Portal Method – Lateral Load Analysis

An interactive Jupyter notebook that applies the Portal Method to approximate the shear forces and bending moments in the beams and columns of a multi-storey, multi-bay rectangular frame subjected to lateral loads (wind / seismic).

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What is the Portal Method?

The Portal Method is a hand-calculation approximation technique used in structural engineering for the preliminary analysis of building frames under lateral (horizontal) loads. It assumes:

1. A point of contra-flexure (zero bending moment) at mid-height of every column.
2. A point of contra-flexure at mid-span of every beam.
3. Interior columns carry twice the shear of exterior columns in the same storey.

These assumptions reduce the frame to a set of simple portals that can be solved using statics alone, making it very fast compared to the cantilever method or a full finite-element analysis.

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Project Structure

portal-method/
│
├── data/
│   └── portal.xlsx          ← Input file (edit this with your frame data)
│
├── portal_method.ipynb      ← Main notebook (run this)
│
├── requirements.txt         ← Python dependencies
├── .gitignore               ← Files Git should ignore
└── README.md                ← You are here

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Input File Format (data/portal.xlsx)

Open data/portal.xlsx and fill in one row per storey, from top to bottom:

Column name	Unit	Description
beams	—	Storey number (1 = top storey)
force	kN	Lateral force applied at this storey level
distance between beams from top	m	Storey height (floor-to-floor)
distance between columns	m	Bay width (column-to-column)

Example (3-storey, 4-column frame):

beams	force	distance between beams from top	distance between columns
1	15	3.5	5
2	30	3.5	7
3	30	4.0	5

The number of bays is inferred from the number of non-empty rows in the distance between columns column (bays = rows with a value, columns = bays + 1).

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Getting Started

Option A – Run on Google Colab (no installation needed)

1. Go to colab.research.google.com.
2. Click File → Upload notebook and upload portal_method.ipynb.
3. Upload portal.xlsx to the Colab file panel (click the folder icon on the left sidebar → upload).
4. In the notebook, update EXCEL_PATH to match where you uploaded the file, e.g. "/content/portal.xlsx".
5. Click Runtime → Run all.

Option B – Run locally

Prerequisites: Python 3.8 or newer.

# 1. Clone the repository
git clone https://github.com/<your-username>/portal-method.git
cd portal-method

# 2. (Recommended) Create a virtual environment
python -m venv venv
source venv/bin/activate        # Windows: venv\Scripts\activate

# 3. Install dependencies
pip install -r requirements.txt

# 4. Launch Jupyter
jupyter notebook portal_method.ipynb

Then run all cells (Kernel → Restart & Run All).

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Output

The notebook produces:

• Printed tables – column shear, column moment, beam shear, beam moment for every storey and bay.
• Shear Force Diagram – plotted on the frame outline and saved as shear_force_diagram.png.
• Bending Moment Diagram – plotted on the frame outline and saved as bending_moment_diagram.png.
• Summary DataFrames – final results in a clean tabular format.

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Requirements

See requirements.txt. The core dependencies are:

Package	Purpose
pandas	Reading the Excel input and building result tables
openpyxl	Excel (.xlsx) engine for pandas
numpy	Numerical helpers
matplotlib	Plotting the SFD and BMD

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Limitations

• The Portal Method is an approximate method. Results may differ from those of an exact analysis (e.g. stiffness-matrix / FEM) by 10–30 % depending on the frame geometry.
• It is best suited for regular, rectangular frames with roughly equal bay widths and storey heights.
• The method does not account for axial deformations, shear deformations, or geometric nonlinearity.

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License

MIT License – see MIT license

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Author

Uddipta Bhaskar Sarma (uddiptasarma361@gmail.com) – feel free to open an issue or pull request if you find a bug or want to suggest an improvement.