In the design of timber structures, the mechanical resistance in fire conditions has to be ensured among others. In the European standards, Eurocodes, the reduced cross-section method is given to determine mechanical resistance of timber structural elements in fire conditions. The reduced cross-section method is based on an effective cross-section that is determined by two key parameters, namely the charring depth and the thickness of zero-strength layer where the latter accounts for the reductions of the stiffness and the strength of timber due to elevated temperatures. Although, the thickness of zero-strength layer of 7 mm is only prescribed for the ISO 834 standard fire exposure longer than 20 min in the Eurocodes, the same value is often used for non-standard fire exposures as well, which might not always be correct. Thus, in the present paper the thickness of zero-strength layer is investigated for a timber beam exposed to 44 different parametric fires by means of advanced numerical models and a simple design method. A hygro-thermal model and a mechanical model are applied to determine the temperature field over a timber beam cross-section and the mechanical resistance of the beam during fire exposure, respectively. The simple design method is based on the reduced cross-section method. The obtained results show that the thickness of zero-strength layer varies between 8.4 mm and 30.5 mm, which are substantially higher values than the value of 7 mm suggested in the Eurocodes for the standard fire exposure.

COBISS.SI-ID: 42650371

Finger joints with standard geometries and adhesives used for joining coniferous wood are not sufficient in terms of strength when gluing beech wood. A numerical model of finger joined beech layers was prepared and simulations of tension tests were made. Parametric studies with multiple variables referring to geometrical properties of finger joints and two different types of applied adhesives were performed. The results showed a high influence of the finger joint geometrical parameters. Experimental tests on the tension strength of the finger joints were performed. To validate the numerical model, 20 mm long finger joints were produced and tested. Two types of adhesives were used, MUF and PRF. The finger joints were pressed under two different pressures. Prior to tension testing, the dynamic modulus of elasticity was measured with a strength grading machine and strength grade assignments were made. The influence of joining of the boards on the dynamic modulus of elasticity was analysed. The results of the tension tests of the finger joints showed a clear influence of the finger joint geometry, where highest strengths were obtained with longer and thinner fingers, while the influence of the adhesive seems to decrease with the length of finger joints.

COBISS.SI-ID: 29094659

The feasibility of estimation of mechanical properties of limestone concrete after exposure to high temperatures, using nondestructive methods, was investigated. Experimental study was carried out on two concrete mixtures that differed in water to cement ratio (w/c). After standard curing time, specimens were exposed to various temperature levels, i.e. 20?°C, 200?°C, 400?°C, 600?°C or 800?°C. Basic mechanical properties of concrete specimens were determined prior heating at ambient temperature. Once the specimens were cooled down to the ambient temperature, various nondestructive tests including ultrasonic (US) method, determination of rebound number, and resonant frequency method were performed. Further on, specimens were visually inspected to assess the damage of the concrete surface. To determine the residual mechanical properties of specimens after exposure to high temperature, the compressive and flexural strengths were determined on cubic and prismatic specimens, respectively. The main goal of presented research was the estimation of the compressive and flexural strengths of limestone concrete after fire based on developed relationships between results of destructive and nondestructive measurements performed on limestone concrete mixtures 1 and 2. The best relationship for estimation of compressive strength was determined based on measured fundamental torsional frequency, whereas the best relationship for estimation of flexural strength was determined based on combination of fundamental flexural frequency and US pulse velocity, both measured on prisms. The analysis of variance (ANOVA) showed that the influence of temperature on experimental results, obtained from destructive and nondestructive tests, is highly statistically significant for both mixtures. Posteriori test revealed that the majority of the measured quantities were significantly different on the temperature interval between 400?°C and 600?°C.

COBISS.SI-ID: 8878945

This paper presents a new mathematical model for analytical investigation of global buckling behavior of slender concrete-filled steel tubular (CFST) columns with circumferential gaps and partial debonding between the concrete core and the steel tube. The analytical buckling load of circular and slender CFST columns with circumferential gaps and partial debonding is derived for the first time. The critical buckling load decreases as the magnitude and length of the circumferential gap increases. Nevertheless, it is shown that if the length of the circumferential gap is smaller than the length of the CFST column, this effect is less than 4 %. On the other hand, for a fully delaminated CFST column, this effect can be up to approximately 40 %. Similarly, the first buckling shape modes proved to be notably affected by the circumferential gap only if its length is greater than 75 % of the CFST column length. The results can be used as a benchmark solution for the buckling problem of slender circular CFST columns with circumferential gaps and partial debonding between the materials.

COBISS.SI-ID: 1538074051

We have presented a novel energy preserving scheme based on velocities and angular velocities. Additionally, we preserve the kinematic compatibility equations. Interpolation of velocities in fixed basis and angular velocities in local basis allows us to use additive type of interpolation without any additional errors due to the non-linearity of configuration space and leads to relatively simple and robust solution method. Numerical examples demonstrate the accuracy, efficiency and long-term stability of the present approach.

COBISS.SI-ID: 8601953