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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