CONNECTIONS FOR CONTINUOUS FRAMING IN PRECAST CONCRETE STRUCTURES

Transcription

1 CONNECTIONS FOR CONTINUOUS FRAMING IN PRECAST CONCRETE STRUCTURES G.Krummel PEIKKO GmbH, Waldeck, Germany Abstract Connections for continuous framing in precast concrete structures have been a problem almost impossible to solve. A frame system is more economical in comparison with to the standard system for precast concrete elements, consisting of a rigid column to foundation connection and a jointed beam to column connection. But the rigid beam to column connection was difficult and expensive to realise. Therefore an easy and economical bolt system has been developed. This system allows a fast assembling of the column to the foundation and a fast assembling of the beam to the column, independent of weather conditions. The roots of this system are in Scandinavia / Finland. 1. Introduction The economical aspect and the details of precast concrete frames have been researched and tested in the ELECON [1] project. The most common ways of rigid column to foundation and column to beam connections in Europe have been compared with a bolt system to screw the precast concrete elements together. Two bolted rigid frame systems were studied to clarify how big material savings can be obtained. Finally the assembling time of the different frame types was compared. The static calculations have been proceeded according to ENV 1992, the anchoring of the headed bolts is calculated according to concrete capacity (cc-designing). 1233

2 2. Bolt based frame system 2.1 Column-foundation connection Details The common ways for rigid column anchorage have been socket bases and grout sleeve bases. Figure 1: Socket foundation Figure 2: Grout sleeve base These systems are expensive and costly. The socket requires a deep foundation. The column is fixed by wedges during assembling time and might need a support (tall columns). The grout- sleeve base carries the loads by overlapping in the foundation and has to be supported during assembling time until the grouting is hardened. The bolted connection consists of the following parts: 1. Anchor bolts (base bolts), which are placed to right position before casting. 2. Column shoes, which transmit forces from the column to the anchor bolts. There can be 4 shoes in every corner and shoes in the middle, too. The shoes are placed entirely inside the column thus the formwork can be done easily. 3. After the column has been erected and the anchor bolts are tightened, the gap between column and foundation is grouted with non shrinkage mortar. 1234

3 Figure 3: Bolted column foundation connection The connection is also suitable for column to column joints Load transfer The column shoes transmit the loads normally by overlapping with the longitudinal reinforcement of the column. The load transfer after grouting can be calculated according to ENV The compression force is taken by the mortar and the bolts, the tensile force from the bending moment is taken by the bolts. The stiffness of the joint depends on the size of the cross section and the diameter of the bolts. The shear force can be taken by the bolts, the concrete section or an extra shear dowel. During assembling time all loads can be taken by the bolts. The anchoring of the bolts depends on the dimensions of the foundation. Anchor bolts with headed studs can be calculated according the German approval (HPM/L, PPM/L anchor bolts) or a truss and tie model. Other anchorings like overlapping etc. according ENV 1992 are also possible (HPM/P, PPM/P anchor bolts). A special software to calculate the load transfer and the anchoring is available. Figure 4: Column shoe Figure 5: HPM/L anchor bolt 1235

4 2.1.3 Assembling A levelling plate is adjusted in the right level on the foundation. The lower nuts are screwed under the right level, the column is screwed with the upper nuts in the right direction. After that the lower nuts are tightened to the base plate of the column shoe. It is a rigid connection already during assembling state and no extra support for the column is needed. Finally the gap between the foundation and the column is grouted with an non shrinkage mortar. 2.2 Beam to Column connection Details The connection between column and beam was basically realised by welding joints or grout sleeves. Both systems are difficult during assembling time and can be very expensive. The bolted joint consists of the following parts 1. Anchor bolts with or without muffs cast in the column 2. Beam shoes cast in the beam (similar like column shoes) 3. Corbel for shear force- if required 4. Niches on the columns face and the end of the beam for shearforces 5. After the beam has been erected and the anchor bolts are tightened, the gap between column and beam is grouted with non shrinkage mortar. Figure 6: Bolted connection Three different connections are mainly used: 1236

5 Connection between a column and a precast beam: Figure 7: The beam is screwed to the column, the beam shoes can have a long hole. The top connection is realised by bolts with muffs or couplers. Connection between a column and a half precast beam: Figure 8: The beam is screwed to the column, the upper connection is realised by couplers and are casted in situ, depending on the floor system (f.e. filigran floor system). 1237

6 Connection between column and precast top beam Figure 9: The top beam is assembled in the same way as the column to column or column to foundation connection. The bars of the column shoes can be bent according to the structure of the top beam Load transfer The beam shoes carrying the normal forces by overlapping with the longitudinal beam reinforcement. The bolts are screwed with the beam shoes and transfer the loads inside the column. The shear forces are normally taken by corbels, depending on loads. Niches in the column and in the beam are also possible to take shear forces after grouting with non shrinkage mortar. The anchoring is mainly calculated to ENV 1992 (f.e. overlapping, anchor plate). The usage of headed studs for anchoring the tensile forces has been tested. Mainly it can be calculated according to cc- method and the tie and truss model Assembling A levelling plate is adjusted in the right level on the corbel. The beam is seated on the levelling plate, the nuts are tightened. The gap and the niches are casted with non shrinkage mortar. 3. Economical aspects When the elements have been screwed together, the connection is rigid and need not to be braced. The assembling is independent of weather conditions. The hardening time of the grout is not critical. The erection time of two one storey high frames (bolted system and socket foundation) has been compared [1], 35% savings of assembling time are possible to reach with a bolted column foundation connection in comparison with a socket foundation. 1238

7 The assembling time for the bolted system to fix a beam rigidly to a column is much shorter than the welding joints or the grouted sleeves. The erection is independent of weather conditions, the system is rigid during assembly time. The material savings for a frame system have been tested and researched [1]. The amounts of concrete and steel can be reduced by up to 30% with a rigid frame system in comparison with the common system with hinged beam to column connections. The material saving starts with the foundation. The quantity of topsoil which has to be removed is smaller in comparison with to the socket foundation, the length of the column can be shorter. The rigid beam to column connection reduces the bending moment for the column foundation connection in comparison with the hinged joint. 4. Conclusion Bolted rigid frame systems for precast concrete elements offer an economical and fast are more than an alternative to the common connections for precast concrete elements. These systems allow an easy and economical assembling for rigid column to foundation and rigid column to beam connections. The assembling advantages for steel constructions are translated to precast reinforced concrete constructions. 5. References 1. ELECON project: Research project for bolted connections (Bolt based connection for precast frames) 1996, Tampere University of Technology 2. Deutsches Institut für Bautechnik (DIBt), Berlin 1997: Bemessungsverfahren für Teräspeikko Ankerbolzen HPM/L 3. Technical data PEIKKO GmbH 1239