Double beams: Comparing the performance26. February 2020
Ceiling loads can increase when buildings are converted, for instance when a dance studio is installed above an apartment. The existing ceiling beams should be preserved and reinforced using the most effective solution while also being as non-invasive as possible. The installation height should also be kept as low as possible so as not to reduce the height of the ceiling too much. Carpenters can opt for double beams in this case. We compared the performance of a full cross-section to a screw-fastened cross-section.
We tested whether a screw-fastened cross-section can keep up with a full cross-section at our fischer academy, with surprising results.
Double beams for experts: We carried out a test with our testing device
During the following experiment we compared the performance of a full cross-section with a screw-fastened cross-section. The full-thread Power-Full screw was used for the experiment.
Step 1: Two 120/120 mm beams lie loosely on top of one another on the testing device.
Two 120/120 mm beams made of GL24h glue-laminated timber are loosely laid on top of one another on the testing device. The beams are then put under pressure up to a deflection of 20 mm with a force of F = 3.6 kN.
Under these parameters this corresponds to a deformation of around l/188 (supporting width – 3.75 m – divided by 188). The permissible calculated load is 7.5 kN at a deflection of 20 mm.
Step 2: The beams are screwed together
The two beams that were lying loosely on top of one another are now screwed together using full-threaded screws (Power-Full, diameter 8.0 x 295 mm). Holes are predrilled into the beam at a 45-degree angle and a depth of approximately 50 mm to fit the screw’s core diameter of 5 mm.
The screws can also be inserted without the need for predrilling by being placed directly on to one of the beams and screwed in at a 45-degree angle using fischer’s screw adapter. Over the course of this step of our experiment, a total of 96 screws are inserted in rows of two.
Step 3: Beam camber on the testing device
The two beams that still need to be connected are now placed back on the testing device, creating a camber or deflection of around 30 mm. This might be caused by jack posts on a construction site, for instance. All of the screws are now fully inserted into both beams.
Intermediate status: After releasing the testing device the deflection goes back down to 20 mm, corresponding to l/188.
Step 4: Beam camber
We now take the screw-fastened beams off the testing device, turn them upside down and place them back on the device with a camber of 20 mm.
The distances between the screws are determined via the structural engineering calculation and tend to follow the shear line. They are arranged diagonally from the bottom of the middle of the beam up towards the support.
The screw-fastened beam cross-section is once again placed under pressure using the testing device.
Intermediate status: The camber has been used up, the beam is horizontal and we have a deflection of 20 millimetres (compares to the initial state). The total force applied during the experiment is 14.5kN.
The beam is now bent even further. When bending the beam by 40 mm, a force of 38.7 kN is applied in our experiment. At a deflection of 20 mm, the calculated load should be around 27 kN in addition to around 22.5 kN of bending stress.
Next, we increase the load until the beam splits. Once we’ve reached a deflection of 50 mm and a force of 40.6 kN it finally happens – a split occurs in the lower part of the 120/120 mm glue-laminated timber beam cross-section.
Comparison between full cross-section and screw-fastened cross-section
Over the course of the next step we compare the screw-fastened cross section with a 120/240 mm full cross-section of GL24h glue-laminated timber. These are ideal conditions from a static point of view, but are often not a viable option, for instance during refurbishments. We now apply force to the full cross-section. At a deflection of 20 mm and a force of 24 kN the beam is no longer fit for use. At a deflection of 20 mm, the calculated load should be around 30 kN in addition to around 25 kN of bending stress. The total force applied is 53.5kN at a deflection of 40 millimetres.
In conclusion: Double beams are effective
The calculated load-bearing capacity of the double cross-section is around 89 per cent of the full cross-section. During our comparison test, the load-bearing capacity of the double cross-section was 1.6 times higher than that of the full cross-section at a deflection of 20 millimetres.
Our comparison was merely a one-off test, of course, and should not be used as a reference for future construction projects. Nevertheless, double beams are certainly effective and ideal for refurbishments of conversions, for instance when installing a photovoltaic system on a roof. This approach also requires barely any structural intervention.