School in Montelabbate

Building use

Other

Country/Region

Italy

Overview

Multiple types of steel products, including composite columns (concrete filled tubes) and composite beams (thin-walled steel profiles with concrete infill) were used to retrofit the structural system of an elementary school that was deemed unsafe according to the new code requirements that came into force in Italy in 2003.

Basic information (construction date, number of stories, gross floor area, adopted design code, engineer(s), Contractor(s), etc.)

Construction date: Original construction 1964, Refurbishment 2012
Number of stories: 2
Gross floor area: 665m2 per floor
Adopted design code: NTC/2018
Engineer(s): Prof. Eng. Paolo Foraboschi
Contractor(s): De Niro Srl

Reason for composite solution

Alternative solutions have been considered, and composite construction was adopted due to the adaptability of steel as a building material to provide structural reinforcement of early RC concrete structures. Concrete was added inside the steel columns to provide the maximum load-carrying capacity and it was then decided to connect it with shear studs to provide the columns with wide hysteresis loops. Steel beams were preferred to concrete beams for their simpler construction. To facilitate the use of smaller construction equipment the primary beams have been designed as thin-walled “U” shaped sections, and part of the positive moment resistance was assigned to the concrete infill.

Specific solution/technical details

Concrete filled columns: 200×200 square profiles, realized with cold-bended 10mm thick sheets, shear studs welded on the inside face before bending. Concrete poured inside with an additional pre-fabricated rebar system inserted into the hollow steel profile before the installation and the concrete pour to prevent concrete cracking due to the concrete-steel bond created between the profile and the pour.
Concrete filled primary beams: each beam was composed by three different segments. The end pieces connecting the beam to the composite columns described above and a central segment, covering most of the span. The beam vertical section was formed by a thin-plate U section that allowed the easy welding of L-shaped shear connections. The concrete pour included the use of additional horizontal rebars. Secondary beams (steel only) were bolted to the primary beams before the concrete was poured.

Impact or effectiveness

Alternative solutions for the structural retrofit of the school have been assessed and discarded due to multiple circumstances. The main objectives of the structural intervention described are 5: 1 add structural resistance to the pre-existing frame that was negatively affected by the low compressive strength of the concrete; 2 Maintain the aesthetic characteristics of the original building; 3 avoid a composite behavior of the new and the old structure, to prevent unpredictable and unwanted interactions between the two systems; 4 relate the lateral stiffness of the new frame to the one of the existing RC frame so as to provide the greatest dissipation capacity; 5 prevent hysteresis loops during the inelastic behavior of the building.

References / Technical Papers Content

Foraboschi P., 2016. “Versatility of steel in correcting construction deficiencies and in seismic retrofitting of RC buildings” J. of Building Engineering, 8 107-122
http://dx.doi.org/10.1016/j.jobe.2016.10.003