Application of the “Parsant” method in a masonry building John MARNERIS Structural Engineer M.Sc, Athens, Greece, john@marneris.gr Paper ID: 10279
A brief presentation of the PARSANT method This method is developed by me some years ago, in order to retrofit existing buildings, mainly made of concrete, in a “non destructive way”, acting on their perimeter, without damages and without interrupting their use. It is based on additional panels, consisting of strong steel composite frames, with diagonal members and independent foundations, which are placed on the perimeter of buildings, acting as shear walls. They are pin connected to their framing system, using an original detail with steel rods going through concrete and steel beams. Left, PARSANT panels– Middle, pin connection detail – Right, dampers can be included in the diagonals of the upper floors The panels can be covered with cement board or with decorative steel panels if it is architecturally required . Paper ID: 10279
Introduction The method, as presented before, is applied to reinforced concrete or steel structures, which are by nature flexible enough to “activate” the PARSANT panels, during an earthquake. This is not the case for masonry buildings, which show very small seismic horizontal deflections. To address this problem, we had to adapt PARSANT method, changing its philosophy, so that the panels instead of acting as shear walls, act as seismic energy absorbing elements. For that the following modifications had to be made: ▪ The diagonal members of the panels are replaced by metallic yielding dampers of BRUB type. ▪ Invented “gear devices”, named MHD (Magnifiers of Horizontal Displacements), are inserted between the building and the panels, which magnify the horizontal deformations of the building by 5 times, before they are applied to the panels. This way the BRUB dampers are activated, exceeding the yielding limit of the steel, being able to form hysteretic loops and absorb seismic energy of more than 20% Paper ID: 10279
Framing system of the existing building It’s about a nursery school, the design of which was assigned to our Company, in order to retrofit it under the condition to use light interventions, only at its outside perimeter. The building consists of a ground floor, a 1 st floor, and a small basement, covering a total area of about 250 m 2 , with a framing system made of masonry walls and reinforced concrete slabs and beams During the Athens earthquake of 1999 the building was damaged, not severely, with cracks in the masonry walls and was characterized as “habitable after repair”, by the authorities. The Municipality carried out the repairs, without following a proper structural design. Paper ID: 10279
PARSANT panels placed to the building Around the building 9 panels are placed (3 extending to two floors and 6 extending to one floor), containing 36 metal yielding seismic dampers. The columns and beams of the panels consist of hollow cross sections SHS 150*5 and the diagonal BRUB metal dampers consist of plates 100*10 mm of low strength. The plates are “enveloped” appropriately by hollow sections SHS 150*3 and cement grout to avoid buckling. At the upper part of the panels, corresponding to the floors of the building, the devices MHD (Magnifiers of Horizontal Deflections) are installed. Paper ID: 10279
MHD device This invented device consist of a system of connected circular gears, one of a diameter of 50mm and the other of 250mm. The small diameter gear moves between 2 horizontal linear gears, connected to a plate, which in turn is attached to the masonry wall. The large diameter gear moves between 2 horizontal linear gears, connected to a plate, which in turn is attached to the beam of the PARSANT panel. The connection of plate to the concrete frieze of the wall is implemented by anchors inserted to it. The plates, at each side of the gear system, are connected together through steel rods. As the masonry wall of the building moves horizontally in its direction by certain distance, this movement is transferred to the panel 5 times larger. Paper ID: 10279
MHD device A clearer view of the MHD device Paper ID: 10279
Conventional retrofitting Apart of the PARSANT panels, additional retrofitting of conventional type is applied only to the external faces of the building, like: ▪ Creation of a concrete frieze at the level of the panels connection. ▪ Plastering of external faces with high quality plaster including reinforcing mesh. The above do not increase considerably the building’s rigidity, and this benefits the application of this method. Paper ID: 10279
Linear dynamic analysis The building is analyzed by using the STAAD PRO program of Bentley. Walls and slabs are described as plate finite elements and the members of the PARSANT panels are described as linear steel elements located at a distance of 20mm from the building’s external walls. The connection between the building and the panels is implemented at the locations of the MHD devices and is described as a steel member of 20mm length with a diameter of 50mm, equal to the small diameter of the MHD gear system. The presence of the MHD devices, is described in the analysis by increasing the panels rigidity by 5 times. This is a reasonable assumption, since as the panels have to move a 5 times greater distance, they need 5 times higher horizontal forces. The increase of the panels’ rigidity is described simply by increasing 5 times their modulus of elasticity . Then we calculate the change of length of diagonals is calculated by a simple excel program, having as data the original and the final location of the diagonal members joints, after multiplying the analysis results by 5. If the strain is equal or greater than 0.11% the diagonals exceed the yielding point and enter the plastic region, being able to form hysteretic loops and hence absorb seismic energy. This is derived from the strain-stress relationship ε = Δ l/l = fy/E= 235.000/(2.1*10 8 ) = 0.11% . Paper ID: 10279
Linear dynamic analysis We evaluate the effective damping ( β eff ) of the structure, according to the provisions of the American Code FEMA 356, Chapter 9, in relation to “displacement-dependent” devices, as expressed by the following equation. β eff = β + [ Σ W j / (4* π *W k )] <30% where: β : damping equal to 0.05, W j : work done by devices W k : The maximum strain energy in the frame. ▪ The work done by each diagonal member, which enters the plastic region and hence works as damper, is derived from the following equation. Wj = 4 * d * As * fy where: d: the change of length, As: the area of the damper, fy: yielding stress, 4 stands for the movements of one seismic cycle i.e. elongation – back to original position – shortening – back to original position . Using a simple excel program we calculate the total work done by all devices Σ W jx = 91.3 KNm, Σ W jz = 104.3 KNm ▪ The maximum energy strain is provided directly as a result from the program analysis as W kx = 153.6 KNm, W kz = 137.1 KNm Paper ID: 10279
Linear dynamic analysis ▪ Applying values of Wi and Wk to equation we get the values of the effective damping in the two directions as: β eff,x = 0.098 and β eff,z = 0.110, where x represents the long dimension and z the short one. ▪ The damping modification factor of the spectrum to account for the energy dissipation is derived using the following equation of the Greek anti-seismic Code. n = [7/(2+ β eff )] 0.5 We get n x = 0.77 and n z = 0.73, which are greater than 0.70, which is the lower limit for the Greek Code. ▪ Taking into account these values we modify the seismic spectra of the structure, using the reduced damping modification factors. ▪ A final analysis is performed, using the new seismic spectra, leading to a reduction of the seismic forces of about 20%. Indeed the base shear in x direction is reduced from 862 KN to 704 KN (82%) and in z direction from 898 KN to 690 KN (77%). The horizontal deformations had a reduction of about 9% i.e. in x direction from 6.5 mm to 5.9 mm and in z direction from 7.1 mm to 6.5 mm. The fundamental periods of the building are 0.19 in x and 0.20 in z direction. Paper ID: 10279
Linear dynamic analysis The stresses are considerably reduced as it can be seen in the stress contours below. As a result the stresses do not exceed the permissible values, as it was the case of the building “as it is”. In Figure above max. tensile stress of the building “as it is” has a value of 0.473 MPa, which is greater than the allowable stress of 0.352 MPa, whereas after the placement of the panels the value is much smaller 0.24 MPa, well below the acceptable limits . Paper ID: 10279
Time history dynamic analysis A supplementary analysis was performed using time history dynamic procedure in order to get additional information in critical issues, like the seismic energy dissipation. The program ABAQUS is used. The walls and slabs are described as plate finite elements of elastic material with the same properties, used for the elastic dynamic analysis. A synthetic accelerogram is used, acting simultaneously in both directions, after it is being adapted according to the Code recommendations, so that its spectrum overlaps the Code’s one, as shown below. Left, the synthetic accelerogram – Right, adapted accelerogram’s spectrum (blue . line), overlapping the Codes one (red line).. Paper ID: 10279
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