Application of the Parsant method in a masonry building John - - PowerPoint PPT Presentation

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

Paper ID: 10279

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

• r with decorative steel panels if it is

architecturally required.

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

• f 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

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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 1st floor, and a small basement, covering a total area of about 250 m2, 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.

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

• f low strength.

The plates are “enveloped” appropriately by hollow sections SHS 150*3 and cement grout to avoid buckling. At the upper part

• f

the panels, corresponding to the floors

• f

the building, the devices MHD (Magnifiers

• f

Horizontal Deflections) are installed.

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MHD device

This invented device consist

• f

a system of connected circular gears,

• ne of a diameter of 50mm and the
• ther 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

• f

plate to the concrete frieze

• f

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

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A clearer view of the MHD device

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.

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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= Δl/l = fy/E= 235.000/(2.1*108) = 0.11%.

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Linear dynamic analysis

We evaluate the effective damping (βeff) 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. βeff = β + [ΣWj / (4*π*Wk)] <30% where: β: damping equal to 0.05, Wj: work done by devices Wk: 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 ΣWjx = 91.3 KNm, ΣWjz = 104.3 KNm ▪The maximum energy strain is provided directly as a result from the program analysis as Wkx = 153.6 KNm, Wkz= 137.1 KNm

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Linear dynamic analysis

▪Applying values of Wi and Wk to equation we get the values of the effective damping in the two directions as: βeff,x = 0.098 and βeff,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+βeff)]0.5 We get nx = 0.77 and nz = 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.

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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”.

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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

• f the panels the value is much smaller 0.24 MPa, well below the acceptable limits

.

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

• verlaps the Code’s one, as shown below.

Left, the synthetic accelerogram – Right, adapted accelerogram’s spectrum (blue line), overlapping the Codes one (red line)..

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.

Time history dynamic analysis

The connection between the building’s walls and the PARSANT panels is achieved by using a special command feature of the program, according to which the connected joints of the panels will have a horizontal deflection in their direction, 5 times larger than the corresponding joints of the walls. This is appears very clearly in the shape of the deflected structure as shown below.

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.

Time history dynamic analysis

The results are more favorable in comparison to the dynamic elastic method. Tensile stresses are about 50% smaller than the ones before the application of the PARSANT panels, and likewise the horizontal displacements are about 30% smaller. Left, hysteretic curve of a damper – Right, the upper red line represents the total seismic energy transferred to the building, the blue line just below represents the plastic energy dissipated by the dampers, the line at the bottom represents the remaining energy to the system.

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Conclusions

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. The PARSANT method is applied for the first time to the outside perimeter of a masonry building, despite the small seismic horizontal deflections of this kind of structures. For this purpose the previously mentioned modifications had to be made, i.e. use

• f metal yielding dampers as diagonals, and installation of MHD (Magnifier of

Horizontal Displacements) devices The results are very satisfactory and there was a seismic energy absorption of more than 20%, which reduced proportionally the seismic forces, and this together with some minor conventional retrofitting of the outside faces of the walls, lead to reduced stresses, within the Code’s limits. The method can be applied to any kind of masonry buildings provided that they are not critically damaged. In historical buildings the panels can be placed at the internal side of the walls. The MHD devices alone, can have a much wider application to any kind of seismic dampers applications.