Excessive Deflection Research Articles

Design, analysis and experimental validation of high static and low dynamic stiffness mounts based on target force curves

In order to improve vibration isolation, soft components can be used in engineering applications, but this can lead to excessive static deflection. An ideal vibration isolator should have a high static stiffness to ensure that it has sufficient load carrying capacity; at the same time, it should have a low dynamic stiffness to maximize the vibration isolation frequency range. Recently, high static and low dynamic stiffness (HSLDS) mounts have been increasingly shown to have significant benefits for various engineering applications. This paper proposes a method for designing HSLDS mounts based on target force curves. In the design method, the HSLDS mount is obtained by placing a negative stiffness structure in parallel with a positive stiffness linear spring. The negative stiffness structure is achieved by using a roller-slider curve which can be designed according to the requirements to achieve the target force curve. HSLDS mounts are proposed with nth-order stiffness behavior​ which are designed using the method presented here. The results show that, compared with lower order HSLDS mounts based on the same static stiffness, higher order HSLDS mounts have lower dynamic stiffness near the equilibrium position. The Average Method is used to analyze the dynamics of a system based on the nth-order HSLDS mounts, and the displacement transmissibility under harmonic excitation is obtained. The effects of different parameters on the transmissibility are studied. The results show that appropriately increasing the damping ratio is beneficial for the isolation performance of the HSLDS mount. Finally, an experimental prototype is designed and manufactured. The proposed design method and the vibration isolation performance of the HSLDS mount are verified by constant-frequency excitation experiments.

Rapid heat-activated post-tensioning of damaged reinforced concrete girders with unbonded near-surface mounted (NSM) NiTiNb shape-memory alloy wires

Concrete girders can suffer from serviceability issues due to excessive cracking and deflection. In response to this problem, a novel heat-induced post-tensioning technique utilizing unbonded near-surface mounted nickel–titanium–niobium (NiTiNb) shape-memory alloy (SMA) wires was proposed and evaluated. SMAs are a class of smart materials that can recover seemingly permanent plastic deformation when heated. The post-tensioning forces, thus, can be generated by restrained heat-induced shape recovery of SMA wires. Material characterization tests showed that 3.92-mm diameter NiTiNb wires with 2.5% prestrain can generate recovery stress of approximately 500 MPa when actuated via Ohmic heating in a restrained condition. The proposed post-tensioning system was experimentally evaluated in reinforced concrete girders measuring 2.3 m in length and 23 × 41 cm in cross-sectional dimensions. The girders were initially cracked to simulate typical girder damage. NiTiNb wires were then installed in the bottom cover of the girders and anchored at both ends. Subsequently, the wires were actuated via Ohmic heating, which triggered shape-recovery and generated post-tensioning stresses in the girder. The post-tensioning technique reduced the crack widths by up to 74% (370 μm) and recovered the residual midspan deflection by up to 49% (1.52 mm) in the cracked girders. Following post-tensioning, flexural loading up to failure showed that the cracked stiffness and ultimate moment capacity of the girders had increased by up to 31% and 45%, respectively, with a relatively small NiTiNb reinforcement ratio of up to 0.17%.

Rheological Effects in the Bridges Constructed with Cantilever Method

Abstract A characteristic feature of bridges made using cantilever concreting technology is their excessive deflections, which are a result of rheological processes in concrete and prestressing steel. These deflections can be caused by the destruction of the material, e.g. concrete cracking, as well as the changing of the static scheme of the bridge structure, such as the subsidence of supports. The final result of a structure’s operation is changes in its grade line, which in this paper are considered as the deflection line of a bridge’s span. The purpose of the paper is to determine the participation of a structure’s rotation over supports in the deformation of the span with the largest length. The authors proposed an algorithm for determining the deflection function and rotation angles, which were obtained on the basis of changes in the curvature of the beam. It is characterised by an accurate mapping of the rheological processes that occur in the bridge, which is calculated on the basis of the changes of the grade line obtained from geodetic measurements on site. The paper proposes a general geometric indicator of the box cross-section cantilever, which is calculated for the construction phase, and a different indicator for the operation phase. They can be used for comparative analyses of various bridges. The analysis of deflections in cantilever bridges during the operation phase of their longest spans indicates that there is a significant influence of the angles of rotation over the supports. In the paper, such a group of bridges is qualified as unstable, in which the static scheme changes from a determinate cantilever state (the construction phase) into a multi-span system with different span lengths (the operation phase).

Underwater blast loading of partially submerged sandwich composite materials in relation to air blast loading response

The research presented in this paper focusses on the underwater blast resilience of a hybrid composite sandwich panel, consisting of both glass-fibre and carbon-fibre. The hybrid fibres were selected to optimise strength and stiffness during blast loading by promoting fibre interactions. In the blast experiment, the aim was to capture full-field panel deflection during large-scale underwater blast using high-speed 3D Digital Image Correlation (DIC). The composite sandwich panel was partially submerged and subjected to a 1 kg PE7 charge at 1 m stand-off. The charge was aligned with the centre of the panel at a depth of 275 mm and mimicked the effect of a near-field subsurface mine. The DIC deflection data shows that the horizontal cross-section of the panel deforms in a parabolic shape until excessive deflection causes core shear cracking. The panel then forms the commonly observed “bathtub” deformation shape. DIC data highlighted the expected differences in initial conditions compared to air-blast experiments, including the pre-strains caused by the mass of water (hydrostatic pressure). Furthermore, water depth was shown to significantly influence panel deflection, strain and hence damage sustained under these conditions. Panel deformations and damage after blast was progressively more severe in regions deeper underwater, as pressures were higher and decayed slower compared to regions near the free surface.An identical hybrid composite sandwich panel was subjected to air blast; one panel underwent two 8 kg PE7 charges in succession at 8 m stand-off. DIC was also implemented to record the panel deformations during air blast. The air and underwater blast tests represent two different regimes of blast loading: one far-field in air and one near-field underwater. The difference in deflection development, caused by the differing fluid mediums and stand-off distances, is apparent from the full-field results. During underwater blast the panel underwent peak pressure loading of approximately 52.6 MPa whilst during air blast the panel was subjected to 67.7 kPa followed by 68.9 kPa peak pressure loads in succession. The two experiments demonstrate the response of the same hybrid composite sandwich panel under two differing blast regimes.The post-blast damage and strength of the hybrid panels following air and underwater blasts were evaluated. Post-blast testing revealed that the underwater blast causes significantly more damage compared to air blast, particularly debonding between the skins and core. The air blast panel sustains no visible rear skin/core debonding, whereas 13 regions of rear-face debonds are identified on the underwater blast panel. Sustaining no front-skin breakage was advantageous for retaining a high proportion of the compressive modulus for this hybrid layup following underwater blast. Damage mechanisms were interrelated. Determining the most detrimental type is not straightforward in real explosive and non-idealised experiments, however debonding was understandably shown to be significant. A further study to isolate failure modes and improve in situ instrumentation is ongoing.

Development of a Novel Type of Open-Web Continuous Reinforced-Concrete Rigid-Frame Bridge

Long-span continuous reinforced-concrete rigid-frame bridges are sensitive to the shrinkage and creep of concrete and the relaxation of prestressing tendons and have experienced problems related to serviceability loss because of excessive multidecade deflections. Moreover, the large negative bending moment and shear force in the box girder near the piers may lead to cracks in the top slabs and webs. To improve the structural performance and mitigate the long-term deflection of bridges, a novel type of open-web continuous reinforced-concrete rigid-frame bridge was developed during the design of the Beipan River Bridge with a 290-m main span. In this paper, the design and construction techniques of the novel bridge type are summarized first. Then, a structural analysis of the Beipan River Bridge is conducted. The results are compared with conventional rigid-frame bridges. Moreover, the long-term girder deflections are checked through the combined application of monitoring data and finite-element analysis. The results show that the bridge alignment is quite stable during the first 5 years in service and that the open-web design philosophy can effectively mitigate the long-term girder deflections.

Saw cutting (or not) of joint spacing in roller-compacted concrete pavement

Natural cracking in unjointed roller-compacted concrete pavement (RCCP) may exhibit poor load transfer efficiency (LTE) across the cracks, leading to the occurrence of faulting, further cracking and excessive deflection, and a dramatic decrease in pavement service life. The aim of this study is to enable pavement designers to make the right decision as to whether or not to saw cut joints for RCCP. Analytical approaches to predicting the natural crack spacing for unjointed RCCP are considered and verified with data observed in the field. The predictive model response for different locations in South Korea was simulated to consider climate effects on natural crack spacing. Furthermore, measurements of LTE using a falling weight deflectometer across natural cracks in RCCP in relation to the crack opening were performed and a back-calculated joint spacing for adequate LTE was derived. Based on the results obtained, the analytical model for natural crack spacing provided accurate results compared to field data. The results also showed that the natural crack spacing is sensitive to ambient temperature and placement time. These spacings were too large to maintain adequate LTE and significantly decrease the LTE across RCCP cracks. Therefore, it was found that saw cutting of the joint spacing is required.

Excessive Deflection in Long-Span Timber Beams of a Historical Building in the South of Italy: Analysis and Retrofitting Design

AbstractIn 2018, under service conditions, excessive deflections were observed in the long-span timber beams of the first floor of the former Sant’Elisabetta Monastery in Palermo (Italy). The build...

Shrinkage test of concrete: Methodology and variation of strain

Evaluation of shrinkage strain in long concrete structures such as prestressed concrete box girder bridges is very important for the design of such structures. A wrong evaluation may contribute to loss of prestressing force, excessive deflections, cambers and cracking of the structure. It may also affect the durability and serviceability of the structure. The value of shrinkage strain is required during design, far before the construction process. However, realistic evaluation is still a difficult problem because shrinkage strain is influenced by the concrete composition and environmental factors such as humidity and temperature. Therefore, to gain the best evaluation, an experimental program was conducted to measure the shrinkage strain of concrete versus concrete age. Several concrete specimens were collected during pouring of several box girder segments of Tauranga Harbour Link, New Zealand. Variation of shrinkage strain versus concrete age for almost two years of data collection was plotted.

Bufferless Network-on-Chips With Bridged Multiple Subnetworks for Deflection Reduction and Energy Savings

A bufferless network-on-chip (NoC) can deliver high energy efficiency, but such a NoC is subject to growing deflection when its traffic load rises. This article proposes Deflection Containment (DeC) for the bufferless NoC to address its notorious shortcomings of excessive deflection for performance improvement and energy savings. With multiple subnetworks bridged by an added link between two corresponding routers, DeC lets a contending flit in one subnetwork be forwarded to another subnetwork instead of deflected. Microarchitecture of DeC routers is rectified to shorten the critical path and lift network bandwidth. Its Cadence RTL implementations with a $15-nm$ 15 - n m process are conducted respectively for mesh-based NoCs and torus-based NoCs. Additionally, different sized DeC-NoCs are evaluated extensively and compared with previous bufferless designs (BLESS and MinBD), uncovering that DeC with two bridged subnetworks (dubbed DeC2) for 8X8 mesh-based NoCs can lower deflection drastically by some 90 percent and energy consumption by upto 51 percent under real benchmark traffic loads, in comparison to BLESS. Under various synthetic traffic models and workloads, 16X16 torus-based DeC2-NoC sustains up to 2.33X loads when compared with its mesh-based counterpart, exhibiting the same clock rate and taking only negligible more power and area according to our full layout results.

Calculation method of creep under decreasing stress considering variation of natural environmental factors

The actual structure is always in the state of natural changing environment and stress decreasing in the section. At present, there is little method for calculating the creep under decreasing stress which takes environmental variation into account. As excessive deflection becomes a serious problem for long span continuous rigid frame bridges during the operation period, in this paper, a creep calculation method is proposed for the continuous decrease of stress in most girder sections, and considers the environmental factors comprehensively. Firstly, the multifactor (temperature, relative humidity and reinforcement ratio) correction was made to creep model of the current code JTG D62. And five creep recovery models were compared and analyzed simultaneously. Based on the elastic creep theory and the superposition principle, the improved creep calculation method was proposed. Then theoretical calculation was carried out for the assumed structure. The results showed that the calculated value obtained by the proposed method were obviously different from those obtained by the code, and were significantly different from those obtained without considering creep recovery, which proves the superiority of the proposed method under the action of multiple environmental factors and decreasing stress.

Structural Evaluation for Iraqi Ministry of Trade Building Exposed to Fire During 2003 War

Existing concrete structures may, for a variety of reasons, be found to be unsatisfactory. This could manifest itself by poor performance under service loading in the form of excessive deflections, cracking or inadequate ultimate strength and changing the main function. In such circumstances, there are two possible solutions i.e. demolish and rebuild or carryout a program of repair and strengthening. In this investigation, the structural evaluation and suggested repairing works were studied for Iraqi Ministry of Trade building, the building was exposed to fire, vandalism and looting during the war on Iraq at 2003. The building consists of twelve stories, ten stories have area about 1428m2 and about 1150 m2 area of middle and basement stories. Each story has height of 3.5m. The structure of building consists of one way ribbed slab with 900mm slab thickness and 0.15×0.7m rib sectional dimensions. The columns have dimensions 0.6×1.2m and height 3.5m. The girders, ribs, floors and columns were peeled and decayed, there are a several cracks wider than 0.4mm distributed on the beams and floors and several columns, the concrete floors were destroyed and that caused appearance of steel reinforcement, parts of concrete of column at the second floor were destroyed and the longitudinal reinforcement steel buckled outside dragging the transverse reinforcement, a high possibility of reduction in concrete strength caused by the fire effects happened during 2003 war but the steel dose not affected by the fire because of the concrete cover which protected reinforcement. From this investigation, there is no serious damage in the sense that it is imperative to demolish the building and re-create. The damage in the building, as indicated in this study require some repairs, known in the field of rehabilitation through chemical additives and steel structures are additional materials available in local markets. It necessary to minimize the additional dead loads and the use of Limestone partitions in their implementation Gypsum Board partitions or imported light partitions.

A practical creep model for concrete elements under eccentric compression

Many prestressed concrete bridges are reported to suffer from excessive vertical deflections and cracking during their service life. Creep softens the structure significantly, and therefore an accurate prediction of creep is necessary to determine long-term deflections in elements under eccentric axial compression such as prestressed concrete girders. This study proposes a modification to the creep damage model of Model Code 2010 to account for the effect of load eccentricity. The modified creep model considers damage due to differential drying shrinkage. Initially, the creep behaviour of small scale concrete specimens under eccentric compression load is investigated experimentally. Twelve small-scale concrete prisms were subjected to eccentric axial loading to assess their shrinkage and creep behaviour. The main parameters investigated include the load eccentricity and exposure conditions. Based on the experimental results, an inverse analysis is conducted to determine the main parameters of the modified creep model. Subsequently, a numerical hygro-mechanical simulation is carried out to examine the effect of load eccentricity on the development of shrinkage and creep, and on the interaction between drying, damage and creep. The results indicate that eccentric loading leads to different tensile and compressive creep through the cross section, which contradicts the current design approach that assumes that tensile and compressive creep are identical. The proposed model also predicts accurately the long-term behaviour of tests on reinforced concrete elements available in the literature. This study contributes towards further understanding of the long-term behaviour of concrete structures, and towards the development of advanced creep models for the design/assessment of concrete structures.

Evaluating the Effect of the Compressive Strength Development Factor on Concrete Creep Deformation in Bridge Construction

Abstract Excessive midspan deflection is often observed in large-span prestressed concrete girder bridges. In particular, the deformation caused by shrinkage and creep of concrete is an important part of deflection that often exceeds expectations. To achieve creep deformation control, extending the curing time and postponing the loading age are frequently adopted during construction. This article proposed an approach based on creep experiments with prismatic and beam specimens and viscoelastic model-based finite element analyses to evaluate the effect of the concrete strength development factor on the ultimate creep coefficient in the Comité Euro-International du Béton-Fédération internationale de la précontrainte (CEB-FIP) model. The concrete strength development factor ξcc(t/tu) is defined as the ratio of the mean compressive strength at the loading time to the maximum mean compressive strength obtained in the test. The strategies for targeted regulation and control of ξcc(t/tu) were discussed by establishing an artificial neural networks model for strength prediction from a database provided in our previous work. Uniaxial compressive prismatic specimens and pure bending beam specimens with a span of 5.0 m were used in the experiment for creep behavior observation. A viscoelastic finite element analysis (FEA) model was established based on solidification theory. The FEA model results were verified with measured data for capturing the creep behavior under ambient conditions to obtain reliable long-term creep deformation predictions. Finally, the proposed method was validated with the Xincheng bridge construction for the optimistic loading age determination, and the results indicated good feasibility in girder deflection control.

Reinforcement methods for compression perpendicular to grain in top/bottom plates of light timber frames

Relatively low load-bearing capacity of top and bottom plates in light timber framed structures under compression perpendicular to grain and the consequential shortening of vertical elements have resulted in their limited applications in construction of mid-rise buildings. Solutions for the enhancement of the compression perpendicular to grain need to be further developed to tackle this problem. This paper investigates the contribution of excessive deflection of horizontal members to the vertical shortening of light timber frames due to compression perpendicular to the grain and evaluates the effectiveness of some different reinforcement techniques for improving the bearing strength of these members. A comprehensive experimental investigation of light timber framed members under compression perpendicular to the grain, provides information on different mechanical properties, different types of reinforcement methods and their effects on the load carrying capacity of a range of structural timber elements. For this purpose, 750 practical tests are carried out to evaluate the effectiveness of the solutions. The techniques include applying timber on timber and steel on timber connections, use of additional reinforcing materials and mechanical fasteners. The results indicate that the bearing capacity under compression perpendicular to grain can be considerably improved by applying some of these solutions; the level of improvement varies for each solution though.

Assessment of reinforced concrete slabs using in-situ load testing: A case study

In-situ load testing allows for assessment of the adequacy of existing structures with insufficient design information or unknown internal material deterioration history. In this study, flat slabs of an existing reinforced concrete (RC) building were evaluated using in-situ load testing per the ACI-318 test procedure. The tested structure had five storeys, among which two were underground. The building was to reach twelve storeys, yet construction was halted for eight years, exposing the structure to environmental damage mechanisms. Construction of the additional seven floors atop the existing ones required detailed analysis of the current condition. Hence, two slab panels on each floor were selected for performing in-situ load testing. Sand bags and loose sand were used to apply an incremental load on eight selected panels. Deflection measurements at critical locations along with thorough visual inspection were conducted during the loading and unloading cycles. Moreover, cores were retrieved from selected panels to investigate the compressive strength of the in-place concrete. No serious damage was observed during the in-situ loading and unloading cycles. However, slab panels of the first floor exhibited excessive cracking and deflection upon load testing. Deflection acceptance criteria were satisfied for all tested RC slab panels, except those of the first floor. The in-situ experimental results were validated through finite element analysis. It was determined that the existing RC structure had adequate strength to accommodate the new construction. However, it was recommended to demolish and reconstruct the first-floor slab panels.

Cracking and Failure Mode of the Self-Stressed Members with FRP Bars

Fiber reinforced polymer (FRP) bars represent a combination of the polymer binder and reinforcing fibers (glass, basalt, aramid, carbon). The main features of FRP bars are high tensile strength on the background of the relatively low elasticity modulus. To prevent development of the excessive both crack opening and deflections in the FRP reinforced concrete structures it can be effective to implement FRP reinforcement pretensioning with a limited level of created stresses. As a good option can be considered a physico-chemical method of FRP bars pretensioning based on the self-stressing concrete utilizing. In the self-stressed FRP reinforced members it is possible to obtain a considerable values of the early age restrained expansion strains (in comparison with steel reinforced self-stressed members because of FRP bars lower elasticity modulus), which will not disappear after air-dry shrinkage strains realization. In addition, another concern that have to be considered in the field of FRP reinforced self-stressed members is bond performance of the different FRP bars types, especially in combination with self-stressing concrete that within its expansion can provoke decompacting of the transit zone «bar-concrete». Moreover, taking into account that FRP bars is a composite material, its bond properties are strongly influenced by the types of the polymer binder, reinforcing fibers, ratio between binder and fibers, bar coating. Presented studies is consisted in the experimental investigations of the features in the crack development and depended on it occurred failure mode of the self-stressed members reinforced by the different types of FRP bars.

Evaluation of crack width in roller-compacted concrete pavement

Crack width is a critical factor influencing the behaviour and performance of a roller-compacted concrete pavement (RCCP) because wide cracks can result in water infiltration and poor load transfer, leading to faulting, excessive deflection and further cracking. The crack width may be affected by temperature changes, concrete drying shrinkage and creep, crack spacing and the base material. An empirical equation to predict the crack width has been proposed by the American Association of State Highway and Transportation Officials (Aashto). However, this equation is quite simplified because a uniform strain distribution is assumed along the slab length. In this work, a mechanistic approach to predict the crack width in RCCP was developed using a three-dimensional finite-element model considering a non-linear friction model between the slab and the base. Moreover, the effect of concrete creep on the crack width was incorporated using the effective modulus method. The mechanistic model was validated with crack width data from two full-scale RCCP test sections in South Korea. The mechanistic model showed good agreement with the field data, whereas the Aashto equation tended to over-predict the crack widths. The proposed model is thus helpful to predict the crack width in a concrete pavement slab for given environmental conditions, slab geometry, material properties and base material.

Development of a windborne debris impact fragility curve for Cross-Laminated Timber using experimental testing

Cross-Laminated Timber (CLT) is an engineered wood product with high rigidity, dimensional stability, and desirable environmental properties. Because of these characteristics, it presents the potential to resist the impact loads associated with windborne debris in both tornadoes and hurricanes. Full-scale experimental debris impact testing was performed on 3-layer CLT in order to determine its response to debris impact loading scenarios. A variety of missile masses, speeds, and impact locations were tested to determine this response. Various forms of failure were observed including missile perforation, excessive backside deflection, and dislodgement of debris on the backside of tested panels. Analysis of the experimental data was then performed with the goal producing a debris impact fragility curve for CLT. The resulting curves were based on relating a missile's kinetic energy with the probability of failure as defined by relevant testing standards. The results of this study point towards 3-ply CLT having a high likelihood of surviving impacts from EF-2 and EF-3 level missiles with a decreased likelihood of surviving debris from EF-4 and EF-5 level events. For qualification as a material for safe rooms, thicker panels and additional connection design would be required.

Experimental Study on Long-Term Ring Deflection of Glass Fiber-Reinforced Polymer Mortar Pipe

Long-term pressurizing of buried glass fiber-reinforced polymer (GFRP) pipe will result in the reduction of stiffness in the pipes. It leads to excessive deflections in long-term design limits. In situ tests were performed for 664 days to measure deflections of buried GFRP pipe with a large diameter of 2,400 mm. Based on the field test results, finite element analysis was conducted to determine the pipe deflections with respect to the soil conditions and buried depth as variables. Regression analysis has been conducted to determine the long-term deflection of the GFRP pipe after 50 years of construction. The long-term deflection of the GFRP pipe was less than 5 percent suggested by the existing specifications including ASTM D5365 and AWWA M45. The comparison indicates the current specifications significantly conservative to predict long-term deflection of the buried GFRP pipe.

Stress Analysis of Slabs Reinforced with GFRP Rebar

Designed constructions must meet a certain level of safety and durability. They should also be designed in accordance to Ultimate Limit State and Serviceability Limit State with respect to profitability for the contractor. During designing stage of reinforced concrete, it is important to remember that the values in static and strength calculations are assumed as ideal. In reality, material properties of steel and particularly concrete are biased with a certain degree of random. Modern technologies allow to utilize construction materials with strictly determined deformation and mechanical parameters. Significant development of concrete technology in recent years poses new challenges. In the study, authors used Glass Fibre Reinforced Polymer rebar as a replacement for standard steel rebar. The article presents the results of the destruction tests of concrete slabs reinforced with steel and GFRP rebar. The tests were performed on 14 slabs with length-to-width ratio of 4. The thickness of slabs ranged from 6 to 10 cm. In nine slabs, authors replaced the standard rebar with GFRP rebar. The tests involved stress analysis in rebar and concrete. The secondary goal was to determine the bearing capacity and deflection of slabs and assess the cracking of the sections during load application. Composite rebar has many advantages over standard rebar. However, due to a relatively low elasticity modulus in comparison to steel, the slabs exhibit excessive deflection and cracking of bended elements.