Direct analysis method - Sap2000

Direct Analysis method for (LRFD) using sap2000 v20
Introduction: Aisc 360 10 code requirements as per chapter C:
The direct analysis method of design, which consists of the calculation of required strengths in
accordance with Section C2, is permitted for all structures.
General Analysis Requirements:
 The analysis shall be a second-order analysis that considers both P- and P- effects.
 Use of Notional Loads to Represent Imperfections:
Notional loads shall be applied as lateral loads at all levels. The notional loads shall be
additive to other lateral loads and shall be applied in all load combinations.
The magnitude of the notional loads shall be:
Ni = 0.002 Yi ⇒ (C2-1)
Where:
=1.0 (LRFD)
Ni = notional load applied at level i, kips (N)
Yi = gravity load applied at level i from the LRFD load combination kips (N)
1.2 1.6 ⟹
1.2 1.6 1.2 1.6
1.2 1.6 1.2 1.6
Note:
For load combinations that include lateral loading, apply all notional loads in the
direction of the resultant of all lateral loads in the combination.
1.2 1.0 1.0 ⟹ 1.2 1.0 1.0 1.2 1.0
1.2 1.0 1.0 ⟹ 1.2 1.0 1.0 1.2 1.0
Exception:

∆ 2
∆ 1
1.70
⇒ apply notional load only in gravity load combinations.
 Adjustments to Stiffness:
The analysis of the structure to determine the required strengths of components shall use
reduced stiffnesses, as follows:
Axial stiffness shall by multiplied by 0.8
Flexure stiffness shall be multiplied by 0.8
1.0
4.0 1
0.5
0.5
1.0
,
∗ ,
Note:
It is permissible to use . if the magnitude of the notional load .

General Steps in Sap2000 for design stage:
Define notional loads for dead load & live loads in both directions as follows:
Using magnitude of the notional loads
Ni = 0.002 Yi ⇒ (C2-1)
In this case the value of should be checked after design

Define automatic load combinations as follows:

Currently, the software creates design combinations that include notional loads and gravity
loads only. If the user needs notional loads that include combinations containing lateral
loads, the user must define such combinations manually. The automation of combinations,
including notional loads, is currently limited to gravity loads only.

Convert all load combinations to nonlinear case to consider second order effect:

Check load cased to see the converted nonlinear load combinations

Run the model:
Go to steel design preference:
The default stiffness reduction method is Tau-b fixed this means that 1.0

Select load combinations for design as follows:
Run the design for the first time, unlock the model and check any member properties using right
click:
You will find that the design process applies the stiffness reduction factors = 0.8 for all members

Note:
Reduction factors to section EI and EA are automatically applied by the program if the DAM Tau-b
variable or Tau-b fixed option is selected. For the initial analysis, no reduction factors are used.
However, as soon as a design is run the reduction factors are used and remain in the model. This
means that the first time the model is analyzed and designed, users must iterate between design and
analysis at least one additional time. Thereafter, both analysis and design will have reduction factors
applied automatically.
Run the model again to consider those effects and run the design again also; right click on every
member especially the columns to check that the default assumption by the program (The default
stiffness reduction method is Tau-b fixed) is ok
From the previous picture:
0.411 0.5 ⇒ 1.0 ⟹ 0.8

If we got the following:
From the previous picture:
0.820 0.5 ⇒ 1.0 ⟹ 0.8

Note that the program still use EI=0.8 (incorrect value), is this case one of two alternative
may be used
 Change national load ratio from 0.002 to 0.003.
 Change stiffness reduction method to Tau-b variable
(Apply design then unlock the model and run analysis again then apply the design for the
second time to get correct values).
Conclusion:
To avoid checking the value when the stiffness reduction method is Tau-b fixed, use
national load ratio =0.003.
(National load ratio =0.003is required if design Tau-b fixed, 0.002 for Tau-b variable).
In general
Please note that the design module does not verify the acceptability of the selected method.
The user is expected to verify the acceptability of the selected method.
For detailed explanations please refer to:
Help/Documentations/Technical notes/AISC direct analysis method practical guide

Direct analysis method - Sap2000

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