LAB TESTING AND FEA MODELLING M.Haimana, M.Rakb, J.Krolob, Lj.Hercegb, V.Čalogovićb a Department for Structures, Faculty of Civil Engineering University of Zagreb, Kaciceva 26, 10000 Zagreb, Croatia, email@example.com
bTechnical Mechanics Department, Faculty of Civil Engineering University of Zagreb, Kaciceva 26, 10000 Zagreb, Croatia, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org,
This paper presents the results of the numeric and laboratory tests of the «T» section of EPS concrete-timber composite ceiling structures. Sheared connections of EPS concrete board and timber girder elements have been tested in several different ways by means of mechanical connectors. Various types of dowels were used for sheared connections. Shearing forces in the connection plane were assumed by means of dowels, then by means of nails, and by gluing the connection of EPS concrete and timber with epoxy glue.
All the tests were numerically modelled by means of FEA software COSMOS/M 2.8 and the results were compared with those of the laboratory tests.
Application of these structures in civil engineering practice is multiple, particularly in reconstruction of old timber girder floor structures or manufacture of new ones at the reconstruction of old buildings in areas exposed to frequent earthquakes.
At the reconstruction of ceiling timber structures, they can be strengthened with reinforced concrete board composed with the existing timber girder. The new lightweight EPS concretes weighting 1200 to 1400 kg/m3 provide new possibilities of application in composite structures. Besides very high solidity achievable with such structures, their advantage is also the increased fireproof quality and very good acoustic properties. Furthermore, such structures are of significantly lower weight applicable in areas exposed to frequent earthquakes in reconstruction of old timber girders or manufacture of new ceiling structures.
In efforts to test and determine the rules of behaviour of such structures at static loads, several types of dowels were tested in the laboratory. The tests were carried out within the framework of the scientific research program supported by the Ministry of Science and Technology of the Republic of Croatia at the Laboratory of the Technical Mechanics Institute Faculty of Civil Engineering, University of Zagreb.
2. LAB TESTS
Sheared connections of a timber beam and an EPS concrete board were done for four different cases of connections in the connection plane.
In Type 1, the connection was done with Ø25 mm steel dowels placed, two of them in a single section. The distance between the dowels in longitudinal direction was 200 mm. In Type 2, the same Ø25 mm steel dowels were used, but they were placed alternatively, 1 dowel in section, at distances of 200 mm. In these two types, the holes for dowels were of a slightly larger diameter than dowel diameter. Epoxy glue was placed in the holes and then the dowels were installed.
In Type 3, Ø6 mm nails were used for connection. In a single section, three nails were driven in up to the depth of 100 mm. Mutual distance between the nail groups was 100 mm.
Type 4 was tested with the connection by means of epoxy glue placed on timber beam elements directly before placement of EPS concrete.
Presentations of all the four tested laboratory samples are at Fig. 1 and Fig. 2.
Features of lightweight EPS concrete are the following:
Eclw=12400 MPa, σcc=14,5 MPa, σct=4,5 MPa, cs=2,5 MPa
and features of glued laminated timber beams are the following:
EIIv=12800 MPa, σII vc=55 MPa, σII vt=72 MPa and σIIvs=15,5 MPa.
Fig. 1. Types of Timber-EPS concrete composite beams
Fig. 2. Types of in-built connectors
The testing of laboratory samples by bending was carried out on beam systems with 3.2 m span.
The cross section of a glulam timber beam element was 160/180 mm, and the thickness of the EPS concrete board was 80 mm. The width of the tested «T» section was 600 mm.
Load was added by linear force at the midspan, and displacements were measured at the place of maximum expected shear and installed dowels, and vertical displacement in the midspan. Four LVDT sensors were symmetrically placed at the connection plane between concrete and timber. The sensors measure the relative deformation between concrete and timber. Seven LVDT sensors were placed at the midspan to measure deformations on concrete and timber. Three LVDT sensors for measuring the displacement were placed at the midspan and on the bearings. Two LVDT sensors were placed at the end of the beam to measure common sliding between concrete and timber. There were total 15 measuring spots connected to A/D converter and a laptop interface. To monitor and record data during the test, the “National-Instruments – Virtual Bench Logger” software was used.
The bending tests were done for two characteristic load levels.
The first load level was with concentrated linear force of 20 kN at the midspan. The force size is the accounting force of the permitted load. The other level of load was by force until failure, corresponding to breaking load of the testing models.
Measuring places are presented on Fig. 3., and test of one of the models is presented at Fig 4.
Fig. 3: Review of the measuring places
Fig. 4: Testing of EPS concrete composite beam Type 1
Behaviour of the tested composite girders within the limits of permitted load up to 20kN force is almost linear, with small differences in midspan deformation, as seen on Fig. 9. Very high differences were obtained by laboratory test of each of the aforesaid types until failure as presented on diagram (Fig. 5). Types 1 and 4 have shown the best bearing capacity characteristics until breaking, while Type 3 with the nails yielded the worst test results.
Failure of all the tested types occurred by exceeding the tensile strength of the glulam timber beams.
Fig. 5: Force – deformation ratio until failure, for tested types 1 to 4