10 Check deflection – code dependent calculation (CDD)

With respect to different modelling of the post-tensioned slab (see "4.1 Three possibilities of prestressing input") the evaluation of CDD deflections is available only for one of them. Code dependent calculation with the real post-tensioned tendons modelled by hanging nodes or allocated in the ribs is not possible in version SCIA Engineer 2011. The code dependent deflections are available only for the prestressing defined by the equivalent load. The background of the CDD calculation is more explained in "2D Concrete design - EC2; SCIA CZ, s.r.o, Prague 2011".

10.1 Concrete combinations

Concrete combinations are special combinations which are used only for the calculation of code dependent deflections (CDD). We have to prepare combinations with the same prescription as EN SLS quasi permanent. There is no special tool for preparation of the code dependent concrete combination in the program. All combinations have to be prepared manually.

Fig. 69 Concrete combination

We need to calculate the total and additional deflections for this slab according to 7.4.1 (4)(5) from "EN 1992-1-1 Eurocode 2, Design of Concrete Structures – Part 1: General rules and rules for buildings, European Committee for Standardization, December 2004.". That is why we have to prepare two concrete combinations. The first combination (CC1) will be used for calculation of the Total deflection and also deflection caused by the creep and long-term loads will be calculated from this combination. The second combination (CC2) will be used for the evaluation of the immediate deflection which will be used for calculation of the additional deflection

The prescriptions for the preparation of both combinations are based on EN SLS quasi-permanent combination with respect to the long term losses and are the following:

SW

self weight

DL

dead load

P

prestressing

LL

life load

Factors for prestressing mean short-term (0.9) and long-term losses (0.85)

Life load is from category F -> Ψ2 =0.6

Only one variable load can be inserted into a concrete combination for the calculation of the nonlinear deflection with creep because these combinations are not envelopes.  We should prepare new combination for each variable loadcase.

The combinations look as the following in the program

Fig. 70 Concrete combinations

10.2 Code dependent calculation (Code Dependent Deflections = CDD)

The nonprestressed reinforcement (user defined or designed) is needed for the calculation of CDD. There are several options in Concrete solver > SLS > Code dependent deflection (CDD) defining how to deal with the reinforcement. The area of the user defined reinforcement (bars representing prestressing and 2D mesh reinforcement) will be taken into account for the calculation of the CDD as we set in concrete solver (see the following figure) – As,user. The designed and the user defined reinforcement in this calculation was used the same, as was done for the model with the real tendon and column heads (see chapter "7.3.3 Summary from Design ULS+SLS of longitudinal reinforcement").

Fig. 71 Settings in Concrete solver

Then everything is prepared for the CDD calculation. The first step is to run a linear calculation. When the linear calculation is finished then the CDD calculation can be run.

Fig. 72 Dialogue for running CDD

Afterwards we get the confirmation dialogue about general deflections.

Fig. 73 Confirmation dialogue after CDD calculation

10.3 Stiffness presentation

The service for stiffness presentation appears only after CDD calculation. This service is in Concrete Advanced tree (2D Member > Member check > Stiffness presentation).

Fig. 74 Service for stiffness presentation

It is possible to evaluate two types of values in this service:

The stiffnesses are the following. Graphical presentation is done for better overview again for the slab S138.

 

Shorterm

Longterm

EA1

EA2

EI1

EI2

The required areas are the following. Graphical presentation is done for better overview again for the slab S138. The values correspond with the user defined reinforcement (free bars representing prestressing and designed user defined 2D reinforcement).

 

Lower

Upper

A1

A2

10.4 Deformation check

After a successful run of the CDD a function for the evaluation of deflections appears in the concrete tree (2D Member > Member check > Deformations).

Fig. 75 Service for deformation check

Two checks of deflections are required according to 7.4.1(4)(5) from "EN 1992-1-1 Eurocode 2, Design of Concrete Structures – Part 1: General rules and rules for buildings, European Committee for Standardization, December 2004.". The deflections are evaluated in the following way. Two required checks are:

The total and additional deflections are not evaluated directly for 2D members. A special algorithm has to be applied to calculate these deflections.

10.4.1 Total deflection

Total deflection is calculated for the combination including also variable loads (CC2) as the sum of the Elastic deformation and Creep deformation. Elastic deformation is nonlinear deformation for combination CC2. Creep deformation can be evaluated as the difference between Nonlinear+creep and Nonlinear for combination CC1 or CC2.

In fact, Total deflection is the deflection for combination CC2 as type Nonlinear+creep

CC2

Fig. 76 Total deflection

The maximum value is -> Check is satisfied.

If the creep deformation is also required then this value is calculated as difference between Nonlinear+creep and Nonlinear deformation for combinations CC1 where check box calculated to creep has been switched ON:

CC1

 

Screen

Values

--3,17mm

-1,40mm

-3,17 - (-1,40) = -1,77 mm

10.4.2 Additional deflection

Additional deflection is calculated for combination CC2 as the difference between the total and immediate deflection. The immediate deflection is evaluated for combination CC1 as the nonlinear deflection.

CC1

CC2

 

Screen

Values

-6,15mm

-1,40mm

-6,15 - (-1,40) = -4,75 mm

The maximum value is -> Check is satisfied.