an uppercase superscript letter S, so this is not applicable here.
Finally, a bit of advice if I may: don't use math, abbreviations and other non-readable things in headings unless really needed. For example, instead of
\section{Proof that $\forall n \in \mathbb N (n > 1 \implies n =
p_1^{\alpha_1}p_2^{\alpha_2}\ldots p_k^{\alpha_k})$}
write
\section{Proof that any natural number greater than 1 can be
represented as a product of prime numbers}