1. El emprendimiento, la apuesta económica
1.1. Antecedentes del emprendimiento
12. P-Delta P-Delta Analysis Analysis
Structures subjected to lateral loads often experience secondary forces due to the movement Structures subjected to lateral loads often experience secondary forces due to the movement of the point of application of vertical loads. This secondary effect, commonly known as the of the point of application of vertical loads. This secondary effect, commonly known as the P-Delta effect, plays an important role in the analysis of the structure. In STAAD, a unique P-Delta effect, plays an important role in the analysis of the structure. In STAAD, a unique procedure has been adopted t
procedure has been adopted to incorporate the P-Delto incorporate the P-Delta effect into a effect into the analysis. The procedurethe analysis. The procedure consists of the following steps:
consists of the following steps:
1.
1. First, the primary deflections are calculated based on the provided external loading.First, the primary deflections are calculated based on the provided external loading.
2.
2. Primary deflections are then combined with the originally applied loading to createPrimary deflections are then combined with the originally applied loading to create the secondary loadings. The load vector is then revised to include the secondary the secondary loadings. The load vector is then revised to include the secondary effects.
effects.
Note
Note that that the the lateral lateral loading loading must must be be present present concurrently concurrently with with the the vertical vertical loadingloading for proper consideration of the P-Delta effect. The REPEAT LOAD facility (see for proper consideration of the P-Delta effect. The REPEAT LOAD facility (see Section 5.32.11) has been created with this requirement in mind. This facility allows Section 5.32.11) has been created with this requirement in mind. This facility allows the user to combine previously defined primary load cases to create a new primary the user to combine previously defined primary load cases to create a new primary load case.
load case.
3.
3. A new stiffness analysis is carried out based on the revised load vector to generateA new stiffness analysis is carried out based on the revised load vector to generate new deflections.
new deflections.
4.
4. Element/Member forces and support reactions are calculated based on the newElement/Member forces and support reactions are calculated based on the new deflections.
deflections.
It may be noted that this procedure yields very accurate results with all small displacement It may be noted that this procedure yields very accurate results with all small displacement problems. STAAD allows
problems. STAAD allows the user the user to go tto go through multiple iterathrough multiple iterations of the ions of the P-Delta procedureP-Delta procedure if necessary. The user is allowed to specify the number of iterations based on the if necessary. The user is allowed to specify the number of iterations based on the requirement. To
requirement. To set the displacement set the displacement convergence tolerance, enter convergence tolerance, enter a SET a SET DISP DISP f f commandcommand before
before the the Joint Joint Coordinates. Coordinates. If If the the change change in in displacement displacement norm norm from from one one iteration iteration to to thethe next
next is less is less than than f f then it then it is converged.is converged.
The P-Delta analysis is recommended by several design codes such as ACI 318, LRFD, The P-Delta analysis is recommended by several design codes such as ACI 318, LRFD, IS456-1978, etc. in lieu of the moment magnification method for the calculation of more IS456-1978, etc. in lieu of the moment magnification method for the calculation of more realistic forces and moments.
realistic forces and moments.
There are two options to carry
There are two options to carry out P-Delta analysis.out P-Delta analysis.
1)
1) When the CONVERGE command is not specified: The When the CONVERGE command is not specified: The member end forces aremember end forces are evaluated by iterating “n” times. The default value
evaluated by iterating “n” times. The default value of “n” is 1 of “n” is 1 (one).(one).
D D O O N N O O T T D D I I S S T T R R I I B B U U T T E E - - P P r r i i n n t t i i n n g f g f o o r r S S t t u u d d e e n n t t U U s s e e i i s s P P e e r r m m i i t t t t e e d d S S t t u d u d e e n n t t W W i i r r a a H H e e r r u u c c a a : : k k r r a a C C o o m m p p a a n n y y : : P P T T D D i i n n a a m m i k i k a a T T e e k k n n i i k k P P e e r r s s a a d d a a C l C l a a s s s s D D a a t t e e 0 0 9 9 O O c c t t 2 2 0 0 1 1 2 2 : : - - -
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2)
2) When the CONVERGE commWhen the CONVERGE command is included: and is included: The member end foThe member end forces arerces are evaluated by performing a convergence check on the joint displacements. In each evaluated by performing a convergence check on the joint displacements. In each step, the displacements are
step, the displacements are compared with those of the previous iteration in ordercompared with those of the previous iteration in order to check whether convergence is attained. In case “m” is specified, the analysis to check whether convergence is attained. In case “m” is specified, the analysis will stop after that iteration even if convergence has not been achieved. If will stop after that iteration even if convergence has not been achieved. If convergence is achieved in less than “m” iterations, the analysis is terminated.
convergence is achieved in less than “m” iterations, the analysis is terminated.
(DO NOT ENTER “n” when CONVERGE is provided) (DO NOT ENTER “n” when CONVERGE is provided)
3)
3) To set convergence displacement tolerance, enter SET DISPLACEMENT fTo set convergence displacement tolerance, enter SET DISPLACEMENT f command. Default is maximum span of the structure divided by 120.
command. Default is maximum span of the structure divided by 120.
Example Example
Followings are some example on use of the command for P-Delta analysis.
Followings are some example on use of the command for P-Delta analysis.
PDELTA ANALYSIS PDELTA ANALYSIS PDELTA 5 ANALYSIS PDELTA 5 ANALYSIS
PDELTA ANALYSIS CONVERGE PDELTA ANALYSIS CONVERGE PDELTA ANALYSIS CONVERGE 5 PDELTA ANALYSIS CONVERGE 5
Without one of these analysis commands, no analysis will be performed. These Without one of these analysis commands, no analysis will be performed. These ANALYSIS commands can be repeated if multiple analyses are needed at different ANALYSIS commands can be repeated if multiple analyses are needed at different ph
phasaseses..
A PDELTA ANALYS
A PDELTA ANALYSIS will correctly IS will correctly reflect the secondary effects reflect the secondary effects of a combinationof a combination of load cases only if they are defined using the REPEAT LOAD specification of load cases only if they are defined using the REPEAT LOAD specification (Section 5.32.11). Secondary effects will not be evaluated correctly for LOAD (Section 5.32.11). Secondary effects will not be evaluated correctly for LOAD COMBINATIONS.
COMBINATIONS.
D D O O N N O O T T D D I I S S T T R R I I B B U U T T E E - - P P r r i i n n t t i i n n g f g f o o r r S S t t u u d d e e n n t t U U s s e e i i s s P P e e r r m m i i t t t t e e d d S S t t u d u d e e n n t t W W i i r r a a H H e e r r u u c c a a : : k k r r a a C C o o m m p p a a n n y y : : P P T T D D i i n n a a m m i k i k a a T T e e k k n n i i k k P P e e r r s s a a d d a a C l C l a a s s s s D D a a t t e e 0 0 9 9 O O c c t t 2 2 0 0 1 1 2 2 : : - - -