Dr. Stelios Antoniou
Managing Director of Seismosoft ltd.
Director of the Repair and Strengthening Section of Alfakat SA
MEng Civil and Structural Engineering
Civil Engineer at Seismosoft ltd.
The current example employs a typical industrial building built in the late 1980s. The building has 2 floors of approximately 880m2 each. The infills are relatively strong with good quality ceramic bricks and mortar of relatively high strength. The combination of strong infill panels and short columns is the most important characteristic of the building and constitutes a serious structural problem related to its seismic behaviour.
The concrete grade is C16/20, the steel grade is S400 for longitudinal bars and S220 for transverse bars, and in general there is adequate longitudinal reinforcement (for instance a typical rectangular or square column has 8Φ20 of rebars). The shear reinforcement however is just Φ8/15 for the beams and Φ8/30 for the columns.
Figure 1: Front and back view of the building under consideration
For this example, the checks will be carried out in shear and plastic hinge rotation. It is noted however that the results will be mainly displayed for the shear checks, since in existing reinforced concrete buildings shear is usually the most critical check. The checks will be done according to ASCE 41-17 for the Performance Objectives of Life Safety (3-C) and Collapse Prevention (5-D), which is probably the performance levels mostly used for assessment purposes. The building will be checked with the Nonlinear Static Procedure (NSP) of ASCE-41 and it will be subjected to pushover analyses with the following combinations
- Modal + X + eccY
- Modal + X – eccY
- Modal – X + eccY
- Modal – X – eccY
- Modal + Y + eccX
- Modal + Y – eccX
- Modal – Y + eccX
- Modal – Y – eccX
All the analyses and code-based checks have been carried out with SeismoBuild v2023 Release-1.
With eigenvalue analysis the fundamental periods in the X and Y directions are equal to 0.361 and 0.335sec, respectively. The shape of the fundamental modes is characteristic of the short columns, with very large deformations at the short columns and much smaller in the columns with infill (Figure 2).
Figure 2: Fundamental mode of the building in the X direction (T=0.361 sec)
With pushover analysis the target displacement of the control node is:
- 3.16cm in the X-axis and 2.29cm in the Y-axis for the Life Safety
- 3.76cm in the X-axis and 2.86cm in the Y-axis for Collapse Prevention
In Table 1 the maximum values of the demand-to-capacity (DCR) ratios for beams and columns are displayed for both selected performance objectives.
Table 1: Code-based checks of Shear Capacity of the existing building
The value of the DCR is an indication of whether or not the member can sustain the imposed demand; with DCR>1 structural failure occurs, and with DCR<1 the member is safe. As expected, almost all the observed failures of the existing building are located at the short columns, confirming the fact that lightly reinforced short columns are indeed an element of increased vulnerability in existing buildings.
Figure 3: Members failed in shear for Life Safety
Figure 4: Members failed in shear Collapse Prevention
STRENGTHENING WITH JACKETING
The retrofit scheme is applied in all the columns of the building, which are strengthened with 10 cm wide shotcrete jackets in their entire height. The jackets’ concrete grade is C25/30, and the steel grade is B500c. The longitudinal reinforcement of a typical rectangular jacket is 4Φ20+4Φ16, and the shear reinforcement of the members is significantly increased with stirrups of Φ10/10. Figure 5 shows an indicative jacket layout, as modelled in SeismoBuild.
Figure 5: Typical layout of the jacketed sections
Eigenvalue analysis indicates a significant increase in the stiffness of the building with a fundamental period for X-axis of 0.231 sec, i.e., a considerable decrease of almost 36% with respect to the 0.361 sec of the initial building (Figure 6).
Figure 6: Fundamental mode of the building strengthened with jackets (T=0.231 sec)
Looking specifically at the shear code-based checks in Table 2, columns do no longer fail in shear, and in none of the columns is the demand larger than the capacity.
The building has become considerably safer. From the Pushover analysis (Figure 7) we now observe a significant increase in the building capacity and smaller target displacements.
Figure 7: Target displacement for Pushover Analysis: modal +X+eccY, for the initial and strengthened building
STRENGTHENING WITH NEW RC WALLS
The retrofit is carried out with five new external reinforced concrete walls at the perimeter of the building, which extend to the full height of the building. The reinforcement of the walls is similar to what would be found in shear walls in a new construction, i.e. with pseudo-columns with a typical reinforcement of 16Φ20 with Φ10/20 stirrups. The walls are placed in symmetrical positions in the perimeter, in order not to introduce undesired torsional effects, and they are connected with the existing beams and the columns through a large number of dowels that are designed to transfer the seismic inertia forces from the weak building to the strong walls.
The proposed strengthening scheme is shown in Figure 8, and it can be very beneficial in the cases when the structural retrofit is not accompanied by a radical architectural renovation. This is because the strengthening is carried out mainly with interventions in the external side of the walls in the perimeter of the building, which cause very limited disruption to the operation of the building.
Figure 8: Front and back view of the building strengthened with new RC walls
Figure 9: Typical configuration of new RC walls
Eigenvalue analysis indicates that the strengthened building is significantly stiffer with respect to the initial building with a fundamental period of 0.184 sec (an impressive decrease of almost 50%).
Figure 10: Fundamental mode of the building strengthened with new RC walls in the X-direction and Y-direction
Pushover analysis shows an improved performance (Figure 11 & 12). However, due to the existence of the entrance, it was not possible to construct walls at the center frame of the front view of the building. As a result, larger deformations are concentrated at the short columns on the right and on the left side of the entrance, causing failure of the short columns with DCRs up to 1.15 for Life Safety and 1.25 for Collapse Prevention (Figure 11 and Figure 12).
Figure 11: Members failed in shear for the Life Safety performance objective
Figure 12: Members failed in shear for the Collapse Prevention performance objective
A better strengthening solution could be with the use of two additional external shear walls at the two sides of the entrance (Figure 13).
Figure 13: Front view of the building strengthened with new RC walls
Looking at the eigenvalue analysis results, there is a significant increase in the stiffness of the building with a fundamental period in the X-axis of 0.130 sec, i.e., a considerable decrease of almost 64% with respect to the 0.361 sec of the initial building (Figure 14).
Figure 14: Fundamental mode of the building strengthened with two more shear walls (T=0.130 sec)
With this new intervention there are no failures in the Life Safety and Collapse Prevention performance objectives with DCRs significantly lower than 1.0.
An interesting observation, which explains the good performance of the strengthened building, is that the new walls near the entrance absorb a large part of the base shear and the storey shear forces (up to 85-90%). This example shows us that, when large shear walls are added in an existing building, it is very common that no other intervention is required in the other structural members, provided of course that these are in relatively good condition, and possess some, non-negligible, existing reinforcement and lateral strength.
Figure 15: No Members fail in shear
With both methods, RC jackets in the columns or new shear walls, the seismic performance of the building has improved significantly. The demand-to-capacity ratios (DCRs) are much lower after the strengthening interventions, especially for the more critical vertical members, and the vulnerability of the structure has decreased considerably.
The SeismoBuild input files for the analyses of this project can be downloaded from this link. All the analyses and code-based checks have been carried out with SeismoBuild v2023 Release-1. A SeismoBuild trial version, with which all the analyses can be run, can be downloaded from here.
Keywords: seismic retrofit, short column, structural strengthening, reinforced concrete jackets, structural assessment, existing buildings, industrial building