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Civil Engineering/Pile - Fixed or Pinned Head

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QUESTION: Dear Arshad,

When is a pile head considered as pinned, and when is it considered as fixed? Can I design a slab/pile connection as pinned considering a slab thickness of 1m or less and pile diameter of 3m.

I would ideally like to consider the pile as pinned so no moments from slab is imparted to the pile, but how do I justify the pinned connection and durability issues when I am queried about opening up of the crack between pile and slab when slab is subject to lateral load or moments.

Your comment on this would be most appreciated.

Best Regards,

Marcus

ANSWER: Hi Marcus,

Whether the joint is considered pinned or fixed depends on the relative stiffnesses of pile and cap/slab (which depend on the spans too) and also the rebar detailing.

Seems like 1m thick slab is rather thin over 3m diameter piles, so not much moments will be transferred to the piles? Lateral load from slabs will need to be all resisted by the piles, unless you have a pile cap and the slab just sits on the cap without any rebar going through.

If you want more advice, send some further details, please.


---------- FOLLOW-UP ----------

QUESTION: Hi Arshad,

Many thanks for your response.  My question was posed in relation to a twin cell culvert/tunnel structure supported by three numbers large diameter bored piles, with single pile located under the central dividing wall and pile under the external walls but with slight offset inwards.

I inherited the existing design as described in my original query but now looking to simplify the design by considering a pinned head connection.  Would provision of rebar with anchorage length of 12D into the base slab be considered as pinned?

The structure is required to be designed as water tight structure so would you consider cracking between pile and slab to be a concern.  The structure is also subject to seismic load therefore a significant lateral force is applied resulting in frame action between structure and pile hence the moments.

The essence of my query therefore really relates to what can be called pinned and what is fixed.  We may pin the detail in analysis and provide minimal anchorage.

Again many thanks for your time and insight.

Regards,

Marcus

Answer
Hi Marcus,

With 3 m diameter piles, all rebar around the edges of the pile that go into the slab will create their own moment of resistance due to the large lever arm of around 2.8m, so I think a pinned connection is not practical, unless you terminate pile rebar below soffit of slab and then have just dowels projecting into the slab from near the centre of the pile diameters.

Also, for waterproofing it would be better to have  a fixed connection.

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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: 40 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 •2013: IEK International Conference, Kisumu, Kenya

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 2010)

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