At the Luleå University of Technology, research has been performed on thermal stresses and thermal cracking in maturing concrete in the last decade. Laboratory tests have been carried out providing material data for calibration of a proposed theoretical model for thermal stress analysis in young concrete. The laboratory tests included creep tests, free thermal volume change tests and relaxation tests. In the theoretical model, the thermal properties and the transient mechanical properties of the young maturing concrete as well as the restraint within the concrete element are considered. The model has been implemented in a computer program permitting the analysis of different concreting scenarios of which some examples are shown. In the examples, influences of different measures against cracking as well as influences of changes in temperature conditions, restraint, and mechanical behavior of concrete are quantified. For some cases in the computational studies it may be concluded that, for control of early-age thermal stresses and cracking in a structure, it is highly inadequate to consider only the early-age distribution of the temperature field within the studied structures.

The present study addresses the loading or deformation rate effect on the yield loads of nailed timber joints. Four different types of joints, which differed with respect to the thickness of the members and the angle of load to grain, were tested. The bearing strength of the wood and the bending strength of the nails were also tested in order to analytically verify the dynamic ultimate capacity of the joints. All tests were run with deformation rates from static loading values up to approximately 1 m/s (40 in./s). The pilot study results show that the strength of the nailed joints can be expressed in terms of the deformation rate. A logarithmic expression for the strength of the joints was obtained from regression analysis of approximately 200 results. The values obtained analytically agreed well with the experimental ones for the various joints tested

Exact first- and second-order analyses for composite beam-columns with partial interaction and subjected to transverse and axial loading are presented. General closed-form solutions for the displacement functions and the various actions in the composite element are presented for the first- and second-order cases. In this paper, the axial loads acting on the composite elements are assumed to be proportioned in accordance with their relative axial stiffnesses so that their resultant acts at the centroid of the transformed cross-sectional area of the fully composite member. Resultant axial loads active at the centroid ensure that, in the first-order analysis, the composite elements are subjected to uniform axial strain through the depth of the member and that no bending is induced by the resultant axial load. The analysis procedures are applied to simply supported beam-columns subjected to an axial force and a uniformly distributed transverse load to obtain closed-form solutions for the internal actions and displacements. The difference in the first- and second-order analysis approaches is illustrated by applying the solutions to a given beam-column problem.

This paper describes tests on a novel composite floor system constructed by connecting prefabricated concrete slabs to timber joists. Seven types of shear connectors have been developed and tested: lag screws, either alone or combined with a notch cut from each timber joist; metal plates embedded in the concrete slab and either nailed or glued to the joists; dowels embedded in the concrete and glued to the timber; and toothed metal plates embedded in the concrete and pressed into the timber. Four-point bending tests to failure were performed on five, full-scale, 4.8 m long specimens connected with lag screws or metal plates nailed to the timber. Values of deflection and relative slip between the concrete slab and the timber obtained in these tests showed high correspondence with values obtained from a uniaxial finite element model developed for nonlinear analyses of composite beams. The model was also used to perform a numerical analysis to failure of composite beams with the other four connection systems that were developed but not tested on full-scale specimens. The outcomes of the experimental tests and numerical analyses show that the newly developed system can provide good structural performance, especially if connections with coach screws and notches in the timber are used. The economic advantages of prefabrication and the possibility to disassemble the structure and reuse the timber beams and concrete panels at the end of the service life make the proposed floor system very promising.

The expanding height of multistory mass timber buildings makes the development of guidance to prevent disproportionate collapse an increasingly important area of engineering design; however, there is no guidance available in building codes or design standards on how to prevent disproportionate collapse of multistory mass timber buildings. In this paper, practical project-specific solutions applied in constructed prominent multistory mass timber buildings to prevent disproportionate collapse are presented. Various approaches have been successfully used for different construction types of multistory mass timber buildings to decrease the potential risk of disproportionate collapse, such as using walls above supports as deep beams and using columns to carry tension forces and hold the floor below in case a supporting column or wall is damaged. Then, recent and ongoing research in the field of prevention of disproportionate collapse of multistory timber buildings is reviewed. Such research is mostly based on numerical simulations of element removal scenarios and pushdown experiments on floor systems and connections and provides valuable insights and guidance to designers on important structural aspects regarding the prevention of disproportionate collapse in multistory mass timber buildings. Finally, the draft provisions for an upcoming standard related to the prevention of disproportionate collapse are reviewed, and the conclusions regarding the state of the art are presented.

Cast-in-place headed anchors with different head sizes embedded in plain and reinforced concrete members of various thicknesses were subjected to pullout tests. The influence of member thickness, size of the anchor head, and orthogonal surface reinforcement on the tensile capacity and performance of anchor bolts was evaluated. The member thickness varied from 1.5 to 3.0 times the anchor embedment depth and headed anchors with small, medium, and large heads were tested.

The experimental results of the present study showed that increasing member thickness and/or the use of orthogonal surface reinforcement lead to increased anchorage capacity and anchorage ductility, whereas the anchorage stiffness decreases slightly. In contrast to the anchorage ductility, the tensile breakout resistance and the anchorage stiffness increase significantly with increasing size of the anchor head.

The experimental results corresponded closely to numerical results from a previous study (Nilforoush et al. 2016 a & b), which suggested a modified model incorporating several modification factors for improving the predictive capability of the Concrete Capacity (CC) method. In the present study, these factors yielded improved prediction of the tensile breakout capacity of the tested headed anchors.

Beams of high-strength concrete (fcc up to 90 MPa) were tested in shear and bending. Various types of fiber (metallic and nonmetallic) were added to the concrete matrix to up to 1% per volume concrete. Also, four different beam depths were investigated. The efficiency of fibers depends on the governing load-carrying mechanisms, which may vary for different beam dimensions. The chosen steel fibers did not improve the capacity of the 250 mm deep beams as they did not for some of the 300 mm beams. They enhanced the toughness in the 500 mm beams, but were not enough for the 700 mm beams. Also, a fiber cocktail may prove more effective than using a single type of fiber. The chosen steel fibers are competitive with conventional stirrups, according to this study. A nonlinear truss model is devised to predict the load-carrying capacity of the beams. The model gave good predictions for beams without stirrups and depths up to 500 mm

Connections with glued-in rods are very efficient and high strength joints, and can be successfully used for e.g. anchorage of vertical timber elements to the foundation. Although extensive research was done on the pull-out strength of single rods, few references can be found on the behavior of multiple glued-in rods. This paper presents the results of an experimental program carried out on timber-to-steel connections with multiple glued-in rods. Two series of ten specimens with four large-diameter glued-in rods made from mild steel (series GB) and ten specimens with twelve small diameter high grade glued-in rods (series GS) were tested to failure under monotonic tensile load. Both types of connections failed ultimately in a brittle manner, however series GB developed some plastic deformations prior to failure by pull-out of two rods and longitudinal splitting of the timber member. By comparing the experimental results with the analytical predictions, no significant group effect could be detected in test series GB, and an overall acceptable approximation was found. In test series GS, no final conclusion on group effect could be drawn as several rods failed by nut stripping. A parametric study using analytical formulas was carried out to investigate the effect of rod strength, rod diameter, glued length, and timber density on the connection performance. Based on the results of the parametric study and experimental tests, it can be concluded that the use of mild steel as well as more rods of larger diameter properly spaced from each other and from the edges are all effective measures to increase the connection ductility. Furthermore, by leaving a suitable length of rod unbonded between the steel plate and the glued length, it is possible to control ductility, providing all brittle failure modes have higher strength capacity than the rod yielding force. Special care should be taken during the manufacturing process to ensure good quality of gluing (required glued length and amount of adhesive) and rod alignment.

Old structures are frequently modified to comply with current living standards and/or legislation. Such modifications may include the addition of new windows or doors and paths for ventilation and heating systems, all of which require openings to be cut into structural walls. However, effects of the required openings are not sufficiently understood. Thus, the objective of the work reported here was to analyze openings’ effects on the axial strength of large concrete wall panels. Three half-scaled walls with two opening configurations, corresponding to “small” and “large” door openings, were subjected to a uniformly distributed axial load with a small eccentricity. The results indicate that the 25% and 50% reductions in cross-sectional area of the solid wall caused by introducing the small and large openings reduced the load-carrying capacity by nearly 36% and 50%, respectively. The failure progression was captured using digital image correlation technique and the results indicated involvement of a plate mechanism rather than uniaxial behavior as adopted in current design codes. Using a simplified procedure, the load-carrying capacity was predicted using existing design models found in the research literature and design codes.

A finite element (FE) model was calibrated using the data obtained from a full-scale test to failure of a 50 year old reinforced concrete (RC) railway bridge. The model was then used to assess the effectiveness of various strengthening schemes to increase the loadcarrying capacity of the bridge. The bridge was a two-span continuous single-track trough bridge with a total length of 30 m, situated in Örnsköldsvik in northern Sweden. It was tested in situ as the bridge had been closed following the construction of a new section of the Railway line. The test was planned to evaluate and calibrate models to predict the load-carrying capacity of the bridge and assess the strengthening schemes originally developed by the European research project called Sustainable bridges. The objective of the test was to investigate shear failure, rather than bending failure for which good calibrated models are already available. To that end, the bridge was strengthened in flexure before the test using near-surface mounted square section carbon fiber reinforced polymer (CFRP) bars. The ultimate failure mechanism turned into an interesting combination of bending, shear, torsion, and bond failures at an applied load of 11.7 MN (2,630 kips). A computer model was developed using specialized software to represent the response of the bridge during the test. It was calibrated using data from the test and was then used to calculate the actual capacity of the bridge in terms of train loading using the current Swedish load model which specifies a 330 kN (74 kips) axle weight. These calculations show that the unstrengthened bridge could sustain a load 4.7 times greater than the current load requirements (which is over six times the original design loading), whilst the strengthened bridge could sustain a load 6.5 times greater than currently required. Comparisons are also made with calculations using codes from Canada, Europe, and the United States.

Flexural buckling is one of the main problems steel structures are faced with in ensuring an economic design. In Europe, the buckling resistance is calculated using an imperfection factor based on the section type, fabrication method, and steel grade. The current European design standards contain guidelines for the imperfection factor for sections made of steels with yield strength up to and including 700 MPa. However, the current design codes are based mainly on tests performed on steels with yield strength below 460 MPa. Therefore, the applicability of the methodology was reassessed. This paper reviewed the background documentation of the European flexural-buckling design methodology and discussed the current design practice described in the American National Standard. A total of 72 flexural-buckling experiments performed on cold-formed, hot-finished, and welded sections made of steel with yield strength in the range 690–960 MPa were collected and analyzed. Four models for estimating the resistance of high-strength steel struts subjected to pure compression were statistically evaluated based on the collected data. Finally, a recommendation for the estimation of flexural-buckling resistance of high-strength steel members is presented.

For reinforced concrete (RC) slabs without shear reinforcement, shear and punching can be the governing failure mode at the ultimate limit state if subjected to large concentrated loads. Shear and punching of RC slabs without shear reinforcement has been a challenging problem in assessment based on current standards. To examine a previously developed enhanced analysis approach, this study was conducted by applying a multilevel assessment strategy to a 55-year old RC bridge deck slab subjected to concentrated loads near the main girder in a field failure test. This strategy clearly provides the engineering community a framework for using successively improved structural analysis methods for enhanced assessment in a straightforward manner. The differences between analysis methods at different levels of assessment were discussed regarding one-way shear and punching shear behavior of the slab. The influences of parameters, such as boundary conditions, location of concentrated loads, and shear force distribution, were investigated.

A nailed joint system, comprising of two slotted-in steel plates with shot-through smooth round steel nails in glulam, was analyzed from the viewpoint of an engineering description of the ductility behavior. Experiments on 15 specimens, with 10 nails in each specimen, were performed. Based on the results, a model describing the ductility behavior is proposed. Ductility is defined as the ratio of ultimate-to-elastic deformations with the ultimate deformation defined as the first load drop corresponding to wood splitting. The model is based on "engineering" parameters and estimates the engineering ductility from the slenderness ratio, timber density, and the fastener yield strength in bending. The model is compared with similar experiments in the literature and the validation shows correlation. The method of solution, based on the plastic theory of the European yield model for dowels, is general, and is applicable to other timber-to-timber or timber-to-steel dowel-type connections. The engineering ductility philosophy can give engineers a tool to assess ductility for connections in timber structures.