Procedure 4 Large Structures Construction Stage Analysis)
5.7 The Pre-stressing Load Case
5.7.1 Definition of the Load Sets for Pre-stressing
Select "LOADS AND CONSTR.SCHEDULE #LOADS !LSET to create a new Load Set. First define a new Load Set in the upper table by selecting a position in the upper table and choosing ‘Insert before’ or ‘Insert after’. The related tendons are then inserted in the load table (lower table in the GUI).
Note: It is also possible to define several Load Sets and to combine them in one Load Case.
The following input must be inserted in the upper table:
• Number Load Set number
Construction stage 1 pre-stressing loading set, Load Set 502 for Stage 2 pre-stressing loading, etc.)
• Location The program creates an ASCII file automatically without any user input.
• Description Descriptive text (max. 80 characters)
The loading data for this new Load Set must then be defined in the lower table:
• Select a position in the upper table
• Select ‘Insert before’ or ‘Insert after’ at the top of the lower table to open the Load Type selection window.
• Select the Load Type
# Stressing
% Select ‘Tendon stressing’ (TEND0)
Define the Tendons to be assigned to this Load Set in the displayed input window:
% From/To/Step Series of tendons to be assigned to the currently se- lected Load Set (501… 599)
Note: It is possible to use in a Load Set respectively Load Case only some of the tendons grouped together in the same “Stress Label”. However, before using the Load Set in the action CALC for calculating the pre-stressing Load Case, the actual stressing forces have to be determined for all related tendons in the stressing action STRESS.
# Increment-Force The difference with respect to a previous stressing process is considered
# Total-Force The total force is considered in the calculation
Note: The setting of the above switch is only important, if a tendon is stressed more than once at different stages in the Construction Schedule. The previous tension force is zero if a tendon is stressed for the first time, therefore the difference will be equal to the total tension force.
If the switch is set to „$ Increment-Force“, then the previous tendon forces are sub- tracted from the new forces when the forces acting on the structural system are calcu- lated, i.e. only the tendon force difference yields a secondary (constraint) stress state in the structural elements. Using this option is required when tendons are partially stressed in one construction stage and re-stressed to the full tendon force in a later stage (e.g.
stressing to 60% in the first stage and to 100% not before the last stage.
Attention has to be paid to the fact, that the 2 stressing cycles have to be grouped in 2
first „STRESS“ action. The “STRESS” action for fully stressing the tendons must be placed in the construction schedule not before the calculation of the related pre-stressing load case (storing the related tendon forces in the database) has been performed. In the ensuing „CALC“ action the previous tendon forces are subtracted from the forces com- puted in the „STRESS“ action.
If the switch is set to „$ Total-Force“, then the full force is applied if the same tendon is stressed in a later Load Case. This is mostly not meaningful. This option may be useful in special cases, e.g. for approximately considering a pre-stressing steel relaxation (as percentage of the initial force), or for simulating later removal of a tendon (see below).
Example:
Construction stage 1: First stressing to the tendon force state “A”
The selection of the switch has no influence on the results
„A“ ... Total-Force = Increment-Force
The stress state “A” is applied to a tendon in the first construction state (function STRESS in the Construction Schedule – see chapter 5.8 “Calculation actions for Pre- stressing in the Construction Schedule). Selecting “Total-Force” or “Increment-Force”
is in this case of no importance, because no previous tendon forces exist.
Construction stage 2: Re-stressing of the tendon to the tendon force state “B”
„B“ ... Total Force „B-A“ ... Increment
The stressing force state “B” is assigned to this tendon in the 2nd construction stage. The total tendon force “B” is used in the Load Set if „$ Total-Force“ is selected. If this Load Case is summarized in a superposition file, then the result is superimposed to that of the first construction stage, and the sum “A+B” will be as final result in superposition file, instead of the correct state “B”.
Spannen A – Keilschlupf A
Anfang Ende
„A“
Stressing A – Wedge slip A Begin End
Stressing B – Wedge slip B
„A“
„B-A“
Stressing – Wedge slip B
„B“ End Begin
If „$ Increment-Force“ is selected, then the force state “B-A” will be used as loading.
The sum of the 1st and 2nd construction state will now be the state due to the tendon force state “B”. I.e. the choice of the switch „$ Increment-Force“ is important if a ten- don is stressed in different construction stages, and a standard Load Case accumulation is performed.
A practical application of the option „$ Total-Force“ may be an approximate considera- tion of the pre-stressing steel relaxation. Considering the Load Set in the Load Case with a factor of –0.2 means a relaxation of 20%. An other application of „Total-Force“
is the removal of tendons (Load Set with Total Force is assigned to the Load Case with factor (-1.0)).
Example for relaxation:
Assumption:
A tendon is stressed up to 100% as usual. The stressing sequence is available, a Load Set (LSET 501 – Tendon number – „Increment“) and a Load Case 501 with factor 1.0 for the Load Set 501 are defined in calculated within the construction schedule.
The relaxation occurs over a certain amount of time (e.g. 8 days). It is ‚0’ at application time of the Load Case and e.g. 10% after 8 days.
A further Load Set (LSET 502 – Tendon number – „Total-force“) and a Load Case 502 with factor (-0.10) for the Load Set 502 are defined and calculated in the construction schedule, 4 days (as approximation) after the Load Case 501.