Splitting a long equation with a curly brace for the equations in two lines

Question

The equation that I have pasted below is very long. I want the first equation (e34a) after dw^2/dx^2 to be split in two lines with a common curly brace for the equation in two lines. My code is

\begin{subequations}\label{e34} 
  The Normal Stress are given by 
  \begin{align}
   q = & \overline{C}_1\overline{C}_3\overline{C}_5\dfrac{\alpha^{*}k_s w}{U\big(1+ \alpha^{*}k_s(\frac{w}{q_u})\big)}- \dfrac{\partial^{2}w}{\partial x^{2}}\Bigg(G_1 H_1+\overline{C}_2(T_p+T_1)\cos\theta+G_2 H_2\overline{C}_1 \label{e34a}\\
&+ G_3 H_3 \overline{C}_1 \overline{C}_3+G_4 H_4 \overline{C}_1 \overline{C}_3\overline{C}_5+ \overline{C}_1\overline{C}_4(T_p+T_2)\cos\theta+
         \overline{C}_1\overline{C}_3\overline{C}_6(T_p+T_3)\cos\theta\Bigg)\nonumber\\
        \intertext{The Mobilised Tension for top reinforcement is}
            \frac{\partial T_1}{\partial x} = & -\Bigg(q+G_1H_1\frac{\partial^{2}w}{\partial x^{2}}\Bigg)\  \overline{D}_1
            - \Bigg(\overline{C}_3\overline{C}_5\dfrac{\alpha^{*}k_s w}{U\big(1+\alpha^{*}k_s(\frac{w}{q_u})\big)} - A_1 \frac{\partial^{2}w}{\partial x^{2}}\Bigg)\  \overline{D}_2\\
        \intertext{The Mobilised Tension for middle reinforcement is}
            \frac{\partial T_2}{\partial x} = & -\Bigg(\frac{1}{\overline{C}_1} \Big(q+A_2\frac{\partial^{2}w}{\partial x^{2}} \Big) +G_2 H_2\frac{\partial^{2}w}{\partial x^{2}} \Bigg)\ \overline{D}_3  \label{e34c} \\
            & - \Bigg( \overline{C}_5 \dfrac{\alpha^{*}k_s w}{U\big(1+\alpha^{*}k_s(\frac{w}{q_u})\big)} - A_3 \frac{\partial^{2}w}{\partial x^{2}} \Bigg)\ \overline{D}_4  \nonumber
            \intertext{The Mobilised Tension for bottom reinforcement is}
            \frac{\partial T_3}{\partial x} = & -\Biggl(\frac{1}{\overline{C}_3}\Bigg(\frac{1}{\overline{C}_1}\Big(q+A_2\frac{\partial^{2}w}{\partial x^{2}} \Big)+A_4\frac{\partial^{2}w}{\partial x^{2}}\Bigg)+G_3 H_3\frac{\partial^{2}w}{\partial x^{2}} \Biggl)\ \overline{D}_5  \label{e34d} \\
            & - \Bigg( \dfrac{\alpha^{*}k_s w}{U\big(1+\alpha^{*}k_s(\frac{w}{q_u})\big)} - G_4H_4 \frac{\partial^{2}w}{\partial x^{2}} \Bigg)\ \overline{D}_6  \nonumber
\end{align}
\end{subequations}

Answer 1

I suggest the following code, which uses the split environment, defined by the amsmath package, to align each of the sub-equatons. Note that I've also attempted to make the code more readable by defining the commands \Cbar and \Dbar, and by replacing the various \Bigg instructions with \bigg. (Bigg seems just too big...)

\documentclass{article}
\newcommand{\Cbar}{\overline{C}}
\newcommand{\Dbar}{\overline{D}}
\usepackage{amsmath}

\begin{document}
\begin{subequations}\label{e34} 
\noindent
The Normal Stress $q$ is given by 
\begin{align}
\begin{split}
q = &\phantom{=} \Cbar_1\Cbar_3\Cbar_5\dfrac{\alpha^{*}k_s w}{U\big(1+ \alpha^{*}k_s 
(w/q_u)\big)}
- \dfrac{\partial^{2}w}{\partial x^{2}}\bigg\{G_1 H_1+\Cbar_2(T_p+T_1)\cos\theta 
\label{e34a}\\
&+G_2 H_2\Cbar_1+ G_3 H_3 \Cbar_1 \Cbar_3+G_4 H_4 \Cbar_1 \Cbar_3\Cbar_5\\
&+\Cbar_1\Cbar_4(T_p+T_2)\cos\theta+\Cbar_1\Cbar_3\Cbar_6(T_p+T_3)\cos\theta
\bigg\}\,.\\
\end{split}
\intertext{The Mobilised Tension for top reinforcement is}
\begin{split}
\frac{\partial T_1}{\partial x} 
= & -\bigg(q+G_1H_1\frac{\partial^{2}w}{\partial x^{2}}\bigg)\  \Dbar_1 \\
&- \bigg(\Cbar_3\Cbar_5\dfrac{\alpha^{*}k_s w}{U\big(1+\alpha^{*}k_s(w/q_u)\big)} 
- A_1 \frac{\partial^{2}w}{\partial x^{2}}\bigg)\  \Dbar_2\\
\end{split}
\intertext{The Mobilised Tension for middle reinforcement is}
\begin{split}
\frac{\partial T_2}{\partial x} 
= & -\bigg(\frac{1}{\Cbar_1} 
\Big(q+A_2\frac{\partial^{2}w}{\partial x^{2}} \Big) 
+G_2 H_2\frac{\partial^{2}w}{\partial x^{2}} \bigg)\ \Dbar_3  \label{e34c} \\
&- \bigg( \Cbar_5 \dfrac{\alpha^{*}k_s w}{U\big(1+\alpha^{*}k_s(w/q_u)\big)} 
- A_3 \dfrac{\partial^{2}w}{\partial x^{2}} \bigg)\ \Dbar_4\\
\end{split}
\intertext{The Mobilised Tension for bottom reinforcement is}
\begin{split}
\frac{\partial T_3}{\partial x} 
= & -\biggl(\frac{1}{\Cbar_3}\bigg(\frac{1}{\Cbar_1}\Big(q+A_2\frac{\partial^{2}w}
{\partial x^{2}} \Big)
+A_4\frac{\partial^{2}w}{\partial x^{2}}\bigg)+G_3 H_3\frac{\partial^{2}w}{\partial
x^{2}} \biggr)\ 
\Dbar_5  \label{e34d} \\
&-\bigg( \dfrac{\alpha^{*}k_s w}{U\big(1+\alpha^{*}k_s(w/q_u)\big)} - G_4H_4 
\frac{\partial^{2}w}{\partial x^{2}} \bigg)\ \Dbar_6 \\
\end{split}
\end{align}
\end{subequations}

\end{document}

Answer 2

I find the use of "\big and friends" to be a quick fix that works 99% of the time, with that 1% being quite frustrating to solve. I took Mico's answer (credit to him for the improved layout of the first two equations) and reverted back to the old trustworthy \left and \right pairs, where LaTeX decides how to stretch/extend the extensible delimiters () and {}. There is problem when stretching has to occur across more than one line in an equation though. But for that, \vphantom{...} allows a vertical (zero-width) stretch of the correct height. By example, here's my code:

\documentclass{article}
\newcommand{\Cbar}{\overline{C}}
\newcommand{\Dbar}{\overline{D}}
\newcommand{\myfrac}{\dfrac{\partial^{2}w}{\partial x^{2}}}% For height purposes
\usepackage{amsmath}

\begin{document}
\begin{subequations}\label{e34} 
\noindent
The Normal Stress $q$ is given by 
\begin{align}
\begin{split}
q = &\phantom{=} \Cbar_1\Cbar_3\Cbar_5\dfrac{\alpha^{*}k_s w}{U\left(1+ \alpha^{*}k_s 
(w/q_u)\right)}
- \myfrac\left\{\vphantom{\myfrac}G_1 H_1+\Cbar_2(T_p+T_1)\cos\theta \right.
\label{e34a}\\
&+G_2 H_2\Cbar_1+ G_3 H_3 \Cbar_1 \Cbar_3+G_4 H_4 \Cbar_1 \Cbar_3\Cbar_5 \\
&+\>\Cbar_1\Cbar_4(T_p+T_2)\cos\theta+\Cbar_1\Cbar_3\Cbar_6(T_p+T_3)\cos\theta
\left.\vphantom{\myfrac}\!\right\}\,.\\
\end{split}
\intertext{The Mobilised Tension for top reinforcement is}
\begin{split}
\frac{\partial T_1}{\partial x} 
= & -\bigg(q+G_1H_1\frac{\partial^{2}w}{\partial x^{2}}\bigg)\  \Dbar_1 \\
&- \bigg(\Cbar_3\Cbar_5\dfrac{\alpha^{*}k_s w}{U\big(1+\alpha^{*}k_s(w/q_u)\big)} 
- A_1 \frac{\partial^{2}w}{\partial x^{2}}\bigg)\  \Dbar_2\\
\end{split}
\intertext{The Mobilised Tension for middle reinforcement is}
\begin{split}
\frac{\partial T_2}{\partial x} 
= & -\left(\frac{1}{\Cbar_1} 
\left(q+A_2\frac{\partial^{2}w}{\partial x^{2}} \right) 
+G_2 H_2\frac{\partial^{2}w}{\partial x^{2}} \right)\ \Dbar_3  \label{e34c} \\
&- \left( \Cbar_5 \dfrac{\alpha^{*}k_s w}{U\left(1+\alpha^{*}k_s(w/q_u)\right)} 
- A_3 \dfrac{\partial^{2}w}{\partial x^{2}} \right)\ \Dbar_4\\
\end{split}
\intertext{The Mobilised Tension for bottom reinforcement is}
\begin{split}
\frac{\partial T_3}{\partial x} 
= & -\left(\frac{1}{\Cbar_3}\left(\frac{1}{\Cbar_1}\left(q+A_2\frac{\partial^{2}w}
{\partial x^{2}} \right)
+A_4\frac{\partial^{2}w}{\partial x^{2}}\right)+G_3 H_3\frac{\partial^{2}w}{\partial
x^{2}} \right)\ 
\Dbar_5  \label{e34d} \\
&-\left( \dfrac{\alpha^{*}k_s w}{U\left(1+\alpha^{*}k_s(w/q_u)\right)} - G_4H_4 
\frac{\partial^{2}w}{\partial x^{2}} \right)\ \Dbar_6 \\
\end{split}
\end{align}
\end{subequations}

\end{document}

The macro \newcommand{\myfrac}{\dfrac{\partial^{2}w}{\partial x^{2}}} is introduced just for the sake of brevity.

For more ideas on how to work with math in LaTeX, Herbert Voss' mathmode document is an invaluable source.