Load Combinations

P-Delta Analysis

Scope & Limitations

ProtaStructure P-delta analysis is based on “Two cycle iterative Method” (Chen and Lui 1991). This method utilizes geometric stiffness matrix in the computation. It takes into account the compression softening or tension stiffening due to axial loads on the members (geometric stiffness matrix) :

1. Firstly, the geometric stiffness matrix is calculated (by iteration) for G, Q, QP1 (Pattern1), QP2 (Pattern2), G+Q
   




2. For design load cases, [Ke + Kg] is used to incorporate second order effects.
   




3. For example, for  G+Q+E (Earthquake) combination :     G, Q are solved using [Ke + Kg (G,Q)] and E loadcases are solved using [Ke + Kg (G+Q)].
   

This method only takes into account P-Small Delta, not the P-Big Delta. The second-order effects are calculated based on the deflected shape and no additional imperfections are considered. Hence, the additional moments calculated from P-Small Delta analysis is usually small, especially for braced concrete building.  If the structure is sway-sensitive or slender, then a nonlinear second-order P-Large Delta analysis is required (out of scope currently). 

For more details regarding second order effect, please refer to 
Sway Sensitivity & Second-Order Effects

Load Combinations Editor

You can choose which combination is used for concrete and steel design separately.

You must also carefully consider the load factors applied in this combination to ensure that the correct ultimate load factors are used in the finite element analyses.

For example, if you want to apply an ultimate load factor of 1.4 to Dead Loads in Finite Element Floor Analysis, the load factor of the first combination must be
1.4G.

The easiest way to define a new set of load combinations within the Load Combinations Editor is to click the
"Loading Generator" button.

Loading Generator

Vertical Loads

The current design code controls which load case(s) are patterned:

1. BS8110 /CP65 – both G and Q are patterned.
   




2. EC 2 – only Q is patterned

The effective ‘adverse’ load factors for dead load and live load are the Max. G Factor and Max. Q Factor values respectively. The effective ‘beneficial’ load factors for dead load and live load are 1.0 and 0 respectively. In other words, certain alternate spans and adjacent spans are loaded and unloaded in different permutation. as stated in selected code of practice. 

Staged Construction

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For further explanation on staged construction analysis, refer here :  
Staged Construction Analysis

Lateral Loads

Seismic Loads

1.  The "Load cases" of the "Modal Response Spectrum" are defined as Sx+, Sx-, Sy+, Sy- while the load cases of "Equivalent Static Load" are defined as Ex+, Ex-, Ey+, Ey-.
2. Additional "Load Combinations" can be generated if the "Create All Possible Combinations for symmetric results" option is checked. This option is used to cater for additional load combinations in the same directions but in negative sign.

Temperature Loading

Manual Definition of Load Cases

1. The second column in the table contains the label of the load case. Any string value can be assigned as the label but short names are preferable.
2. The third column graphically explains the load case type by the use of a symbol.
3. The forth column defines the pattern of the vertical load case ( _ empty, = full ).
4. The fifth column shows the direction of the lateral load cases, whereas sixth column specifies the eccentricity (if any) of the lateral load case.
5. The seventh column is allow users to check the option "Use Cracked Section". The stiffness table will only be applied for load cases with this option checked, except for seismic load cases where cracked sections are always assumed.
6. The last column includes the definition of the load case. Always give a brief explanatory description to the load cases that you create.

Vertical Loads

1. “User Defined Vertical Load” allows you to define manual vertical loads other than the dead and live load category. These loads can be assigned onto beams, slabs and column/wall nodes.
2. “Dead Load Case” generally consists of member self weights and additional dead loads. However, you are free to assign additional distributed loads or point loads under the dead load case category.
3. “Live Load Case” includes vertical temporary imposed loads on the building other than dead load and other stationary loads.
4. "Roof Live Load Case"  consists of the live loading acting on the roof.
5. "Snow Load Case"  and "Rain Load Case"  includes the forces/ weight of the snow or rain when it is falling on the structural members.
6. "Vertical Earthquake Load Case"  considers the vertical forces that is induced by the earthquake.

Pattern loads will have fully loaded spans specified as
‘1’, and unloaded spans as
‘0’. Leave this field blank for cases that all spans are loaded.

For example, in order to create a dead or live load with a
“Full-Empty-Full-Full-Empty” pattern, enter 10110 in this field. If all spans will be fully loaded, then leave this field empty.

For example, if you want direction-1 beams to have pattern loading while direction-2 beams with no pattern loads (if they are not sitting on a direction 1 beam) then, select
“Dir-1”
beams.

Select
“None” to apply the pattern loading to both direction beams.

Lateral Loads

In summary, to create a load case that will consist of a loading in the X direction, select
“Primary”. For a Y direction loading select
“Secondary (Primary+90)”.

Thermal Load

Adjusting Construction Stage Content and Duration

Adjusting Construction Stage Content

For example, to combine storey 1 and storey 2 into a single stage – edit Stage-2 to start at storey 3.

Adjusting Construction Stage Duration