Evi Visviki
MEng Civil and Structural Engineering
Civil Engineer at Seismosoft ltd.

Zoi Gronti
Civil Engineer, MSc

The current example employs a typical building built in the late 1980s. The building has 5 floors of approximately 180 m² each, and a soft storey at the ground level. The infills of the upper levels are relatively strong with good quality ceramic bricks and mortar of relatively high strength. The combination of the soft ground storey and strong infill panels in upper floors is the most important characteristic of the building and constitutes a serious structural problem related to its seismic behaviour.

The concrete grade is C12/15 the steel grade is S400 for longitudinal reinforcement and S220 for transverse reinforcement, and in general there is adequate longitudinal reinforcement (for instance a typical rectangular or square column has 4Φ20 of rebars). The shear reinforcement however is just Φ8/20 for all the beams and Φ8/25 for the columns.

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Figure 1: Front and back view of the building under consideration

For this example, checks will be carried out only in shear. This is a reasonable and realistic simplification, considering that in the vast majority of cases of such existing buildings, shear is the most critical check. The checks will be done according to EC8 for a single Limit State, Significant Damage SD, which is probably the performance level mostly used for assessment purposes. The building will be checked with the Nonlinear Static Procedure (NSP) of EC-8 and it will be subjected to pushover analyses with the 16 following combinations:

1. Uniform + X + eccY
2. Uniform + X – eccY
3. Uniform – X + eccY
4. Uniform – X – eccY
5. Uniform + Y + eccX
6. Uniform + Y – eccX
7. Uniform – Y + eccX
8. Uniform – Y – eccX

9. Modal + X + eccY
10. Modal + X – eccY
11. Modal – X + eccY
12. Modal – X – eccY
13. Modal + Y + eccX
14. Modal + Y – eccX
15. Modal – Y + eccX
16. 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 0.390sec and 0.429sec, respectively. The shape of the fundamental modes is characteristic of the weak storey, with very large deformations at the ground level and significantly smaller in upper floors (Figure 2).

Figure 2: Fundamental mode of the building in the Y direction (T=0.429 sec)

With pushover analysis the target displacement of the control node is: 3.93cm in the X-axis and 5.06cm in the Y-axis for the Life Safety limit state.

In Table 1 the maximum values of the demand-to-capacity (DCR) ratios for beams and columns are displayed. The maximum demand to capacity ratio DCR is 1.44 for the columns and 1.06 for the beams. 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, all the observed failures of the existing building are located at the ground level, confirming the fact that soft ground storeys are indeed an element of increased vulnerability.

Table 1: Code-based checks in shear for the existing building
(Significant Damage limit state)

Figure 3: Members failed in shear for the -X+eccY combination

The retrofit scheme is applied in all the ground storey columns, which were strengthened with 10 cm wide shotcrete jackets. The jackets’ concrete grade is C25/30, and the steel grade is B500c. The longitudinal reinforcement of a typical rectangular jacket is 4Φ20+8Φ16, and the shear reinforcement of the members is significantly increased in all columns with stirrups of Φ10/10. Figure 4 shows indicative jacket layouts, as modelled in SeismoBuild.

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Figure 4: Typical layout of the jacketed sections

Eigenvalue analysis indicates a significant increase in the stiffness of the building with a fundamental period of 0.322 sec, i.e. a considerable decrease of almost 25% with respect to the 0.429 sec of the initial building. What is more important is that the shape of the fundamental mode has changed with larger deformations at the upper stories, rather than at the ground floor (Figure 5).

Figure 5: Fundamental mode of the building strengthened with jackets (T=0.322 sec)

Looking specifically at the code-based checks in shear 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.

Table 2: Code-based checks of Shear Capacity of the Jacketed building

Note that, according to the analyses, 5 the DCRs of 5 beams exceed unity. In four of them the DCR is close to one, the exceedance by less than 5%, and this can be accepted without strengthening measures. In one beam the DCR is 1.26. This beam can be strengthened either with RC jackets or FRP fabrics.

The retrofit is carried out at the ground level: X-shaped steel braces are added at 6 different locations in the perimeter of the building, and, FRP wrapping is applied to all the ground storey columns adjacent to steel braces. FRP wrapping was considered necessary for these columns, in order to strengthen them in shear, due to the increased shear demand, cause by the loads transferred to them from the diagonal beams. For the braces 150×8 hollow rectangular sections are employed, which are pinned to steel plates that are fixed at the corners of each concrete bay with steel anchors and epoxy adhesives. For the FRP wrapping, 1 layer of a typical FRP fabric is employed (dry fibre thickness approx. 0.129 mm, area density of the carbon fibres approx. 235 g/m2 and dry fibre tensile strength approx. 3,200 N/mm2). The braces should be placed as symmetrically as possible to avoid unwanted torsional effects.

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Figure 6: Front and back view of the building strengthened with steel braces and FRPs

Eigenvalue analysis indicates a moderate increase in the stiffness of the building with a fundamental period of 0.350 sec, but now the shape of the fundamental mode has changed with uniformly distributed deformations along the height, rather than large deformation concentrations at the ground level. What is important is that the stiffness of the braces can be easily calibrated (i.e. changing the cross-sections of the braces), in order to avoid irregularities in elevation, when the strengthening is not applied to the entire building height, as in this case.

Figure 7: Fundamental mode of the building strengthened with steel braces and FRP Wrapping (T=0.350 sec)

With this new intervention there are no failures in the Significant Damage limit state, with DCRs significantly lower than 1.0 for both columns and beams.

Table 3: Code-based checks of Shear Capacity of the building with steel braces and FRPs

With both methods, RC jackets in the columns or Steel Braces and FRPs, 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.

NOTE:

The SeismoBuild input file for the analyses 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 can be downloaded from here.

Keywords: seismic retrofit, soft storey, structural strengthening, reinforced concrete jackets, FRP fabrics, FRP wrapping, steel braces, pilotis, structural assessment, existing buildings, infills.

• Structural Assessment, Strengthening & Retrofitting carried out using SeismoSoft Earthquake Engineering Software.