ProtaStructure : Column/Wall Model Types, Stiffness factors, Cracking and Creep

Stiffness factors, Cracking and Creep

Stiffness Factors

Beam, Slab, Column and Wall stiffness factors can be set automatically to make an allowance for cracking and creep. An automated calculation is provided for this purpose: see Cracking & Creep.

Alternatively the stiffness factors can be defined individually if required for maximum flexibility.
Users are also advised to read through the Effective Stiffness Modifiers Design Guide.
The example given in the design guide would be helpful for explaining the concept to users.  

Beam Stiffness Multiplier

Beam bending stiffnesses will be multiplied with the factor provided in this field. Occasionally, you may like to have beams to provide exaggerated restrain effect to the slabs and rib beams. In order to realise this effect, you can increase the stiffness factor of the beams by entering values greater than “1” in this field.

Note that, this factor is not applied to the ribs or grillage beams.

Slab Stiffness Multiplier

Slab bending stiffnesses will be multiplied with the factor provided in this field. Occasionally, you may like to reduce slab stiffnesses to provide exaggerated restrain effect to the slabs and rib beams. In order to realise this effect, you can decrease the stiffness factor of the slabs by entering values less than “1” in this field.

Column Stiffness Multiplier

Column stiffnesses will be multiplied with the factor provided in this field.

Wall Stiffness Multiplier

Wall stiffnesses will be multiplied with the factor provided in this field.

Include Column and Wall Sections in FE Model

This setting controls the inclusion of column and wall outlines in the finite element analytical model.

A single node will be used for modeling the columns if this option is not checked. This option is very useful for reducing the support moments at column and wall nodes by considering the section of the column.

If you check this option, rigid links are provided to create a rigid column outline to reduce stress concentrations and spikes occurring around columns and walls.

Note that, this modeling option is created specifically for flat slab structures. Models may not be created successfully for beam/slab systems.

Include Slab Plates in FE Model

This setting controls the inclusion of slabs as Finite Element Plate Bending elements.

If you check “Include Plates in FE Model”, slabs will be included in the Finite Element Floor Model. When slab are included, calculated slab loads that are transferred to beams are ignored and beams are divided into segments using the nodes created by the plate elements.

Consider Beam Torsional Stiffnesses

Beam Torsional Stiffnesses are calculated and used in the finite element model if this option is checked. Otherwise, the torsional stiffness of all beams in the model will be ignored.

Include Upper Storey Column Loads

If this option is checked, the calculated column axial loads of the upper storey Finite Element Model are transferred as point loads at the related column locations. Using this option, it is possible to make 3D-Short Frame analyses and include the column and wall axial loads of the upper stories.

Note that, if this option is checked, the analyses should be carried in order from upper to lower floor levels.

Upper Storey Column Loads Table

Column and wall loads that are calculated in the upper floor levels analysis can be viewed and edited using the “Upper Storey Column Loads Table”.

Note that, the self weight of the columns and walls are added to the axial force results of the upper storey.

Use ‘Sparse Solver’ for FE Analysis

The purpose of the sparse solver is to reduce the time required for analysis. For certain model types a dramatic reduction in the analysis time can be achieved, for other models it may be less significant.

Cracking & Creep

The "Cracking & Creep" button is used to calculate a stiffness factor which is then applied to the FE model. The purpose of this factor is to make an allowance for creep, cracking and shrinkage when determining an estimate of total long term concrete slab deflection. 

Key points to note when considering total long term concrete slab deflection are:
  1. The total deflection estimate is obtained by reviewing the G+Q*F combination.
  2. The stiffness factors suggested also take account of the load factors –the deflections displayed are a serviceability estimate.(There is no requirement to make further adjustments to the deflections).
  3. If you are using the stiffness factor adjustment as suggested then there is absolutely no value in looking at the deflections for G or Q individually.  (These results are only left visible to accommodate engineers with their own methods of estimating the deflection based on different adjustment factors.)
  4. It needs to be understood that creep and cracking effects do not apply equally to dead and imposed loads – this is another reason why the individual G and Q deflection plots should be viewed with extreme caution if at all.
  5. If the total deflection determined by this method is greater than span/250 then in general the slab may be regarded as being too thin.
  6. It is Prota’s view that considering deflection in this simple, well established and accepted fashion remains the most pragmatic approach in most situations. (Refer to concrete centre guide “How to design reinforced concrete flat slabs using Finite Element Analysis” for further information.)
  7. The possibility of adding reinforcement to control deflection is sometimes raised.  This requires a more theoretical approach where once again creep cracking and shrinkage must all be considered.  Our research indicates that this approach cannot readily prove slab depths which have always been accepted without the addition of reinforcement, are in fact acceptable.  You also have to add a lot of reinforcement to make a small % difference to the deflection.  Therefore, it seems that indiscriminate use of such an approach may lead to wasteful specification of additional reinforcement.

To calculate the stiffness factor to apply:

  1. Choose the “Load Type” (either Domestic/Office or Storage).
  2. Enter the “Average/Typical Dead Load” on the current storey.
  3. Enter the “Average/Typical Live Load” on the current storey.
    1. A suggested range for the stiffness factor is calculated based on the above input. This range also depends on the code being designed to.
  4. The “Stiffness Factor to Apply” is displayed at the bottom of the dialog. The default is the lowest value in the suggested range.
  5. Click the “OK” button to apply the stiffness factor.
On the Model Preparation page, the Beam, Slab, Column and Wall stiffness factors will now be set to the calculated value.


Stiffness adjustments are in accordance to reference : ‘The Concrete Centre publication ‘How to design reinforced flat slabs using finite element analysis’ by O Brooker, May 2006 (HTFS for short).
  1. Creep cracking and shrinkage are all factors that affect the long term deflection of concrete.
  2. Short term E (Est) should be reduced to allow for these factors, a long term value (Elt) should be used in analysis.
  3. ProtaStructure auto cracking & creep allowance follows the above guidance. The coefficient range suggested effectively convert the factored combination displacement to a serviceability estimate for total long term deflection.

Calculation for Stiffness Factor to Apply

Notation

G
Average Typical Dead Load
Q
Average Typical Live Load
Cg
Dead Load Factor
Cq
Live Load Factor
MCSF
More Conservative Stiffness Factor
LCSF
Less Conservative Stiffness Factor
SR_lower
Lower Value of Suggested Range
SR_upper
Upper Value of Suggested Range

1) Calculate Load Ratio


2) Calculate Stiffness Factors
     
      a) Domestic/ Office
        

      b) Storage
      

3)  Calculate Suggested Range

      a) Eurocode and National Annexes
            
      b) Other Codes (British Standard and ACI)
            

4) Stiffness Factor to Apply

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