WHFF Project: 2019.10
Authors: Ernst Gehri ( ETH Zurich), Pedro Palma & René Steiger (Empa) and Thomas Strahm (neue Holzbau AG)
The short video about the project on Youtube can be watched under the following link (only available in German): https://youtu.be/q2eJPn3biCY
The most important facts in brief
Test methods for the experimental determination of shear stiffness and shear strength of beams (esp. of hardwood glulam) were investigated.
In 3-point bending tests on spruce glulam beams of strength class GL 32c with beam heights of 480, 600 and 800 mm, mean values of shear strength and shear modulus of 4.1 N/mm2 and 600 N/mm2, respectively, were determined.
From the few preliminary tests on one each of 400, 500, 600 and 700 mm high beech glulam beams of strength class GL 48c, mean values of shear strength and shear modulus of 9 N/mm2 and 1000 - 1100 N/mm2, respectively, were determined.
From the 3-point and 4-point bending tests on ash glulam beams of strength classes GL 40c, GL 48c and GL 48c "Special", mean values of shear strength fv,mean or shear modulus Gmean of 10 to 12 N/mm2 or 1120 to 1160 N/mm2 were obtained.
For building practice, it is usually sufficient to prove that the specified product values are met or that the test values exceed these requirements. For this purpose, the simpler 3-point flexure test configuration is sufficient
Project description
In building practice, wood is very often used in the form of flexural beams made of solid wood or glulam in timber structures. Engineers have to verify in structural calculations that a flexural beam meets the requirements of the building standards regarding structural safety and serviceability.
This includes, among other things, the verification of sufficient bending and shear strength and a calculation of the expected deflection as well as a comparison of this value with specified limit values.
In addition to the flexural strength fm and the flexural modulus of elasticity Em, the central mechanical properties in the above-mentioned verifications are the shear strength fv and the shear modulus G. These values must be determined for the wood products used in building practice in accordance with the specifications of testing standards. While the experimental determination of flexural strength and flexural modulus of elasticity can be carried out in a relatively simple manner with a 4-point flexural test, it is significantly more difficult to determine a value for the shear strength and shear modulus that is suitable for building practice and standardization.
This research project therefore aimed to investigate test methods for the experimental determination of the shear stiffness and shear modulus of beams (esp. of hardwood glulam) for two areas of application, namely.
for research, where it is important that exclusively shear fractures occure in the tests and where it is possible to realize complex test setups and to use advanced measurement methods (e.g. image correlation measurements); and
for glulam production operations, where it is important to operate with simple, low-cost test setups and measurement and evaluation methods, and where quality control during initial testing (and, if indicated, during factory production control) focuses on verifying the declared properties of the glulam product.
Furthermore, more precise knowledge of the influence of the component size on the shear strength (influence of the cross-sectional dimensions, influence of the geometric dimensions of the test configuration) of glulam beams made of softwood (spruce) and of hardwood (beech and, in particular, ash) should be developed and, from this, valid relationships for recording the size influence in the shear strength should be derived for construction practice.
Conclusions
The results of the research work carried out can be summarized as follows:
In 3-point bending tests on spruce glulam beams of strength class GL 32c with beam heights of 480, 600 and 800 mm, mean values of shear strength and shear modulus of 4.1 N/mm2 and 600 N/mm2, respectively, were determined. The dependence of the shear strength (shear stress at the location of shear failure) on the beam height can be described, starting from a reference beam height of 600 mm, by the function fv = 4.0⋅(600/h)0.76.
From the few preliminary tests on one each of 400, 500, 600 and 700 mm high beech glulam beams of strength class GL 48c, mean values of shear strength and shear modulus of 9 N/mm2 and 1000 - 1100 N/mm2, respectively, were determined. The dependence of shear strength (shear stress at the point of shear failure) on beam height can be described by a function in the form fv = constant-h-0.40.
The focus of the investigations was on 3-point and 4-point bending tests on ash glulam beams of strength classes GL 40c, GL 48c and GL 48c "Special", the latter with 3 lamellae each of strength class T50 without finger-joints on the bending tensile side to prevent the beams from failing in bending before a shear failure occurs.
From the tests, the average values of the shear strength fv,mean and the shear modulus Gmean were 10 to 12 N/mm2 and 1120 to 1160 N/mm2, respectively. The dependence of the shear strength on the beam height h can be described with a function in the form fv = constant-(600/h)0.35, starting from a reference beam height of 600 mm, based on the results of the 4-point bending tests.
From the 3-point bending tests, the relationship fv = constant-(600/h)0.20 was obtained. These research results on ash glulam were put into practice in the publication Lignatec 33/2021 "Bonded hardwood products for structural use".
In Lignatec 33/2021, the mean value of the shear modulus of ash glulam is given as Gmean = 1000 N/mm2 and the shear strength, lying on the safe side, with a design value of fv,d = 3.2 N/mm2. The correction function to account for the influence of component size on shear strength was given in Lignatec 33/2021 in the form fv = constant-(600/h)0.25. The decisive factor for all shear tests on glulam beams is to ensure that the force application corresponds as closely as possible to the shear flow. This is not possible, or only possible for small beam heights (and for beams made of spruce), by means of a force application exclusively via transverse pressure in the wood.
Due to the higher shear strength of hardwood glulam, the introduction of the high forces required to generate shear fractures in hardwood glulam can practically only be carried out with glued-in threaded rods. In the tests carried out as part of the present research project, it was shown that it is possible to introduce the necessary high forces into the test specimens by reinforcing the force introduction zones in the area of the load introductions and the supports by means of glued-in threaded rods of the GSA® system, supplemented in some cases by fully threaded screws screwed into pre-drilled holes.
In order to investigate the influence of the ratio of the shear field length to the beam height α = Lv/h, or to determine the corresponding optimum α-value, tests were carried out in the present research project on beams with different α-values between 1.11 and 2.5. As reconfirmed by the tests, a standardized basic test configuration in the form of a 3-point bending test with a ratio between shear field length and beam height of α = 1.6 - 1.8 is recommended for determining the shear strength in order to avoid bending fractures and to obtain a shear stress distribution in the shear field that is as pure as possible. For this range of α values, similar shear moduli were also obtained from the 3-point and 4-point bending tests.
For construction practice, it is usually sufficient to demonstrate that the specified product values are met, or that the test values exceed these requirements. The simpler 3-point flexural test configuration is sufficient for this purpose. However, a deeper insight into the shear behavior cannot be gained with this test configuration. The so-called "alternative 4-point flexure test configuration" used in this project is suitable for this purpose. Although this is more complex, it makes it possible to generate higher shear stresses and test longer shear fields with the same required forces. Compared to 3-point bend tests, this type of test allows the field length to be doubled for the same bending moment and identical transverse force. The influence of the transverse compressive stresses on the shear stress can thus be virtually eliminated.
Test methods for the experimental determination of shear stiffness and shear strength of beams (esp. of hardwood glulam) were investigated. value linked to a regulated procedure (testing, dimensions, evaluation), in research the shear behavior of glulam beams is to be recorded as holistically as possible.
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You can find more information about the project on ARAMIS.
The project was supported by the Forest and Wood Research Funding Switzerland FOWO-CH of the Federal Office for the Environment BAFU and the Conference for Forests, Wildlife and Landscape KWL of the cantons.