rehabilitation of buildings-tec 2019

Rehabilitation of Buildings-TEC 2019

• The operations required to overcome the problems determined by an assessment procedure and improve the seismic behavior of the structural system (such that a better response is obtained as a result of re-assessment of the rehabilitated structure):

▫ Strengthening of the existing members for a certain type of behavior (axial, shear, flexural, etc.)

▫ Strengthening of the entire structure (i.e. addition of new structural members)

▫ Reducing the mass of the structure

• Demand increase caused by the increased lateral rigidity should be considered by providing sufficient increase in strength (global or local).

• The strengthening of one member or against one type of response should not trigger a different type of failure (weakness). For instance, the application of diagonal CFRP sheets on the existing infill walls may trigger a base rocking response in case of insufficient lapped column reinforcements (plain bars) or shear failure at the columns ends and beam-column joints. In those cases, the weakness of the structure should be recovered by a proper rehabilitation in different parts (e.g. careful welding of lapped column bars or shear strengthening of column ends and beam column joints).

Assoc.Prof.Dr. Emre Akın


• Types of rehabilitation:▫ Member strengthening: applied in order to improve the strength and deformation capacity of the members

Confinement of the columns: to improve the ductility, axial strength and shear strength Confinement provided by reinforced concrete layer Confinement provided by steel sections (braces) Confinement provided by fiber-reinforced polymers (FRP): Carbon (CFRP), Glass (GFRP), Aramid (AFRP), Basalt

(BFRP) Improving the flexural response of the columns (increasing the sectional dimensions of the reinforced concrete

columns) Confinement of the beams: to improve the ductility and shear capacity of the beams

By addition of external strirrups Wrapping by FRP’s

▫ System rehabilitation: Addition of shear walls Addition of new RC frames Strengthening of existing infill walls and



• Confinement of the columns:

Confinement provided by reinforced concrete layer: The concrete cover should be removed or stratched. Both longitudinal and lateral reinforcements (stirrups) should be placed around the existing columns (starting from the top of the lower slab level to the bottom of the upper slab). The thickness of the confining reinforced concrete section should be at least 100 mm. The rules for the details of the stirrups along the mid-length of the highly ductile columns (section 7.3.4.2) is valid all along the length of confinement. The axial (and shear) strength of the strengthened new member is 0.90 times the summation of axial (and shear) strength of existing (old) section and confining layer.




Confinement provided by steel sections (braces): The steel sections are placed at each corner of the column which are then connected (via welding) by lateral steel plates with a certain spacing. The steel sections at the four corners should be in full contact with the concrete surface. The lateral steel plates should be continous all over the column height. The steel confinement should be in contact with the slabs (at the bottom and top) such as to provide axial load transfer (this is required to increase the axial and flexural capacities of the column). The additional shear strength provided by the steel confinement:


tj, b and s: the thickness, width and spacing of lateral steel platesd: effecttive depth of RC column sectionfyw: yield strength of steel plates



Confinement provided by FRP’s:

FRP: composite materials made of polymer matrix reinforced with fibers (carbon, glass, aramid or basalt fibers). These materials are applied by using wet-lay-up process. In this process, the concrete surface is cleaned from dust (by using air compressor and application of a primer layer). Then putty material is applied for rough surfaces to obtain a smooth surface and provide a better bond to the concrete. Epoxy is applied on the concrete (or putty) surface by using a roller at the final stage. Then the epoxy impregnated FRP fabrics (tailored in pre-determined sizes) are wrapped on the column. The hardening of the epoxy during this application stage should be avoided. Thehardened epoxy constitutes the organic matrix which provide the stress transfer from the concrete to the fibers of FRP.


Wet Lay-up Process

Application of «putty» over the surface where «primer» was applied in the previous stage.

Epoxy application on the surface.

Epoxy impregnation for the FRP fabrics.

Tailored FRP fabrics (in pre-determined sizes).

Placing the epoxy impregnated FRP fabrics on the surface

Inserting anchor dowels (in case of strengthening of infill walls)

FRP Materials

• Advantages of FRP materials:▫ high tensile strength per weight,▫ corrosion resistance,▫ superior mechanical characteristics,▫ ease of application,▫ provides flexibility in design and application (can be formed into any geometry),▫ organic epoxy matrix is very successful in the stress transfer from the concrete surface to the fibers.

• Disadvantages:▫ high cost (especially for CFRP),▫ inconsistent mechanical properties under high temperatures,▫ susceptibility to fire, ▫ cannot be applied onto the wet surfaces,▫ impermeability of moisture,▫ irreversible,▫ include some toxic materials (e.g. furan).

FRP Confinement-Experimental Studies

Axial Compression Test Setup:

• Linear Variable Displacement Transducers (LVDT’s): to measure the axial deformations (shorthening of concrete)

• Strain Gauges: to measure the lateral (hoop) strain on the FRP jacket (sometimes also used for measuring the axial strains)

• Load Cell: to measure the applied axial compressive loads (required if load data cannot be obtained from the test system)

• Data Acquisition System: to collect the measured data (converts voltage output from the measuring devices into convenient form)

P (axial compressive load)

P


• The type of loading:

▫ Monotonic: the axial compressive loads are applied from zero up to failure (rupture of FRP jacket) continuously.

▫ Cyclic: the axial compressive loads are applied from zero up to a certain defined strain level and then the specimen is unloaded at that strain level on the envelope curve (εun,env). Generally, a small amount of load is maintained on the specimen in order to prevent any dislocation of member during the test. If the elastic limit is exceeded, there occurs a certain residual strain (plastic strain, εpl). Then, the specimen is re-loaded from that plastic strain up to a new point on the envelope curve. These load cycles are repeated with previously determined strain intervals (e.g. 0.2% strain intervals).

εun,env=0.044

εpl=0.038


• The parameters which affect the performance of FRP confinement:

▫ Type of FRP

▫ Amount of confinement (number of FRP layers)

▫ Unconfined concrete compressive strength (fco)

▫ Geometry of the confined concrete section (circular, rectangular, square)

▫ The inclination of the fibers (generally fibers are oriented along the lateral direction)

▫ The type of applied loading (monotonic or cyclic)

▫ The length of overlapping zones of FRP sheets (the diameter, D of the circular sections is adequate)

▫ Wrapping or tube confinement


2-Layer of CFRP confinement

1-Layer of CFRP confinement

Unconfined Concrete

Mechanics of Confinement

Up to approximately «0.7~0.9fco»: A volume reduction of concrete takes place, deformations are limited and the behavior is generally no different than the axial response of unconfined concrete.

After approximately «0.7~0.9fco»: The concrete starts to expand (volume expansion) under increasing axial stresses due to poisson’s effect. The expansion of concrete is restrained by the confinement. In case of FRP confinement, the FRP jacket responds back to the expanding concrete and applies a reaction (equal) stresses on the concrete core. This reaction continues linearly up to the rupture of FRP. In case of steel confinement, this reaction of the steel ends after yielding.


The difference between these three cases is caused by the confinement ratio. When the confinement ratio is larger than a threshold value, an improvement in the axial strength can be provided (efficient confinement). This threshold confinement ratio is a function of the unconfined concrete strength (fco). As the fco increases, the number of FRP layers required to provide the same amount of confinement efficiency increases.

εcc

1. fcu<fcc no improvement in axial strength 2. fcu=fcc 3. improved axial strength

Mechanics of Confinement𝑓𝑙 × 𝐷 = 2 × 𝑓𝐹𝑅𝑃 × 𝑡

𝑓𝑙 =2 × 𝑓𝐹𝑅𝑃 × 𝑡

𝐷

𝑓𝐹𝑅𝑃 = 𝐸𝐹𝑅𝑃 × 𝜀ℎ,𝑟𝑢𝑝𝑡

……(1)

……(2)

……(3)

sr

t

D (diameter)

𝜎ℎ 𝜎ℎ

FRP

Concrete

sr : lateral pressure on the concrete (produced by confinement)sh : hoop strain on the confining FRP jacketfl : ultimate confining pressure on the concrete fFRP : ultimate hoop strain

eh,rupt: hoop rupture strain of FRP jacketEFRP : modulus of elasiticity of FRP

𝑓𝑙𝑓𝑐𝑜

: Confinement Ratio

𝑓𝑐𝑐𝑓𝑐𝑜

= 1 + 𝑘1𝑓𝑙𝑓𝑐𝑜

Richart ve diğ. (1928),

[k1=4.1 : aktif sargılanmış betonda]

Note: A uniform distribution of sr and sh

is assumed.


• The rupture strain of FRP jacket (εh,rupt) is generally smaller than both the rupture strain of FRP material provided by the manufacturer and the rupture strain determined by tensile coupon tests of FRP samples (εfu) . Therefore, we may say that FRP may not be utilized efficiently up to its ultimate strain when used as a confining material. There may be various reasons of this; one of which may be caused by the negative effect of non-uniform deformations of concrete.

A strain-reduction factor is generally used for FRP jacket: 𝑘𝜀 =𝜀ℎ,𝑟𝑢𝑝

𝜀𝑓𝑢


• There are many models that predicts the axial behavior of FRP confined concrete (overall response or only the ultimate strength and strain):

Turkish Earthquake Code (2019):

Rehabilitation of Buildings-TEC 2019• Improving the flexural response of the columns : The column sections may be enlarged for this purpose. An additional RC layer should be formed around the column. The longitudinal bars of this additional layer should be continuous along the stories (holes should be drilled through the slabs). The lateral reinforcement should also be provided at the beam-column joints by either drilling holes through the beams or lateral reinforcing bars should be anchored into the beams. The total cross-sectional properties should be considered by multiplying with «0.9» during the calculation of strength and rigidities.



• Confinement of the beams: Aim is to increase the shear capacity and ductility (in certain cases) of the beams

Confinement provided by reinforced concrete layer: The lateral reinforcing bars are added at both faces of the beam and connected to a bottom steel plate by means of bolts. The upper ends of the lateral bars should be anchored into the concrete of the upper slab as shown below. The total amount of lateral reinforcement (existing and added) is considered while calculating the shear capacity of the confined beam. It is assumed that no additional ductility is provided.



• Confinement of the beams:

Confinement provided by FRP’s: The wrapping of FRP around the beam should be applied all along the circumference. If FRP sheets are applied in a discontinuous manner with a width of wf, the spacing between seperate FRP sheets should not exceed «wf+d/4» (where d: effective depth of the beam). The minimum overlapping length of FRP sheets should be 200 mm. The corners of beam should be rounded by a radius of 30 mm.


rehabilitation of buildings-tec 2019

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