A pharaonic architectural project in Makkah

The Omar Jabal project includes plans for the construction of about fifteen towers in Makkah; the lowest will be 60 metres high and the tallest almost 200 metres.  The two tallest form a hotel complex to accommodate pilgrims, and will face each other and be connected by a walkway 150 metres above the ground forming an arch facing Al-Masjid Al-Haram, the Sacred Mosque.  CSTB was called in to help with the wind design of these two towers and improve pedestrian comfort under wind.

A wind design appropriate for the project

CSTB has carried out wind tunnel tests for the wind design of the two tower buildings.  Engineers collected weather data from the Makkah weather station and then carried out statistical studies in order to define reference winds "striking" the structure.  Since winds will be modulated by the architectural environment, an instrumented model of the two towers and nearby towers was used to make very precise aerodynamic pressure measurement tests.  "The results are used to characterise forces applied to the two towers.  We were thus able to advise the Oger International design office that will be able to make an optimum design of the buildings, foundations, glazing thickness, etc.,” says Julien Berthaut, CSTB engineer.  Special attention was paid to the connecting walkway:  relative movements between the tops of the twin towers make a certain degree of freedom necessary so that these movements can be tolerated without introducing any problems.

Comfort, an essential concept

Comfort is a concept that had to be considered objectively based on precise criteria.  This is why winds were measured at pedestrian level near the base of the two towers and quantified.  "There is a limit to the wind speed that does not annoy passers-by, continues Julien Berthaut.  This limiting speed depends on how the space is used (as a passageway, for standing still or prolonged seating, etc.), but also on the temperature in the place concerned.  A slight breeze in northern France would potentially be more unpleasant than in Saudi Arabia where it might refresh the population."  Since the project does not include anything that would reduce comfort, no recommendations about adding vegetation or a wind break were necessary.

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The towers tends to initiate vibrations, particularly at a height of 200 metres.  The tolerance threshold to these vibrations is much lower for a hotel which is a place of rest and relaxation than for a tall office building, in which human activity is more important.  CSTB engineers are currently studying the concept of the pilgrim comfort inside the two towers, based on their stiffness and the wind loading.