52 research outputs found
A new hold-down device for seismic applications in CLT buildings: Design, testing, analytical and numerical assessment
No full textAn hold-down device was designed and tested experimentally for seismic applications (Seismic Hold-Down, SHD). The magnitude of axial strength as well as the up-lift displacement capacity is mainly oriented to cross-laminated-timber (CLT) buildings with medium-to-high ductility. There is consensus in the design practice of hold-down devices for promoting, as dissipative mechanism, flexural yielding of the dowel-type connectors (e.g. nails, screws) combined to timber's embedment. Few are the applications aiming at hold-down's optimization as it emerges from up-to-date literature review. In this framework, SHD was conceived such that tension break-out of steel was promoted at a reduced segment (fuse) of the vertical flange. In the context of capacity design, a convenient overstrength was assumed for other failure mechanisms, e.g. anchor's pullout, laterally-loaded steel-to-timber connection, shear-plug of timber. The mechanical response under axial load was consistently predicted by an application of the component method, i.e. global stiffness and strength were analytically-derived using simplified series of non-linear mechanical springs. Three-dimensional finite elements model was employed to investigate buckling of the fuse which might occur, during cycles, when up-lift displacement is recovering. In practical circumstances, it is recommend to design SHD devices of a CLT wall similarly to diagonal elements of steel bracings, i.e. neglecting the contribution of the compressed element. Finally, SHD is compared to traditionally-designed hold-down, which mostly dissipates (and fails) at steel-to-timber connection. Overall hysteretic response is somehow similar, although the inelastic mechanisms involved are different. For example, both the devices show pinching during cycles which for SHD is mainly caused by buckling of the fuse while, for traditional hold-down, is dominated by timber's embedment
Haunch retrofit of RC beam–column joints: Linear stress field analysis and Strut-and-Tie method application
No full textThis paper addresses the stress field of reinforced concrete (RC) beam–column joints retrofitted with haunches. Design of such solution currently assumes internal forces evaluated by the so called β-factor approach, which was originally conceived targeting the enhancement of steel moment-resisting frames. Extension to RC is subsequent as it emerges from the literature survey. The analytical model is first critically rediscussed. Inconsistencies of the adopted structural scheme, with respect to the actual mechanical behavior, may lie on the compatibility conditions which are imposed between the haunch and concrete beam (or column). In this regard, two-dimensional finite element models (FEM), using linear-elastic materials, are employed to study the stress field of two benchmark specimens derived from literature. A partial validation is carried out against experimentally derived internal forces. Results show that, for haunches with extended flat plates and stiff diagonals, compressive diffusion affects the entire haunch region. Consequently, beam's kinematic hypothesis of linear strains is no longer valid. The predicted joint shear demand resulted underestimated by β-factor approach by 50%. Since 2D FEM may be not efficient for many practical circumstances, an application of Strut-and-Tie is alternatively proposed. Finally, both the limitations and possible extensions of the proposed approaches are stated transparently
Influence of Specimen Geometry on the Response of Post-Installed Anchors Subjected to Constant Load under Crack Opening and Crack Cycles
No full textIt is generally acknowledged that one of the most critical tests to capture the behavior of a post-installed fastener under seismic action is the so-called “seismic crack movement test,” which consists in applying a constant load to a single fastener installed in a crack subjected to opening and full closing cycles. This article presents experimental results of crack movement tests on large size post-installed anchors that show a strong influence of the geometry of the concrete specimen in which the anchor is installed. To improve the regularity of the crack plane, a feedback control using the crack opening signal is applied to the servo-hydraulic actuators. Results of seismic crack movement tests using two different test setups were compared. The major aspects are as follows: (i) splitting force generated by the anchor affects the restoring of the zero crack opening when increasing the number of cycles, and (ii) increasing the size of concrete element limits the effects of bending induced in the concrete specimen. The issue of residual crack opening at the zero actuator’s load is observed experimentally and is further approached both analytically and numerically. The parameters that mostly affect the crack closure phase, i.e., steel ratio, transfer length, and de-bonding length, are finally discussed
RC beam-column joints, discussion of the provisions in the second generation Eurocode 8
No full textThis paper reviews the provisions given in the draft of the second-generation Eurocode 8 (EC8) for the design and assessment of RC beam-column joint against seismic conditions. The analytical bases were recently published by Michael Fardis. A critical discussion of the analytical models, supported by a numerical example, is given. Validation against an independent database of exterior joints is made. A final comparison with respect to (i) current EC8 provisions and (ii) other Building Codes is presented
Comparative Assessment of Shear Demand for RC Beam-Column Joints under Earthquake Loading
No full textThis paper focuses on the evaluation of bi-axial shear demand for reinforced concrete (RC) beam–column joints assuming: (i) the SPEAR frame as a benchmark; and (ii) different structural analysis methods which share the same seismic input. A numerical model was implemented using lumped plasticity. The joints were modeled as rigid offsets of beams and columns. The shear demand at a joint is evaluated as a post-process of the beam’s nodal moment. The discussion focuses on the differences between the estimated shear demand considering modal-response-spectrum analysis (MRSA), non-linear static analysis (NLSA) and non-linear time history (NLTH). Strength assessment of joints is discussed as well. Significant strength differences were recognized by using different building codes targeted to existing structures which, in general, behaved on the safe side. The elliptical shear strength domain resulted in being conservative when compared to NLTH shear demand orbits. NLSA, using modal combination, proved to estimate the larger shear demand with respect to MRSA and NLTH
HAUNCH RETROFIT OF RC BEAM-COLUMN JOINT: FASTENING ASSESSMENT
No full textHaunch retrofit of RC beam-column joint is made, essentially, by introducing steel diagonal elements at the beam-column joint location to reduce the shear demand in seismic deficient RC frames. Steel-to-concrete connection is often made using groups of post-installed fasteners, e.g. bonded anchors. This paper addresses the evaluation of anchor’s forces using a two-steps structural analysis. First, haunch’s diagonal force is evaluated by applying strut-and-tie model to the beam-column sub-assemblage. Second, a FEM model is employed for the sub-structure formed by the steel haunch and anchorages. Validation of both the structural analysis steps is made with respect to experimental results (e.g. measured anchors’ forces, reinforcement strains) obtained by the Authors and published elsewhere. Agreement is found when numerically-derived internal forces are compared to experimental ones. Finally, some suggestions are given for practical design cases where estimated forces need to be compared to nominal concrete break-out
SEISMIC DESIGN AND EXPERIMENTAL INVESTIGATION OF A NEW TIMBER HOLD-DOWN CONNECTION
No full textTimber connections made with hold-down are usually adopted for timber wall to foundation connection at the edges of
the wall to restraint the possible overturning moment (due to rocking) which can occur by applying a horizontal force to
the panel (e.g. under seismic action). The main components of the connection are the steel plate fastened to the panel
via threaded screws or nails and the anchor system to the concrete foundation. As such, the connection behavior under
seismic loading is characterized by the contemporarily contribution of different resistant mechanisms, i.e. (i) laterally
loaded timber screws or nails; (ii) axially loaded steel plate; (iii) axially loaded anchor in concrete. The former has been
basically neglected in past investigations having anchored the hold-down with steel bolt directly to the strong floor.
However, the design of the anchor should be considered as crucial when it comes to capacity design, especially in case
of narrow foundations (edge failure) and post-installed anchors when the resistance is highly reduced with respect to
steel bolt capacity. Moreover, the overall dissipative performance might be affected by the anchor’s displacement. An
innovative hold-down connection has been designed to promote steel failure of the axially loaded plate. The plate’s
geometry has been optimized such that capacity design rules according to Eurocode 8 can be applied. Additionally, a
target displacement requirement is addressed, and the stretch length is defined accordingly. The whole connection is
tested against cyclic loading and results show enhanced performances with respect to standard configuration. In
particular, brittle mechanism such as concrete-cone failure of the anchors and splitting of timber are prevented. The
paper discusses the experimental results for different type and size of the anchor, also the size of the optimized steel
plate is considered as a parameter
The effect of very low bearing pressure on the behavior of cast-in anchors
No full textThe effect of bearing pressure on the behavior of cast-in anchors is addressed presenting the results of an experimental campaign on cast-in anchors with different head type and same embedment depth. In particular, three cast-in anchor solutions were tested with the anchor-head ranging from relatively small head (high bearing pressure) to very large head-size (low bearing pressure). For the small head-size only, the anchor is installed using high strength grouting mortar after the hardening of the base concrete material. The concrete base member was lightly reinforced. Anchors were tested under axial force and different mechanical response (load-displacement) are observed. Failure modes change depending on the anchor’s type. The force transfer mechanism might migrate from pure concrete cone formation to structural collapse of the concrete base member. In some cases, the cone surfaces can be clearly recognized despite of the presence of a splitting crack. In other tests a plate failure was obtained, characterized by the presence large triangular segments between cracks radially arranged. This aspect is strictly related to the bearing pressure at the anchor-head location. Indeed, a hydrostatic stress-strain field is developed with different gradients according to the head-size. Small head-size leads to an increasing of the bearing pressure with severe crushing of concrete and consequent reduction of the expected load capacity. Furthermore, the observed failure mechanism suggests that the crack pattern propagation is unaffected by the presence of grouting mortar
Comparison of post-installed and cast-in rebars under monotonic and cyclic loads
No full textIn current design practice post - installed rebars are widely used in structural applications. Their design rules are limited by international codes to the performance of the corresponding cast-in reinforcement bars, albeit they show a higher bond strength. Moreover, the use of straight bars is definitely forbidden in dissipation zone such as beam-to-column joints. The presented paper is intended to assess the performance of cast-in and post-installed rebars designed according to reference codes under the application of cyclic and monotonic loads. To this purpose, experimental tests using a modified beam-end specimen were performed. Two different diameters were selected (db = 12, 16 mm); bonding lengths were determined to account for the promotion of steel failure. Cyclic tests were conducted by applying a repeated stepwise increasing load history (up to fifty-nine repetitions), adapted from test protocols used in the seismic assessment of post-installed anchors
NUMERICAL INVESTIGATION OF DISSIPATIVE BEHAVIOR OF CONNECTION USING POST-INSTALLED ANCHORS
No full textSteel to concrete connections using post-installed anchors are nowadays used in seismic-prone countries worldwide. In the case of structures where steel to concrete connections link primarily members (e.g. resistant to seismic loads) or they are used for seismic retrofit, some questions may arise:(a) what is the stiffness of the structure; (b) what is q factor in case of new building (essential in commonly adopted design with response spectra). A lack of knowledge is recognized in the contribution of the connection to the seismic performance at whole structural level. Ductile behavior and hysteretic energy dissipation of the connection become fundamental aspects to be investigated complying with capacity design approach, namely to ensure local dissipation (plastic hinges) preserving the structure as a whole. The present paper addresses the problem of column-to-foundation connection using bonded anchors with stretch length, subjected to seismic loads. Numerical models, using simple beam elements with lumped plasticity, are calibrated starting from experimental results on the sub-structure namely (i) steel column, (ii) steel base plate; (iii) anchors. Changes in anchors’ non-linear behavior are investigated. An application to one-story frame subjected to seismic artificial accelerograms is then presented
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