04.08 Composite sections
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Composite Action
Composite action is of primary concern when dealing with built up sections subjected to bending, which are common within mullions and transoms. The issue with composite action being to determine how the various discrete elements within the section interact and how they may be combined in order to ascertain the section performance. In most cases there will be a primary structural member with additional components attached via screws, gaskets and other fixings, image. If the primary section is sufficient in its own right to carry the applied loads then there is little reason to consider composite action. However, within mullion and transom sections the primary structural component is rarely directly loaded, particularly when considering suction wind loads. In such cases the load paths through the various components need careful thought in order to assess the performance of the overall section, and composite action may need consideration.
The two extremes or boundary conditions associated with composite action are the layered condition and the fully composite condition. The layered case considers each component to act independently whereas the fully composite case assumes the section to behave as if all the components are effectively contiguous or solid. Most practical sections lie somewhere between these extremes, although the majority closer to the layered state. Detailed analysis shows that the transition from layered to fully composite is not linear, being in fact almost a step function, thus for normal design purposes the identification of one of the boundary states will be a valid solution, presuming the correct condition is selected. If in doubt the layered condition will represent the worst case or weakest solution.
Layered condition
The layered condition considers each element within the cross section as acting independently with no interaction between adjacent ‘layers’ In effect simply laying one on top the other. A beam consisting of layered planks subjected to bending deforms with each layer sliding relative to its neighbour(s), image. In terms of stresses, there is no transfer of shear stress between the layers. Negating the effects of friction each layer acts independently carrying an appropriate percentage of the total applied load. In this case the performance of the beam as a whole, in terms of stiffness, is the simple sum of each layer in turn. Numerically this is achieved by adding the respective I values for each element. Thus for a layered section consisting of n layers the total I value becomes
Ilayered = I1 + I2+ I3 + ……… + In
The deflection response of the beam is then calculated through elastic theory and if a standard case simply using the appropriate equation, e.g. PL3/48EI. This presumes that the various layers are all of the same material, because the deflection equation links E and I. Only those I values with equivalent E values can be added. If different then the I values can be normalised in proportion to the stiffnesses.
Fully Composite condition
In this case all layers are considered contiguous with one another, and a fully composite beam would deform as its solid equivalent. The deformed beam would present no relative slippage between components. For no relative sliding to occur there must be full effective transfer of shear between the components. It is this transfer of shear that determines the composite nature of a section. The performance of the section will be determined through the I value of the section and calculated as if contiguous.
It is this condition of relative slippage and transfer of shear stress between and within components that is the critical criterion in the assessment of composite action.
The following presents a numerical example illustrating the numerical variation between layered and fully composite action.
Consider two beams of equivalent depth and breadth, but one being solid the other built up out of four equal layers with no effective connection between the layers. The respective I values for the solid and layered case will be as follows.
I value of individual layer = b ( d / 4 )3 / 12
I value for layered beam = 4 x b ( d / 4 )3 / 12
I value for solid beam = b d3 / 12
Thus the I value for the solid or fully composite section is 16 times greater than the layered section.
Percentage Composite action
The percentage composite action is determined with respect to the two boundary conditions. 100% composite action means fully composite i.e. an effectively contiguous section, and 0% composite action represents the layered case. The practical determination of the percentage composite action for a given section is at present best determined by test. This requires establishing the actual deformation of the section in question and comparing this with the calculated deflections for each boundary condition. There are analytical procedures available but they require detailed information about the shear stress capacity within and transfer between components which itself requires test data, which at present is not widely available.
Practical Design considerations
An example of a typical mullion section is shown here, image. Illustrating an aluminium box plus a thermal break, various gaskets and a pressure plate detail. Clearly the section is built up out of a series of individual elements or layers. In order to consider fully composite action no relative slip can occur and shear stress has to be transferred within and between all the individual elements as if contiguous. Any slippage and shear stress continuity is removed and the layered case dictates the solution. Clearly there is a disparity in stiffness between the aluminium components and the thermal break, plus what level of fixity is provided by the crimping action that connects the two. In most cases these factors are variable and uncertain, and the safest solution would be to ignore all elements in ‘front’ of the core mullion box section when considering the structural response of the overall section.
The wind suction load case is of particular concern. In this situation all the loading on the glazing panels is transferred to the pressure plate. Due consideration is required as to how this load is transferred to the structural box section. This will require a detailed assessment of the stiffness, shear and tensile capacity of the pressure plate components, fixing screws, thermal break and the crimped junction to the thermal break.