Staged Construction Analysis
Introduction
Staged construction analysis is a sophisticated method of solution which, if used with a reasonable degree of care can more closely reflect the real behaviour of the structure.
A standard 3D analysis takes no account of time dependant effects ‐ it assumes the structure has been constructed instantaneously. In the real world a concrete building has to be constructed in stages ‐ typically one floor at a time. Depending on the number of storeys within the building a significant period of time can elapse from the commencement of first floor to completion of the top floor.
Because concrete matures with time, this means that at the top floor the concrete has properties which ‘lag behind’ those of the first floor (and to a reducing degree each intermediate floor).
Another aspect of the staged nature of concrete construction is that any settlement of existing lower floors which takes place prior to the pouring of the current floor is allowed for on site when establishing the current floor level. This effect can not be allowed for in a standard 3D analysis, however it is catered for automatically in a staged construction analysis.
Simple Example
A model consisting of two blocks is constructed as shown below. The first floor plan and loading is identical in both blocks. A second floor framework is added to the right hand block but with no imposed load applied to that level.
A standard 3D analysis can be performed for the above, in which all members are analysed simultaneously with all the loads applied.
A staged construction analysis can also be performed. If the option were selected to stage the imposed load only, the analysis would proceed as follows:
- In the first stage, only the first floors are analysed for their imposed load.
- In the second stage, the second floor is added and the analysis repeated (for the undeformed model) but this time only for the imposed load at second floor level.
- In this way, each stage can only affect the result in it’s own and previous stages.
Results for Q - Unstaged
In the unstaged result you can see that Q load applied at first floor is causing moments to develop in the structure above. If that frame existed when the load was applied then this is completely logical.
Results for Q ‐ Staged
In the staged result no moments develop in the frame above the first floor members and in fact the forces in the first floor structure are identical in the single storey frame and the two storey frame. This is logical if all the load is applied before the frame above exists AND assuming that all deflection is instantaneous.
The effect of creep is not considered in the above ‐ in reality the first floor beams would continue to deflect and the joints continue to rotate after the frame has been built and loaded. Therefore, although there are no moments in the second floor frame at the point at which it is constructed, the creep of the first floor structure should cause some moments to develop over time in the second floor.
Model Creation
No additional steps are required when creating a model for staged construction. Stages are created automatically, (each storey being treated as a separate stage). Stages can be adjusted once the model has been created as described in the section Stage Control.
Staged Loading Creation
Staged loading is specified within the Load Combination Editor by clicking the Loading Generator button : Staged Construction Load Combination
Duration of Each Stage
The duration of each stage controls the modulus of elasticity used at that stage ‐ it is assumed that a normal cement is used so that concrete elastic modulus, Ec at age t is estimated by:
The significance of the stage duration on Ec is illustrated below:
- Duration 1 day 0.59 Ec(28)
- Duration 15 days 0.96 Ec(28)
- Duration 28 days 1.00 Ec(28)
- Duration 99 days 1.06 Ec(28)
From the above it can be seen that changes due to duration do not have a large impact on the results after the first few days.
Analysis Properties
As per unstaged construction, factors are applied via the Material and Section Effective Factor Stiffness. These are applied across all construction stages.
The Modulus of Elasticity Factor in particular requires careful consideration as it should be employed in order to make any allowances for creep and cracking. The value entered here is applied to the short term modulus of elasticity Ec(28) defined in Material dialog.
Any allowance for creep is only dealt with effectively in the context of an unstaged analysis.
The E value adopted is very important in relation to sway sensitivity assessment for EC2 (and for other codes also if the ACI sway option is used).
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