How to Strengthen a Reinforced Concrete Building
This article describes how to use StatiCAD’s strengthening module for TBDY 2018 Section 15.10, Annex 15B and Annex 15C. The module is built on top of the seismic performance assessment chain for existing buildings: strengthening is a property of the element, and a single main option decides whether it is taken into account. For the theoretical background see Theoretical Basis of Reinforced Concrete Strengthening, and for the verification studies see Reinforced Concrete Strengthening Verification Studies.
Which editions? Reinforced concrete performance assessment and strengthening are available in the Ultimate, Profesyonel and Betonarme editions. In other editions the strengthening data in the project is preserved but is not shown in the interface and is not included in the calculation.
1. General workflow: before and after in the same file
- Model the existing building; in Project General Settings → Code Selection → Performance Ch.15 group, turn on the reinforced concrete performance assessment, run the analysis and produce the report. This is the before strengthening state.
- In the Properties window of the elements to be strengthened (right-click → Properties), enter the method and its parameters in the Strengthening tab (see Section 2). This information is saved with the project.
- In the Project General Settings → Performance Ch.15 group, tick the “Include strengthening in calculation” box and run the analysis again. The whole chain (stiffness, capacity, damage limits, building decision, report) now calculates the strengthened state.
- If you wish, use the “Add before/after strengthening comparison to the report” box to place a summary of the two states side by side on the first page of the report (see Section 4).
While the box is cleared, the program calculates the building in its before-strengthening state; the results are exactly identical to those of a building with no strengthening data entered.
2. The Strengthening tab
The tab is found in the column (Tie Column Properties), beam (Bond Beam Properties) and wall/shear wall (Wall Properties) windows. The three options at the top prevent accidental application in a multiple selection:
| Option | Meaning |
|---|---|
| None (appears only in multiple selection) | When OK is pressed, the strengthening setting of the selected elements is left untouched. It is the default in multiple selection. |
| Do not apply | The strengthening flag is removed from the selected elements. It is the default for a single element. |
| Apply | The method and parameters selected below are written to the selected elements. |
Changes are saved only with OK; Cancel reverts everything. The summary box below the tab instantly shows the code quantities for the entered values (confinement effectiveness factor, FRP ratio, lateral pressure, confined strength and strain, shear contributions, anchorage capacity, etc.) and writes the applicability warnings.
2.1 Column strengthening
| Method | Code | Inputs | Effect on the calculation |
|---|---|---|---|
| Fibre-reinforced polymer (FRP) wrapping | 15.10.1.3 / Annex 15B | Number of layers and thickness, modulus of elasticity, rupture strain, corner radius, continuous or strip wrapping (strip width and spacing); optionally the existing lap-splice length | Confined concrete model, damage limits (GÖ = εcc, KH = 0.75εcc, SH = 0.004) and shear contribution Vf. Flexural capacity does not increase. If the lap-splice length is entered, the required FRP thickness is calculated per Annex 15B.4 and compared with the provided one; the result is added to the strengthening label as “bind OK / YETERSİZ / UYGULANAMAZ” (OK / insufficient / not applicable). |
| Section enlargement (concrete jacketing) | 15.10.2 | Jacket thickness (≥ 10 cm), jacket concrete grade, number of faces (1–4); jacket longitudinal reinforcement (number, diameter, cover); jacket stirrups (diameter, spacing); calculation model: Composite (default), Code letter or Jacket only | Stiffness and strength from the jacketed section, with a 0.9 factor. The jacket longitudinal reinforcement contributes to flexural capacity; the jacket stirrups are added to the shear strength. For 1-, 2- and 3-face jackets the section is calculated with its actual one-sided geometry and the smaller of the ± capacities is taken. |
| RC jacketing (shear) | 15.10.1.1 | Thickness, concrete grade, number of faces | Shear and axial only: shear strength from the gross jacketed section, with a 0.9 factor. Flexure and stiffness do not change. |
| Steel jacketing | 15.10.1.2, Eq. 15.3 | Plate thickness, width, spacing, yield strength | Vj is added to the shear strength. Ductility and flexure do not change. |
Continuity of the jacket between storeys and the stirrup requirements are the user’s responsibility; they are reminded in the summary box.
2.2 Beam strengthening
| Method | Code | Inputs | Effect on the calculation |
|---|---|---|---|
| FRP wrapping | 15.10.3.2 / Annex 15B | FRP parameters, strip width and spacing | Confined model and Vf. In strip wrapping, if the strip spacing condition (sf ≤ wf + d/4) is not satisfied, the shear contribution is not taken and a warning is given. |
| External stirrups | 15.10.3.1 | Diameter, spacing, number of legs, yield strength | Only the TS 500 stirrup contribution is added to the shear strength; there is no confinement effect and the ductility limits do not change. |
2.3 Shear wall and infill wall strengthening
| Method | Element | Inputs | Effect on the calculation |
|---|---|---|---|
| FRP boundary region wrapping | RC shear wall | FRP parameters | Confined model of the boundary region and Vf. |
| New strengthening shear wall | RC shear wall (15.10.5) | Wall concrete and reinforcement grade, within frame / adjacent, anchor diameter (≥ 16 mm), spacing (≤ 400 mm), depth (≥ 10Ø), friction coefficient, number of rows | While the option is off, the wall is not in the model (before strengthening). When on, it is treated as a new element (knowledge level factor 1); the anchorage check, a boundary region note and a foundation warning are given. |
| Steel-mesh reinforced plaster | Infill wall (Annex 15C) | Wall material (from the code table or custom values), plaster thickness (≥ 30 mm) and number of faces, mesh reinforcement ratio, opening ratio | Equivalent diagonal struts in the model; shear capacity; performance by shear angle (TBDY Table 15.2). |
| FRP diagonal strip | Infill wall (Annex 15C) | Wall material and FRP parameters, strip width | Compression and tension struts. |
In the wall window the method options become active according to the element type: FRP and new shear wall for an RC shear wall, the Annex 15C methods for an infill wall. A new strengthening shear wall is drawn in the model as an ordinary RC shear wall; it is marked as “new” only from this tab.
3. Reading the results
- Report: The column and beam section and confinement tables contain a “Strengthening” column (for example “LP nf=2 tf=0.17”, “Manto t=10”, “Celik sargi”, “Yeni perde ank 1Ø16/200 r=0.91 OK”, “Dolgu hasir siva”). If the main option is off, “(kapali)” is appended to the label.
- New shear wall: While the option is off, its row is excluded from the assessment with the note “öncesi modelinde yok” (not in the before model); when on, the anchorage ratio and the boundary column note are in the label.
- Strengthened infill wall: It is counted as a vertical element; the damage region is determined by the shear angle (TBDY Table 15.2: SH 0.003 / KH 0.005 / GÖ 0.010). The horizontal component of the diagonal strut forces is compared with the shear capacity; if it is exceeded, the element is considered brittle.
4. Before / after comparison
If the “Add before/after strengthening comparison to the report” box in the Performance Ch.15 group is ticked, a snapshot is saved according to the state of the main option when the report is prepared. If the records of both states exist, the following block is printed on the first page of the report: building performance level X/Y (before → after), 15.8 criteria percentages, the list of strengthened elements, the damage distribution and the elements whose damage region changed.
Correct order:
- Main option off → analysis → report (before record)
- Main option on → analysis → report (after and comparison)
The two runs can be done one after the other in the same session.
5. Design reports in performance mode
When linear performance analysis is selected (Ra = 1), the column, beam and shear wall design reports are produced with unreduced earthquake forces: “(Ra=1 - bilgi amaçlıdır)” (for information only) is added to the section headings, and the lines of error type are shown as “Bilgi (Ra=1)” (Information). In this mode, capacity exceedance messages are not design errors; the assessment is made according to the Chapter 15 performance report.
6. Where do code conditions appear?
Strengthening inputs that do not satisfy a code condition drop into the Project Error Checks list after the analysis as a “Hata: TBDY2018 M.…” (Error) or “Uyarı: TBDY2018 …” (Warning) line. The main conditions checked are:
- In FRP, corner radius < 30 mm and section aspect ratio > 2.5 (confinement increase is not applied),
- In lap-splice strengthening, section ratio > 2, plain-surface reinforcement or insufficient FRP thickness,
- Wrapping (jacketing) and jacket thickness < 10 cm,
- Diameter, spacing, depth and shear capacity in the new shear wall anchorage,
- In infill walls, plaster thickness < 30 mm, opening ratio > 10%, diagonal/thickness ≥ 30, length/height ratio outside 0.5–2 in FRP, the storey limit in 15.10.4,
- Shear demand exceeding capacity (brittle behaviour).
Rule violations do not stop the calculation; the decision is left to the engineer.
7. Frequently asked questions
I entered strengthening but the result did not change. Is “Include strengthening in calculation” ticked in Project General Settings, and did you re-run the analysis? In FRP, for a column whose aspect ratio exceeds 2.5 or whose corner radius is smaller than 30 mm, the confinement increase is not applied as required by the code (a warning appears in the summary box).
Is the mass of the new shear wall also counted in the before-strengthening model? No. While the option is off, the self-weight of the wall and the additional loads defined on it are zeroed; storey weights, the center of mass and the finite element masses are calculated without the wall. The wall is also not counted as a support for the slab (the slab load is distributed to the remaining supports). The “before” building has no shear wall.
In an infill wall I see the note “EK 15C çubuğu oluşmadı” (Annex 15C strut was not formed). No existing column could be found at the two ends of the wall, or the material inputs are missing. The diagonal struts are set up between the nodes of the end columns of the wall.
Does strengthening also apply in pushover (nonlinear) analysis? Yes. The stiffness and section changes of FRP wrapping, jacketing, the new shear wall and the infill struts are reflected in the pushover analysis as well. Additional shear checks such as steel jacketing, external stirrups and anchorage are reported in the linear assessment.