Bridge Builder

Authors: Rohin Gosling, Casey Chadwick
Version: 1.4a
Year: 2000

   

🌉 Photo of the physical bridge entered into the competition

An interactive bridge design utility for modelling parabolic tied-arch bridges. Renders real-time 2D orthographic and 3D perspective views of parameterized, parabolic, tied-arch bridge designs.

This software was written to facilitate the design, budgeting, and construction of a model bridge for an academic bridge building competition. The physical bridge built using the software to guide design, placed 2nd in the competition for load capacity, and 1st for design architecture.

🧭 Table of Contents

• ✨ Features
• 📐 Bridge Geometry
• 📋 Requirements
• 🔨 Building from Source
• ▶️ Running
• ⌨️ Keyboard Controls
• 💾 Bridge File Format (.BRG)
• 📄 License

✨ Features

• Parametric design of parabolic tied-arch bridges with adjustable length, height, width, and segment count.
• Three structural configurations: Hanging, Support, and Zig-zag.
• Real-time 2D orthographic and 3D perspective rendering with smooth rotation.
• Double-buffered 3D display with vertical retrace synchronization for flicker-free animation.
• Individual beam inspection showing length, gradient, and vertex coordinates.
• Ten save/load slots for bridge designs.

📐 Bridge Geometry

The bridge is a symmetric parabolic truss centered at the origin. All geometry is derived from three user parameters -- length $l$, height $h$, and width $w$ -- plus a segment count $s$ that controls the resolution of the arch.

Parabolic Arch

The arch profile follows a downward-opening parabola that passes through the endpoints of the road bed and reaches its apex at the bridge midpoint. With the bridge centered on the $x$-axis so that the road bed spans from $-\tfrac{l}{2}$ to $+\tfrac{l}{2}$, the arch height at any position $x$ is:

$$y(x) = ax^2 + h, \qquad a = -\frac{h}{\left(\tfrac{l}{2}\right)^2}$$

At $x = 0$ (midpoint) the arch reaches its full height $h$. At $x = \pm\tfrac{l}{2}$ (the ends) the arch meets the road bed at $y = 0$.

Structural Elements

The arch is sampled at $s$ equally-spaced segments of width $\Delta x = \tfrac{l}{s}$. At each sample point the program evaluates the parabola to obtain the arch height, then generates five categories of beam:

Element	Description
Vertical beams	Connect the road bed ($y = 0$) to the arch point $y(x)$ at each sample.
Arch beams	Connect consecutive arch points, forming the curved top chord.
Diagonal beams	Brace between the road bed and the arch in one of three configurations: Hanging (road to farther arch point), Support (road to nearer arch point), or Zig-zag (alternating).
Cross beams	Span the bridge width $w$ at road level and at the arch, tying the two sides together.
Road bed	Two longitudinal beams running the full length of the bridge, one on each side.

All elements are generated for both sides of the bridge (offset by $\pm\tfrac{w}{2}$ on the $z$-axis), producing a complete 3D truss from the 2D arch profile.

Material Length

Total construction material is the sum of all beam lengths computed via the 3D Euclidean distance:

$$L_{\text{total}} = \sum_{i=1}^{N} \sqrt{\Delta x_i^2 + \Delta y_i^2 + \Delta z_i^2}$$

When the double arch beams option is enabled, arch beam lengths are doubled and extra material is added for splice joints at regular intervals.

📋 Requirements

• DOS-compatible environment (or a DOS emulator such as DOSBox).
• Borland BGI runtime files in the same directory as the executable (already provided in src/):
  • EGAVGA.BGI -- VGA graphics driver
  • LITT.CHR -- Small font
  • TRIP.CHR -- Triplex font
  • TSCR.CHR -- Script font

🔨 Building from Source

All source files, build scripts, and runtime assets live in the src/ directory. Compile with the Borland C++ compiler (targeting 16-bit DOS) by running the provided batch script from within src/:

cd src
build

BUILD.BAT compiles each .CPP module with the large memory model (-ml) and links them with GRAPHICS.LIB to produce BRIDGE.EXE. Run clean to remove the build artifacts.

▶️ Running

Launch the executable in DOSBox or any DOS-compatible environment. BRIDGE.EXE and its runtime assets live in src/, so run it from there:

mount c /path/to/bridge-builder
c:
cd src
BRIDGE.EXE

The BGI runtime files (EGAVGA.BGI, LITT.CHR, TRIP.CHR, TSCR.CHR) and the BRIDGE_*.BRG save slots reside in src/ alongside BRIDGE.EXE, so the program locates them automatically when run from that directory.

⌨️ Keyboard Controls

General

Key	Action
Esc	Exit program
Tab	Toggle active window (Dialog / Model)
V	Toggle 2D / 3D view
*	Reset program to defaults
0 - 9	Load bridge from file BRIDGE_X.BRG
Alt + 0 - 9	Save bridge to file BRIDGE_X.BRG

Dialog Window (Parameter Editing)

Key	Action
Up / Down	Scroll through design and construction parameters
Enter	Edit a parameter / toggle a setting
Left / Right	Adjust design parameter by 1 mm, or cycle segment values
Ctrl + Left/Right	Adjust design parameter by 10 mm

Editable Parameters

#	Parameter	Type	Range
1	Bridge Length	Numeric	0 -- 9999 mm
2	Bridge Height	Numeric	0 -- 9999 mm
3	Bridge Width	Numeric	0 -- 9999 mm
6	Segments	Numeric	2 -- 20 (step 2)
7	Cross-supported Segments	Numeric	2 -- 20
8	Configuration	Toggle	Hanging / Support / Zig-zag
9	Double Arch Beams (Vert.)	Toggle	Yes / No
10	Double Arch Beams (Diag.)	Toggle	Yes / No

2D View (Model Window Active)

Key	Action
+	Select next construction beam
-	Select previous construction beam

3D View (Model Window Active)

Key	Action
Up / Down	Rotate model about the X axis (pitch)
Left / Right	Rotate model about the Y axis (yaw)

💾 Bridge File Format (.BRG)

Bridge designs are saved as binary files named BRIDGE_0.BRG through BRIDGE_9.BRG. Each file is 62 bytes and contains a file header followed by the bridge data structure.

File Layout

Offset	Size (bytes)	Field	Description
0	20	ID String	"Bridge Builder" followed by CR, LF, NUL, padded to 20 bytes
20	2	Major Version	Major version number (16-bit integer, e.g. 1 for version 1.4)
22	2	Minor Version	Minor version number (16-bit integer, e.g. 4 for version 1.4)
24	4	Height	Bridge height in mm (32-bit float)
28	4	Length	Bridge length in mm (32-bit float)
32	4	Width	Bridge width in mm (32-bit float)
36	2	Material Length	Total construction material length in mm (16-bit integer)
38	4	Joint Overlap	Joint overlap distance in mm (32-bit float)
42	2	Num Segments	Number of arch segments (16-bit integer)
44	2	Num Cross-Supported Segs	Number of cross-supported segments (16-bit integer)
46	2	Configuration	0 = Hanging, 1 = Support, 2 = Zig-zag (16-bit integer)
48	2	Split Vertical Beams	0 = No, 1 = Yes (16-bit enum)
50	2	Split Diagonal Beams	0 = No, 1 = Yes (16-bit enum)
52	2	Num Vertical Beams	Total vertical beams (16-bit integer, computed)
54	2	Num Diagonal Beams	Total diagonal beams (16-bit integer, computed)
56	2	Num Cross Beams	Total cross-support beams (16-bit integer, computed)
58	2	Num Arch Beams	Total arch beams (16-bit integer, computed)
60	2	Num Joints	Total joints (16-bit integer, computed)

Total file size: 62 bytes.

All multi-byte values are stored in little-endian byte order (x86). The 16-bit integer and float sizes reflect the Borland C++ 16-bit DOS memory model, where int is 2 bytes and float is 4 bytes (IEEE 754 single-precision).

📄 License

This project is licensed under the MIT License.