 

\subsection{BIS sMDT chamber design}
%
\begin{figure}[th]
\centering
	\includegraphics[angle=90,width=\textwidth]{BIS78_3D_sMDT_RPC.pdf}
	\caption{Mechanical drawings of a BIS~7/8 sMDT chamber with integrated triple thin-gap RPC. The RPC chamber is supported independently on the same rails 
	in the BIS layer as the sMDT chamber. The same support scheme will be used also for the BIS~1-6 chambers.}
	\label{fig:BIS78_3D_sMDT_RPC} 
\end{figure}
%
\begin{figure}[th]
\centering
 	\includegraphics[angle=90,width=\textwidth]{BIS78_RPC.pdf}
	\caption{Mechanical drawing of a BIS triple thin-gap RPC chamber with only 50 mm thickness fitting into the BIS layer together with the BIS sMDTs.}
	\label{fig:BIS78_RPC} 
\end{figure}

\begin{figure}[th]
\centering
	\includegraphics[width=\textwidth]{BIS78_3D_RPC_support}
	\caption{Mechanical drawings of a BIS~7/8 RPC chamber with rail supports.}
	\label{fig:BIS78_3D_RPC_support} 
\end{figure}
%


The design and construction of new BIS78 sMDT chambers with 2 x 4 tube layers and integrated thin-gap RPCs with three gas gaps for the replacement of the current BIS~7 and BIS~8 MDT chambers 
in the 2019-20 LHC shutdown is a pilot project for the complete replacement of the BIS MDT chambers by sMDT + RPC chambers for HL-LHC. 
There are 12 different types of BIS~78 sMDT chambers with complex shapes maximising the coverage in the overlap region with the New Small Wheels~\cite{sMDT_parameterbook}.
The drawings of a BIS~7/8 sMDT detector with integrated triple thin-gap RPC chamber of only 50~mm thickness are shown in 
Figure~\ref{fig:BIS78_3D_sMDT}. 
and \ref{fig:BIS78_3D_sMDT_RPC} and \ref{fig:BIS78_RPC}. 
The design of the remaining new BIS~1-6 sMDT and RPC chambers will be very similar to the BIS~7/8 design, 
with the simplifications of rectangular shape and only two chamber types differing only in their width in z direction.  
The parameters of the BIS~1-6 sMDT chambers are summarised in Tables~\ref{tab:bis16_parameters} and \ref{tab:bis16_numbers}.
The same assembly procedure and precision tooling will be used for both the BIS~7/8 and the BIS~1-6 sMDT chambers.

The common challenge for the mechanical design is the small available space 
and the control of gravitational deformations of the thin chambers with minimal space for support structure. 
The RPC chambers are mounted on their own support frame and will be installed on the existing rails in the BIS layer independently from the sMDTs (see Figure~\ref{fig:BIS78_3D_RPC_support}).
Otherwise the weight of the RPCs would distort too much the BIS sMDT chambers with their thin support structure. 
In order to provide space for the RPC rail supports in between the sMDT chambers, a gap of 20~mm is required between adjacent sMDT chambers which is achieved by removing one tube per layer of each BIS~1-6 chamber
compared to the maximum possible. This does not affect width in z of the Faraday cages on the Readoud and high-voltage ends of the chambers which contain the unchanged number of 
readout and HV distribution boards since the RPC supports have to pass between the sMDT chambers only in two locations along the tubes (see Figure~\ref{fig:BIS_RPC}).  
Also, there needs to be no extra gap between adjacent RPCs. 
The resulting tube layout of a sector of the new BIS sMDT layer for LHC phase 2, including the BIS~7/8 chambers, is shown in Figures~\ref{fig:BIS_layout}.


