| doi:10.3850/978-981-08-6218-3_CC-We019 |
 Final Paper PDF
|
SHEAR CONNECTIONS AND THEIR CLASSIFICATION FOR SHALLOW FLOOR COMPOSITE BEAMS
M. Leskela1 and S. Peltonen2
1Engineering Mechanics Laboratory, University of Oulu, Oulu, Finland.
matti.leskela@oulu.fi
2Peikko Finland, Peikko Group Oy, Lahti, Finland.
simo.peltonen@peikko.com
EXTENDED ABSTRACT
Composite steel and concrete flexural members are frequently considered as merely consisting of steel joists supporting the concrete decking on their upper flange and the identification of the shear connection is simple. However, there are also other types of composite members: in shallow floor beams the steel members support the concrete decking on their bottom flange. The objectives of the paper are to discuss the effect of various loadslip characteristics of the shear connection on the overall behaviour of shallow floor beams.
Due to the governing role of conventional composite beams in any design guidance the basic ideas of shear connection between material parts may be forgotten. However: for having composite interaction in any kind of composite member, a shear connection is required for balancing the changes in the normal stress resultants, but its location is not important and the connection can also be distributed in several locations within the interfaces between sectional parts. The state of bending stresses of the cross-section dictates the total connection force required for equilibrium, but the load-slip characteristics of the connection gives raise to variable longitudinal shear distributions. Three basic types of the shear-slip characteristics of the connections can be identified: hardening, plastic and softening ones. The hardening and plastic types are clearly ductile, as the shear force does not decrease for increasing slips but within the softening connections both brittle and ductile behaviours are identified. The most brittle connection behaviour is characterized as unzipping, as in such connections the resistance is gradually lost. When comparing the responses of composite beams with ductile and unzipping connections the most important features are not seen until slipping in the connection interfaces efficiently starts.
In beams with ductile connections it is possible to develop full plastic bending resistance of a composite section, whereas in the opposite case only the bending resistance of the steel section can develop in the worst case. Bending resistances between steel and full composite values are possible for partial ductile connections, for which a theory modified from that for the composite slabs may be employed. The development of the bending resistance in relation to the degree of connection is different from that for the ordinary composite beam: no lower bound for the degree of connection is required and a fair improvement of the resistance is seen even for small degrees of connection. A tendency sometimes forgotten is that any composite behaviour will improve when the span length or shear span is increased. Thus, for longer spans also the connections with unzipping characteristics will work better in maintaining composite interaction and resistance.
The unzipping connections appear typically in bond interfaces between steel surface and concrete, but the bond strength depends on the surface characteristics and appearance of friction and mechanical interlock, which influence the softening rate in the post-peak range of slipping. Unzipping interfaces frequently appear in the grouted joints of the hollow core decking supported on bottom flanges or ledges of shallow floor beams: although not generally understood, the unzipping connection is favourable for the resistance of the hollow core decking and can be controlled by the amount of joint reinforcement specified for keeping the ends of the decking in contact with the vertical shear interface of the beam. The critical issue in hollow core decking supported on beams is frequently the shear-tension failure in the webs of the hollow core units where additional shear stresses are induced by the composite interaction with the beams. These stresses increase in proportion to the efficiency of the interaction and are highest at the ends of the decking at locations where the vertical shear forces of the beam are highest.
Ductile efficient connections should only be employed in beams with solid types of decking, i.e. decking other than hollow core ones. In the CEN-standard EN 1994-1-1 a minimum slip capacity of 6 mm is required for ductile connectors independent of the span length. The requirement should not be considered general and justified for all span lengths and types of the composite member, however. This may be seen by looking at the slips in the connection interface at the ends of the member. Slips that appear in members with no shear connection are the greatest and depend on the span of the beam and they will give an estimate for the slip capacity required.
Shallow floor beams where most of the steel section is embedded in the concrete are ideal for a span range of moderate lengths and they also have good natural fire resistance. However, there should be no need to force the effective parts of shallow floor systems to be look-alikes of the conventional composite beams: the composite interaction will appear also in sections where the connection interfaces are distributed elsewhere than in the horizontal upper surface of the top flange. Special finite element method has been used to study the effect of having the shear connection in various possible interfaces and the outcome from this is that the location is not important, but only the total resistance in the interfaces. The partial connection theory for shallow floor members does not indicate any dependence on the location of the connection either.
There are two main types of the decking that can be integrated with the beams, hollow-core and solid types. The design principles for these are different and opposite to each other; when employing hollow core decking, the additional stresses induced to the decking units require to keep the interaction as inefficient as possible so as to improve the resistance of the decking. In case of solid types of the decking, the interaction should be designed for highest flexural resistance. When employing mechanical connection, it is not necessary to arrange it on the top flange of the steel section. In cases of partial shear connection there is no need to have a definite lower bound for the degree of connection (such as for ordinary composite beams, e.g. 0.4) and even for degrees below 0.4 the resistance of the composite section is fairly higher than the resistance of the steel section. For degrees above 0.8 the resistance is practically equal with the maximum theoretical value, i.e. improvement of the connection resistance has a negligible influence. Depending on the type of the steel member transverse reinforcing bars provide a natural way of arranging a ductile shear connection, where the resistance of bar may be comparable to that of the headed stud.