Joints in lattice girder structures

Report, 2005

To enable load-carrying in two directions in lattice girder systems, transverse reinforcement in the precast concrete panels needs to be complemented with lapped reinforcement across the joints at the construction site. To ease production, it would be beneficial not to have any reinforcement across the cast joint between the precast concrete and the in situ cast concrete within the splice region. However, this raises questions whether the cast joint will manage to transfer the needed forces. This was studied in the present project. Two different surface treatments of the precast elements were studied: one was brushed and the other had single grooves. In the studied detailing of the joint, a reinforcement mesh was placed in the in situ concrete across the joint, directly on the surface of the precast elements between the lattice girder trusses. Two types of detail tests of the cast joint were carried out: one type where the cast joint was loaded in shear and one in tension. Furthermore, the detailing of the joint between two precast concrete panels was tested in bending in full-scale tests. The detail tests were used to calibrate a model of the cast joint, which was then used in non-linear finite element analyses of the full-scale tests. The test specimens with surfaces with single grooves showed a large scatter in the detail tests loaded in shear; in all other tests the scatter was relatively low. Furthermore, the capacity of the cast joint was markedly higher for the brushed surface than for the surface with single grooves. In the full-scale tests, the joints were strong enough to carry the applied load. In all full-scale tests the failure mode was rupture of the reinforcement, and only one crack occurred: in the in situ cast concrete above the joint between the precast elements. However, the finite element analyses of the full-scale tests revealed that the detailing was sensitive for secondary cracking; when restraints from the cross-bars of the reinforcement mesh initiated bending cracks, the failure mode changed to fracture of the cast joint in the analyses. This happened in the analyses where the precast surface was modelled with single grooves. In the analysis where the surface was modelled as brushed, no secondary cracking occurred even when the restraints from the crossbars were included in the analyses. It is worth to note that in the full-scale tests, the cross-bars were placed directly on the surface of the precast concrete; accordingly they were most likely not so well encased and did not cause any larger restraints. Measurements of the strains in the reinforcement support this. Analyses of whole slabs were carried out to investigate the demands of the deformation capacity of the lattice girder joint. They showed that the load-carrying capacity of a slab depends on the rotation capacity of the lattice girder joints. Therefore, it is recommended that the rotation capacity is not limited by the horizontal cast joint. From the results in this study, it was concluded that the studied detailing of load-carrying joints between lattice girder slabs without any reinforcement across the cast joint is very sensitive to the roughness of the surface of the prefabricated elements. There is a risk of brittle failures; thus it raises questions whether reinforcement across the cast joint is needed to guarantee the structural integrity of a structure. It is further concluded that not all surfaces of the prefabricated elements used in Sweden today can be used. Considering this, it might however still be possible to use the studied detailing for load-carrying purposes. Very important demands are then that both the production of the surface of the prefabricated elements and the conditions at the work site must be controlled and checked on regular basis. Long-term effects such as shrinkage and creep were not included in the present study. This needs to be investigated. Furthermore, it is recommended that detailings with reinforcement across the cast joint are investigated, as the structure would become a lot more robust if that is included.