Bohumil Kasal

Heavy Laminated Timber Frames with Rigid Three-Dimensional Beam-to-Column Connections

AbstractThis article presents the seismic performance of a timber frame with three-dimensional (3D) rigid connections. The connections were made with self-tapping screws and hardwood blocks were used to support the beams. The frame was designed to resist high seismic excitations with the goal of controlling the drift. The moment-rotation characteristics of the connections were measured in the laboratory by applying static cyclic loads. The frame made of laminated wood beams and columns, and cross-laminated lumber deck, was subjected to seismic, white noise, snapback, and sinusoidal sweep excitations. The synthetic seismic excitation was designed to contain a considerable amount of energy close to the frame’s first natural frequency. The structure showed no significant damage up to a peak ground acceleration of 1.25g. Failure of the frame occurred due to shearing of the columns with a peak ground acceleration of 1.5g. The designed structure fulfilled with current serviceability limits up to 0.8g.

Reliability assessment of confinement models of carbon fiber reinforced polymer-confined concrete

This paper presented a review of 84 confinement strength models developed for predicting the ultimate compressive strength of carbon fiber-reinforced polymer-confined concrete subjected to uniaxial compression. Among these models, 64 design-oriented models and 12 analysis-oriented models were selected and evaluated by a comprehensive database including the experimental results of 1475 carbon fiber-reinforced polymer-confined concrete specimens through three statistical indicators: the mean (μ), the coefficient of determination ( R 2 ) , and the root mean square error (σ) of the predicted ultimate compressive strength f cc * and the experimental ultimate strength f cc . Based on the results of evaluation, 27 design-oriented models were further considered for reliability assessment and structural reliability analysis. Given the performance of strength predictions and the reliability assessment, it was found that for the design purpose, the 27 design-oriented models are reliable for practical predictions of carbon fiber-reinforced polymer-confined concrete structures subjected to uniaxial compression.

Adhesion force mapping on wood by atomic force microscopy: influence of surface roughness and tip geometry

This study attempts to address the interpretation of atomic force microscopy (AFM) adhesion force measurements conducted on the heterogeneous rough surface of wood and natural fibre materials. The influences of wood surface roughness, tip geometry and wear on the adhesion force distribution are examined by cyclic measurements conducted on wood surface under dry inert conditions. It was found that both the variation of tip and surface roughness of wood can widen the distribution of adhesion forces, which are essential for data interpretation. When a common Si AFM tip with nanometre size is used, the influence of tip wear can be significant. Therefore, control experiments should take the sequence of measurements into consideration, e.g. repeated experiments with used tip. In comparison, colloidal tips provide highly reproducible results. Similar average values but different distributions are shown for the adhesion measured on two major components of wood surface (cell wall and lumen). Evidence supports the hypothesis that the difference of the adhesion force distribution on these two locations was mainly induced by their surface roughness.

Confinement models of GFRP-confined concrete: Statistical analysis and unified stress–strain models

This paper presented a comprehensive assessment of 22 existing stress models and 13 strain models, which were developed for glass fiber reinforced polymer (GFRP)-confined concrete in uniaxial compression. In addition, a reliability evaluation of these stress and strain models was performed. A database including 212 GFRP-confined cylindrical concrete specimens was collected and analyzed to evaluate the performance of these stress and strain models. The accuracy and applicability assessment of these models were carried out by χ 2 test and the Pearson correlation coefficient r, and the Monte Carlo–JC method was used by for the reliability assessment. Based on the evaluation, it was found that the unconfined concrete strength f co is the most important parameter which determines the accuracy of these stress and strain models for GFRP-confined concrete. Most stress and strain models used in this study can predict the ultimate axial strength and strain of the GFRP-confined concrete appropriately, although some scattered points exist for some models. In addition, it was found that all these models satisfy the command of reliability even some models are too conservative. Based on the database, unified stress and strain models were proposed showing better applicability.

Can Plant-Based Natural Flax Replace Basalt and E-Glass for Fiber-Reinforced Polymer Tubular Energy Absorbers? A Comparative Study on Quasi-Static Axial Crushing

Using plant-based natural fibers to substitute glass fibers as reinforcement of composite materials is of particular interest due to their economic, technical, and environmental significance. One potential application of plant-based natural fiber reinforced polymer (FRP) composites is in automotive engineering as crushable energy absorbers. Current study experimentally investigated and compared the energy absorption efficiency of plant-based natural flax, mineral-based basalt, and glass FRP (GFRP) composite tubular energy absorbers subjected to quasi-static axial crushing. The effects of number of flax fabric layer, the use of foam filler and the type of fiber materials on the crashworthiness characteristics, and energy absorption capacities were discussed. In addition, the failure mechanisms of the hollow and foam-filled flax, basalt, and GFRP tubes in quasi-static axial crushing were analyzed and compared. The test results showed that the energy absorption capabilities of both hollow and foam-filled energy absorbers made of flax were superior to the corresponding energy absorbers made of basalt and were close to energy absorbers made of glass. This study, therefore, indicated that flax fiber has the great potential to be suitable replacement of basalt and glass fibers for crushable energy absorber application.

Low-Velocity Transverse Impact of Small, Clear Spruce and Pine Specimens with Additional Energy Absorbing Treatments

AbstractSingle-blow impact testing of spruce and pine specimens were done to assess the effects of additional E-glass reinforcing on the tension face of a beam along with a rubber lamina adhered to...

Structural health assessment of in situ timber: An interface between service life planning and timber engineering

Abstract The in situ assessment of timber structures has gained considerable attention in recent years due to some unexpected failures of public buildings. The assessment of timber, however, has been used in the evaluation of historic structures for a number of years, and the methods employed have evolved from visual observation (which is still one of the most effective ways of evaluating in situ timber) to more sophisticated methods that use various physical phenomena such as stress-wave or X-ray energy attenuation. In the health assessment of timber, effects of biotic elements such as insects and fungi are of interest, which, of course, is always connected with the presence of water in wood. The structural assessment encompasses questions related to the structural integrity of in situ members and the performance of components and the system. The structural health assessment not only focuses on biotic elements but also attempts to quantify engineering properties of the material such as strength degradation, modulus of elasticity, loss of cross-section, extent of checks, and other quantitative parameters needed for subsequent evaluation of the structural system, frequently expressed as load-bearing capacity. Service life planning of a structure is a complex issue that is related not only to the materials but also the environment and the use of the structure. Assessment of the health and properties of existing timber elements yields a piece of information that is necessary but not sufficient for the service life estimate. In the evaluation of structural timber, a mere use of various assessment techniques is not sufficient and usually an involvement of disciplines such as wood anatomy, wood physics, and statistics is needed. A reliable estimate of the parameters of in situ timber requires careful planning of measurements (experiments) since the material is highly variable and any statement about the properties of an element or even the entire system must reflect the random character of the wood properties. This paper will summarize the state-of-the art methods used in the assessment of in situ timber and analyze the strengths and the weaknesses of selected methods. An attempt will be made to outline future directions in the development of in situ assessment methods.

What was Timoshenko’s Small-Increment Method? With an Application to Low-Velocity Impact of a Wood Beam

This study reviews the article by Timoshenko (Z Angew Math Phys 62(1–4):198–209, 5), and how two other researchers have interpreted the numerical method he employed. The numerical method is then compared to some standard contemporary methods, showing the continuous historical development. Additional important contributions in the impact literature are systematically compared and then applied to the low-velocity impact prediction of a wood beam, which has not been done yet. How to obtain the material parameters needed for the prediction are discussed and also how the impact testing can be used to obtain dynamic material properties are discussed in detail. The predictions using the methods developed by past researchers for metals are qualitatively a good starting point, showing where areas in impact testing of wood can be improved.

State-of-the-art: intermediate and high strain rate testing of solid wood

The following state-of-the-art report summarizes important work done in wood science concerning intermediate and high strain rate testing. Intermediate testing may be done with hydraulic machines, which are generally classified as rapid loading. Intermediate testing may also be done using a transverse impact of a beam specimen with a pendulum, drop mass, or toughness tester, whereas high strain rate testing is generally done with the Kolsky bar. The article analyzes the different experimental testing apparatuses, the conclusions past researchers have made about them, and the toughness and strength measurements which were usually done. The transverse impact test is examined in detail because of its adjustability for specimen sizes. The value of this research is it delves into certain impact mechanics principles which are missing from the analysis of impact testing in wood science, which must be included to validate previously held assumptions. The physical response of wood to a dynamic load is not any different from other materials such as metals or rigid foams and is governed by the same principles. Nevertheless, over the years the application of impact mechanics principles to wood testing has been scarce and “black box” experimental, where empirical approaches such as the duration-of-load were often preferred. An example of these principles is the influence of higher modes of vibration plays a greater role on the stress state in testing than its influence on deflections. This principle has been thoroughly investigated for small strain rate vibrations, however, not applied to impact testing. Presently, there is no consensus on a constitutive strain rate model for wood under intermediate and high strain rates. This article provides direction for obtaining dynamic Young’s modulus and yield strength, which can be normally expected in the design of structures subjected to dynamical loadings, for the future creation of applicable constitutive strain rate models.

Experimental study and numerical simulation on bond between FFRP and CFRC components

Natural flax fabric-reinforced polymer (FFRP) tube encased coir fibre-reinforced concrete (CFRC) structure (termed as FFRP-CFRC) is steel-free hybrid structure that has shown its potential as axial, flexural and earthquake-resistant structural members. An FFRP plate with a CFRC overlay has great potential to be light and environmentally friendly wall panel or pedestrian bridge deck. The overall structural performance of this panel or deck is highly dependent on the bond at the FFRP and CFRC interface. Therefore, this study proposed a novel interlocking at their interface to improve the bond and thus the composite action of the hybrid structures composed of FFRP and CFRC components. This interlocking was generated by creating numbers of perforations on the FFRP component (tube and plate) surface. To evaluate the effectiveness of using this interlocking on the bond behaviour between FFRP and CFRC, two stages experimental studies were conducted. In the first stage, 18 FFRP-CFRC cylindrical specimens were constructed and tested under push-out bond, bending and axial compression. In the second stage, 30 FFRP plate and CFRC sandwich block specimens were constructed and tested under push-out bond considering different experimental parameters, i.e. depth, diameter and number of perforations. Additionally, numerical simulation was performed to verify the failure modes of FFRP plate and CFRC sandwich blocks under push-out. This study revealed that the presence of interlocking is an effective way to improve the interfacial bond and composite action between FFRP (either tube or plate) and CFRC components.