İstanbul Hotel — Concrete Strengthening and Column Jacketing

Buildings such as hotels, schools and hospitals are assessed against a higher performance target in the seismic code. On these buildings the decision to strengthen usually follows an assessment of the existing structure rather than damage.

At this hotel in İstanbul the scope was the structural system itself: columns were jacketed with added reinforcement, new bars were tied into the existing concrete with chemical anchors, and shear walls were added on the grid lines the calculation called for.

The critical step in jacketing: bond

Jacketing means forming a new reinforced concrete shell around an existing column. Whether it works depends on old and new concrete acting together.

• The existing face is opened up to the cover layer, dedusted and wetted.
• Links and longitudinal bars are tied into the existing section with chemical anchors.
• Epoxy injected into an anchor hole that has not been cleaned of dust loses much of its pull-out capacity.
• If reinforcement continuity is not carried through at floor level, the jacket becomes a shell cut off at each storey.

Which method, and where?

There is no single right method in strengthening; the capacity increase the calculation demands determines it. Carbon fibre raises capacity without enlarging the section but contributes little to ductility. Jacketing gives both strength and stiffness but costs floor area. A new shear wall provides the greatest lateral strength, but calls for intervention at foundation level too.