The main objective of this report was to experimentally investigate and describe the flexural behavior of multiple beams when tested in the laboratory under 4 point bending. The observations will be made by observing several failure modes including yielding, cracking or crushing behaviors.
APPARATUS AND EQUIPMENT:
- Timber beam
- Timber beam which has been laminated.
- Metal beam (steel)
- Beam made of concrete and that has little reinforcement
- Beam made of concrete and that has much reinforcement
- 4-point bend testing machine
- Each of the beams’ widths and thicknesses were measured and recorded
- The loading block was set on the 4-point bend apparatus and gripped at the upper and lower gripping heads respectively.
- The first beam (timber beam) among the five was placed in position at the center of the machine with its upper surface to the side.
- The machine was then operated to grip the top of the beam surface. It was ensured that there was full contact between the specimen and the apparatus.
- The necessary parameters on the testing apparatus were fixed including setting the pointers at the zero mark.
- When everything was determined to be okay, the machine was switched on to start the test.
- Immediately the apparatus started applying force, the beam was observed with increasing loading.
- Readings were taken on the amount of loading as the process continued until the beam failed.
- The same procedure was repeated for the other four beams which were: laminated timber, steel, under-reinforced, and over-reinforced beams.
RESULTS AND DISCUSSION:
The four-point bending test is such that two transverse vertical loads are applied to a simply supported horizontal beam such that a constant bending moment is obtained in between the two inner load positions (Kim et al, 2008). The upper loads thus thrusts the beam downwards against the static roller supports. For the timber member, some deflection was observed which increased gradually with the increase in loading. As such, the strain increment on the beam was observed within a certain linear elastic range, after which cracks started to occur. Loading is done using the two loading cells that have high units of force. Strain gauges were used to measure this deflection on the timber beam. The loading was done at a steady rate which ensured a smooth application of weight. Thereafter, a load deformation graph which enabled the visual observation for the different phases such as elasticity, plastic limit, cracking, and finally failure during the testing.
According to the graph that was obtained, the deformation behavior for the timber beam was linear till failure occurred. The distribution of strain as seen from the graph was linear until failure occured. This is illustrated in the photo below:
LAMINATED TIMBER BEAM
The same procedure was carried out on the laminated timber beam and observations made. It was noted that the behavior of the two was almost similar but the only difference is that the ultimate failure for the laminated beam occurred at a higher value as compared to the timber member. Also, the strain recorded per unit loaded was minimal in the laminated member as compared to the former. As such, similar graphs were obtained for this case as above.
In the case of the steel beam, the procedure for loading was taken and observations made. The steel beam underwent deflection whereby the strain was recorded until a maximum yield point was reached. Steel undergoes two phases which are: the elastic phase and the plastic phase before failure finally occurs. In regard to this, a graph was drawn that showed the relationship between the flexural load against the flexure extension which is shown below.
From 0-250 N – (A-B) Steel behaved in an elastic manner
From 250-400N – Steel underwent plastic phase
From 400- Steel was strain hardening
At point D- There was failure/fracture of the steel
The under-reinforced beam is a concrete reinforced beam that contained a slightly lower quantity of reinforcement per unit cross-section area. As such, it has more concrete surface area and relies more on the strength of concrete than that of the reinforcement. The same procedure for loading was applied on the beam and observations made. In this case, the concrete behaved in a ductile manner as it is not elastic and since it was in large amount the reinforcement did not have an effect on it. As such, the beam failed by cracking first and then it crushed at an instant on gradual application of the loading without notable or significant deflection. There were clear, large, open flexural cracks. The beam showed ductile behavior due to steel yielding. The load vs deflection curve that was obtained for this beam is as shown below:
Using a conservative coefficient and a realistic coefficient, the moment that caused cracking is determined. Also, the ultimate moment Mult was determined from sectional analysis (where fsy = 550 MPa, bar is 10 mm diam) (Kim et al, 2008) The values obtained from these two procedures were used to record the load vs deflection graph.
This beam is taken through the same procedure as the under-reinforced one and the observations noted. In the case of the over-reinforced beam, the amount of reinforcement in that member is quite substantial hence the beam behaves in a brittle behavior since the reinforcement does not yield. The concrete here is thus the weaker link in this case since the bar diameter is large. There were fewer flexural cracks observed in this case.
There are several observations that were made as a result of the observations made in these tests. To begin with, the under-reinforced beam reached the ultimate stress limit at a lower value as compared to the over-reinforced beam. Also, it was evident that different beams behaved differently depending on the material they were composed of.
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Kim, Y., Gordon Wight, R., and Green, M. (2008). Flexural Strengthening of RC Beams with Prestressed CFRP Sheets: Using Nonmetallic Anchor Systems. J. Compos. Constr. 12, 44–52
Nilson, A. H., Darwin, D., and Dolan C. W.,Edition, (2006), “Design of Concrete Structure”, McGraw
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