Stathis N. Bousias

Seismic Retrofitting of Columns with Lap Spliced Smooth Bars Through FRP or Concrete Jackets

The effectiveness of RC jacketing or FRP wrapping for seismic retrofitting of rectangular columns having smooth (plain) bars with 180° hooks lap-spliced at floor level is experimentally investigated. The relatively low deformation capacity and energy dissipation of five unretrofitted columns is found not to depend on lap length, if lapping is not less than 15 bar-diameters. Six columns cyclically tested up to ultimate deformation after RC concrete jacketing demonstrate force and deformation capacity and energy dissipation sufficient for earthquake resistance, regardless of the presence or length of lap splicing in the original column. Another ten columns cyclically tested to ultimate deformation after wrapping of the plastic hinge region with CFRP show that FRP wrapping of the splice region is more effective than concrete jackets for enhancement of the deformation and energy dissipation capacity of old-type columns with smooth bars lap-spliced at floor level, provided that wrapping extends over the member length sufficiently to preclude plastic hinging and early member failure outside the FRP-wrapped length of the column.

Fiber-Reinforced Polymer Retrofitting of Rectangular Reinforced Concrete Columns with or without Corrosion

20 concrete columns, with a 250 x 500 mm section and materials and detailing emulating older construction, were tested to study, in a systematic way, the effect of important parameters of seismic retrofit with fiber-reinforced polymer (FRP) wraps, as well as effects of reinforcing bar corrosion on the effectiveness of the retrofitting. As far as the number of FRP layers and fiber material is concerned, it is concluded that replacing carbon fibers by glass fibers, while maintaining the same extensional stiffness of the FRP jacket in the circumferential direction, leads to about the same performance. Nonetheless, FRP extensional stiffness seems to be the controlling factor up to a certain limit, as increasing the number of carbon FRP (CFRP) layers from 2 to 5 does not materially improve performance. Prior damage left unrepaired reduces effectiveness of rehabilitation with FRP wrap. Confinement by the FRP is very effective in increasing concrete strain capacity to levels of 5-6% even in the middle of a wide side of the column. Even so, rectangular columns tested in the strong direction (with a 250 mm-wide compression zone) are found to benefit more from FRP wrapping than when tested in their weak direction (w/ a 500 mm-wide compression zone). Although wrapping with FRP is found to greatly improve seismic performance of columns that suffer from both lack of seismic detailing and reinforcement corrosion, such corrosion materially reduces the effectiveness of FRP wraps as a strengthening measure, as the corroded bars become the weak link of the column, instead of the confined compression zone.

Seismic response and design of RC structures with plan-eccentric masonry infills

The bidirectional response of a two-storey RC frame structure with two adjacent sides infilled is studied through shaking table tests and non-linear dynamic analyses. The pre-cracking stiffness of the infills is large enough to impose twisting of the infilled structure about the common corner of the two infilled sides, with predominant period close to that of translation of the symmetric bare structure in the two horizontal directions. Parametric analyses and test results show that the peak displacement components of the corner column of the two open sides are about the same as (or slightly less than) those of the bare structure under the same bidirectional excitation, but take place simultaneously. This simultaneity of peak local demands from the two components of the motion seems to be the only effect of plan-eccentric infilling that needs to be taken into account in the design of the RC structure. Despite their very high slenderness (height-to-thickness ratio of about 30), infill panels survive out-of-plane peak accelerations of 0.6g at the base of the structure or 1.3-1.75g at their centre.

Strength, Stiffness, and Cyclic Deformation Capacity of Concrete Jacketed Members

This paper highlights an experimental program on rectangular reinforced concrete-jacketed columns, focused on the impact of measures taken to enhance the shear transfer at the interface of the old and new concrete. Columns with plain vertical bars and no detailing were rehabilitated for earthquake resistance with shotcrete jackets connected to the old column through various means and subjected to cyclic uniaxial lateral loading up to ultimate conditions. The effect of different means of connection at the interface on the effectiveness of the jacket was investigated. These test results are supplemented with data from the literature for different means of connecting the jacket to the old column to develop simple rules and expressions for the calculation of the yield moment, the drift at yielding, the secant-to-yield stiffness, and the ultimate drift in cyclic loading of jacketed columns with or without lap splicing at the base of the old column. These rules give the key properties of jacketed columns as ratios of the corresponding quantities of an equivalent monolithic member. These ratios do not systematically depend on the type of measures taken to enhance the shear transfer at the interface of the old and new concrete.

MODELLING OF RC MEMBERS UNDER CYCLIC BIAXIAL FLEXURE AND AXIAL FORCE

A model is proposed for the incremental force-deformation behaviour of reinforced concrete sections and members, under generalised load or deformation histories in 3D, including cyclic loading, up to ultimate deformation. At the section level the model is of the Bounding Surface type and accounts for the coupling between the two directions of bending and between them and the axial direction. For the construction of the member tangent flexibility matrix on the basis of the section tangent flexibility matrix, a piecewise-linear variation along the member is assumed for the nine terms of the tangent section flexibility matrix. Model parameters are derived on the basis of available test results for: (a) the force-deformation response under cyclic biaxial bending with normal force; (b) the hysteretic energy dissipation; (c) the secant-to-yield member stiffness, and (d) the ultimate deformation of the member under cyclic biaxial load paths.

Experimental Investigation of Concrete Frames Infilled with rc for Seismic Rehabilitation

AbstractNew reinforced concrete (RC) walls are popular in seismic rehabilitation of RC buildings as they reduce seismic-deformation demands. Their effectiveness was experimentally investigated by pseudodynamically testing 3∶4 scaled models of three four-story frames infilled with RC. The specimens were considered part of a frame building, the rest of which was substructured in the test as elastic. Unlike in past tests, the wall web was not thinner than the frame members and the specimens were tall enough to be governed by flexure. Two code-conforming designs were tried for the connection of the web to the surrounding frame members, one of them being less labor-intensive. Behavior and failure were dominated by flexure, but open, u-shaped FRP jackets at the two edges of the composite wall were essential to prevent premature failure near the base because of poor detailing of the columns. Slippage/separation at interfaces between the web and surrounding frame members was minor for both connection details. In ...

SEISMIC DESIGN OF OPEN-STOREY INFILLED RC BUILDINGS

For RC frame buildings with heightwise irregularities of the infills, Eurocode 8 (EC8) requires increasing the resistance of beams and columns in the less-infilled storey(s) in proportion to the deficit in infill strength there. For the columns of the less-infilled storey, a less demanding alternative rule is proposed, based on the observation that bending of the columns in the storeys above and below is often opposite to that in the less-infilled storey. It is proposed then to modify the capacity design rule at beam column-joints, in such a way that the capacity of the column of the more-infilled storey is added to that of the beams rather than to the capacity of the column of the less-infilled (or open) storey. A three-storey two-bay infilled RC test frame was designed on the basis of this concept, for pseudodynamic testing at the ELSA reaction wall facility in Ispra (I), in two configurations, each one with absence of infills from a different storey. Pre-test nonlinear dynamic analyses showing satisfac...

Tests and Simple Models of RC Frame Subassemblies for Postulated Loss of Column

AbstractThe impact of the loss of an intermediate column on the most adversely affected subassembly of a multistory, multibay reinforced concrete (RC) frame was studied experimentally and analytica...

Continuous One-Way RC Slabs with Sinking Outer Support: Tests and Simple Model

AbstractTests were carried out on two continuous one-way slab specimens, representing, at a scale of 1:1.5, the floor slabs of a multistory building most affected by a postulated instant loss of a ...

Dry-jointed precast concrete frame on rocking or fixed footings under cyclic lateral loading

Precast concrete frames can sustain earthquakes with little damage, if their beams are not monolithically connected to strong columns but via: (a) unbonded post-tensioned cables, continuous and concentric along all beam spans in the frame, and (b) top and bottom bars which cross the joint unbonded while being well anchored in the beam at both sides of the joint. Such a frame can sway thanks to intermittent opening and closing of the dry interfaces between the beam ends and the columns, instead of plastic hinging and damage at the ends of the beams. A pocket in the foundation typically allows the precast column to sway, without plastic hinging at the base. As this detail is inefficient and increases the vulnerability of the column to blast or impact, it is replaced in the paper with monolithic connection of the column to an isolated footing, which is free to rock and uplift, unrestrained by tie-beams. The lateral load response of such a rocking type frame with two stories and two bays was compared to that of a fixed-base frame. The latter suffered heavy damage at the base of the columns and at weak links along the beam spans, whereas the former confirmed the ability of a dry-jointed rocking frame to significantly reduce structural damage. Certain yielding at the top of the ground-story columns in the rocking frame was due to the artificially low column axial loads, lower than those found in practice even in low-rise buildings. An analysis model appropriately accounting for the flexibility due to the opening of the dry joints and for uplifting (modeled by a pair of no-tension springs under each footing) was found to give good agreement with test results.