Expert: Arshad Khan - 1/9/2010

Question
QUESTION: Dear Sir,
I hope that you are fine and doing good.
My Question is regarding Minaret Design. While desiging minaret, the main load is the wind and eq load.

for both cases how to check it against overturning stability???
What is the criteria against which we check such free standing structures..

One thing that comes to my mind is to lower the CG of such stuctures by providing a heavy base and going more deep into the soil... but I dont know what checks shall i performe??

Best Regards,
Muhammad Umar

ANSWER: Hi Umar,

You are correct that a lower C of G and wider deeper base will assist for stability, especially if the minaret is stand-alone. However, most minarets are part of the mosque building, so you can attach them at the base or part way up the height with the mosque structure, and then overturning stability is assured.

If it is stand-alone, then you have to ensure that the resultant reaction due to gravity and lateral loading falls within the middle third of the base. It is best to avoid tension under the base, which can be done by either making it wider or dropping the base down and relying on over-burden weight of the soil above the base for part of the stability (taking factor of safety into account).



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

QUESTION: Thankyou for the answer..
What checks do i need to apply against seismic and wind design...
do i need to check maximum inelastic deflection??? against maxmium permissible.. but displacement of what point along the length of minaret to be checked???  to of minaret or cg?

what about wind sir???


Best Regards,
Muhammad Umar

Answer
Hi Umar,

As the minaret is not normally considered to be habitable, the overall deflection due to wind/seismic loading should not be an issue for serviceability- all you need to ensure is that the structure is stable against overturning, shear and bending- the latter two need to be checked along the whole height of the section is non-uniform (i.e. tapering, or reduced section thickness vs height).

For seismic design, you should follow code recommendations for chimneys.