Construction-Stage Torsion, Propping and Deflections in Composite Beam Design
Composite beam design - Torsion - Construction (#3594)
q:
I have composite ASB beams in MasterSeries frame model. The building is a large multi-story office building. The beam length is 9.6m, with span one side 11.2m and 6.4m the other side, therefore I am concerned with torsion particularly during construction.
The beam I am looking at fails on torsion during construction. I understand there is an option to prop the beam, this then makes the error disappear. Is there a way of inputting/ seeing the number of props that are required and at what centres?
In addition, could you please clarify what calculations are being carried out in the unpropped case e.g. is the 'side 1' in the calculations the longest span e.g. 11.2m and 'side 2' the shorter span e.g. 6.4m? - Is there a way of seeing these calculations in more detail as it is hard to follow the symbols and where each number is coming from?
a:
MasterSeries allows you to prop the beam during construction – this assumes the beam is supported sufficiently along its length. However, MasterSeries doesn’t check the number or centres of props required. That is left to the user’s discretion.
If the flooring is ‘solid’ and dims of a precast concrete unit entered, then an out-of-balance combined torsion and bending check is produced, checking each side loaded as the possible worst case. These calcs are shown in some detail. Further information on the design process can be found in SCI P401 Design of Composite Beams using Precast Concrete Slabs.
Composite beam design - maximum moment capacity (#4231)
q:
I am designing a multistory office structure with composite ASBs. The span on either side of the loaded ASB is 9.6m and 6.4m, hence torsion during construction is a concern. The sequence of construction is to erect the beam, place plank units on both sides and then prop prior to the topping being poured (hence only the loading case to consider is when one side is loaded with the planks and construction).
The construction stage calculations in composite beam design calculates the out of balance moments when the longest side is loaded with the plank unit and construction and says that my ASB passes. However, above that it calculates a 'max moment' which has a warning. I am hoping you could explain to me how this moment is calculated (with what loads) and if this warning is relevant to my design given the sequence of construction described above?
a:
The construction stage checks look at the following:
1. Loaded on one side - reduced dead and construction load but torsional loading due to eccentric loading
2. Full construction stage load - loaded on both sides so increased load but no torsion.
Your beam is adequate for the loaded on one side case, but not for the full temporary construction stage load when the PC units etc are loading the beam on both sides but the compression flange is not yet restrained (as it's the temporary condition).
q:
The maximum moment value in the full construction stage load (which fails in my model) has loading from all planks, construction, and topping as this would be the worst case.
However, in my design, the maximum loading on the beam before propping is planks and construction only. Do the calculations confirm that the beam passes with plank (and construction) only load?
a:
The case of planks, construction load and no propping (prior to the addition of props) is not checked.
q:
I am wondering if you could also help me with the propping options for the same project composite beam. Again, the construction sequence is the steel beam is erected, the planks/ slabs are placed on, the slab is propped, then topping is poured.
I am hoping you could confirm how it effects the calculations if 'propped' or 'unpropped' is chosen. Does this mean that the beam is propped?
I notice there is an option to choose a number of slab props, should the 'propped' and 'number of slab props' options be used at the same time, and what is the consequence for this?
In addition, in my model - looking at Serviceability Limit State super imposed dead load deflection - when the number of slab props increases, the superimposed dead deflection increases. As I understand it, the super imposed dead is the finishes load applied, could you please further explain why this might be?
a:
The software provides two propping options aimed at modifying the behaviour at the construction stage.
1. Propping of the beam - this stops the deflection of the beam under the slab and topping at the construction stage when the beam will be acting in a non-composite case.
2. Propping of the slab - this reduces the amount of load on the beam at construction stage by reducing the span of the slab.
It's possible for both to be used, but this is a construction issue and should be confirmed with the contractor.
When the beam is propped at construction stage, then self-weight of the slab and finishes are applied at the same time in the composite stage. Thus the stress in the beam is different at the point in time when the superimposed dead is applied, which affects the partial shear connection. The dead load (self-weight) will be lower since the self-weight is taken to only be applied to the beam once the props are removed so overall the deflection is reduced. A similar effect can occur with the slab propping since again the load on the beam at different stages affects the stresses and so the partial fixity that occurs at the various stages of loading.
The following TEch Note is on a slightly different topic, but might give some insight into how the deflections on a composite beam are compiled at the various stages. You can see how this compares with the case when propping is use,d since whether loads are applied at the non-composite stage or after the concrete has cured and composite action is considered to have developed can change the results.
The construction stage checks in the software are derived from the various SCI composite design guides. There are a number of these and it may be worth reviewing these if you have access to them.
q:
The technical note says that in graphical analysis - from load case 006 onwards, composite beams are analysed based on composite cross section.
Live loads are applied after construction when the beam is acting compositely. So I would expect the composite live load from graphical analysis to be the same as the live load in the composite beam analysis. But the live load in composite beam design is much higher. It also cant be explained by construction live loads. Could you explain why this might be please?
In addition, the slab is propped during construction. - The superimposed deflection when slab not propped is 7mm, but when 2 propped slab it is 20.5mm. Could you please clarify why superimposed load changes with props?
a:
The underlying general issue here is how much slippage there is in the shear studs when different construction sequences are considered. When the beam or slabs are unpropped during construction the beam is taking the full wet concrete load and deflects under this load. The studs at this point are not taking load and so are not slipping.
However, if the beam or slabs are propped during construction, the beam is not taking the full dead load. After the concrete has hardened and the props are removed, the beam is now taking the full dead load and some stud slippage will occur even before the live load is applied resulting in an increase in deflection. When the live load is applied the deflection will increase further.
Most of this information is available to read in SCI publications P401 and P405. Could I suggest you have a read through these publications to clarify the issues, if you haven't already read them.
q:
a:
When using un-propped construction, the beam will be deflecting under its self-weight and the weight of the wet concrete, and due to the concrete being wet it will not be acting compositely.
When the concrete 'sets', you will have the added benefit of the slab acting compositely with the beam and therefore increased stiffness of the composite section and less deflection under the final stage live loads.
Hence, the dead and live loads may be similar values, but the beam resisting the live loads is now a composite beam.