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Expert: - 11/25/2002


Question
Hi,
I did some experiments with folding paper and found that paper shaped into a cylinder with a circular cross sectional area could support more weight than a paper folded with a triangluar crossectional area before it collapsed. Is this result theoretically correct or did I do something wrong? Could you also please explain why?

Thanks for your time,
Steven

Answer
Hi Steven,

Yes, your experiments are correct in principal, but the results can vary depending on the relative cross-sectional area or sizes.

Any thin member in compression is much weaker than in tension because of lateral instability caused by buckling. The compressive strength can be increased by restraining the thin member laterally at regular intervals. In a cylinder, the vertical strips are laterally restrained due to the curvature. In a triangle, only the strips next to each corner are restrained, but near the centre of each of the 3 sides, the vertical strips are not restrained and therefore they buckle laterally. If you reduce the size of the triangle, you will notice an improvement in the compressive capacity. The other way to improve it's performance is to attach stiffeners horizontally along each of the 3 sides, from apex to apex. The closer spacing you have for these stiffeners, the better it will perform.

Another useful test for demonstrating lateral instability is by using a plastic straw. Take one and cut of a length of 1 cm. Then compare the axial compressive capacity of this short bit with the full length straw!

Hope this helps!

Kind regards,

Arshad  

Civil Engineering

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Arshad Khan

Expertise

I can answer any questions to do with civil and structural engineering consultancy and construction industry in East Africa and the Middle East, and specifically with the analysis and design of reinforced concrete structures. My particular expertise is in the aseismic design and optimisation of tall buildings.

Experience

Employment history: 36 years in Construction and consultancy in the UK, Kenya, Tanzania, Uganda, Ethiopia, Eritrea, South Africa, Somalia, Zambia, Austria, Bahrain, Kuwait, Doha and the U.A.E.

Organizations
.Fellow of Institution of Structural Engineers (UK) .Fellow of Institution of Civil Engineers(UK) .Member of the Institution of Engineers, Kenya .Registered Engineer, ERB, Kenya .Member of the Architectural Association of Kenya (Engineers Chapter) .Chartered Engineer (UK)

Publications
•1984: International Conference on the Art and Practice of Structural Design, London •1994: 3rd Int. Kerensky Conference in Structural Engineering, Singapore •2008: International Conference on High-Rise Towers, Abu Dhabi

Education/Credentials
BSc, 1st Class Hons, in Building Engineering, University of Bath, UK MSc in Concrete Structures and Technology, University of London. Diploma of Imperial College, UK.

Awards and Honors
•Science Congress Special Award (for 2-seater Hovercraft - 1968) •Institution of Civil Engineers Award for outstanding performance at Bath University (1975) •Concrete Society Postgraduate study Bursary Award (1976) •Consular Representative for British High Commission, Nairobi. (1995 to 1998) •Examiner for Institution of Civil Engineers Professional Interviews, Nairobi. (1997 to 1998) •Branch Representative in Vienna for PI assessment for Inst. of Struct. Engrs. (1999 to date)

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