SAP2000 vs. STAAD PRO Introduction To Piperack Modeling in SAP2000

This brief introduction to SAP2000 is intended for STAAD Pro users wishing to make the transition to SAP2000. It is focused exclusively on steel design.

The primary focus is to highlight the main areas where SAP2000 workflow will differ from STAAD Pro.

Member Geometry Defining member geometry in SAP2000 can be much simpler compared to STAAD Pro. SAP2000 allows for intermediate joints within a continuous member, automatically calculates length adjustment factors in cases where a continuous member is divided, and automatically calculates effective length factors for columns based on the structure geometry.

Figure 1: Frame member with intermediate joint at bracing connection

The highlighted frame member in Fig.1 is one continuous member, despite having an intermediate joint where the bracing connects. In contrast, STAAD Pro would treat this member as two separate beams. This functionality in SAP2000 results in the following:

a. Simpler geometry, with fewer members, simplifies property assignments. b. Connecting new structural members to an existing structure does not affect load assignments.

Figure 2: Subdivided frame with L-factor applied for design check

Furthermore, SAP2000 is capable of automatically recognizing when one continuous member is divided into several smaller lengths, and applies an L-factor accordingly, in order to compute the correct member capacity.

Fig.2 shows the same structure as Fig.1, with the beam in question divided into two, at the joint with the bracing. The steel section properties for one of the beam parts show that an L-factor of 2.0 is used in moment design, to account for the full length of the member.

Similarly to the calculation of L-factors, SAP2000 will also automatically compute K-factors (effective length) for columns based on end conditions and connectivity. SAP2000 documentation does mention that K-factors tend to be conservative, so they should be checked after analysis is performed, and overridden when necessary.

Defining loading in SAP2000 Defining loading in a SAP2000 model requires three steps: defining the load patterns, defining the load cases, and defining the load combinations.

Load Patterns Load patterns are simply a collection of loads. When defining load patterns, it’s possible to be quite granular, thus making the model easier to understand. For instance, when applying piping loads to a model, it may be beneficial to define a separate load pattern for each line in the system. The resulting model would be self-documenting. Other users should be able to make updates to such a model with minimal effort.

Load patterns for notional, wind and seismic loads have the option of defining lateral loads automatically (with some limitations), according to various international codes.

Figure 3: Load Pattern definition screen

Load Cases Load cases in SAP2000 are a collection of one or more load patterns. Each contributing load pattern can be further factored. Also, the load case definition allows the user to specify the type of analysis required (linear or non-linear). Analysis in SAP2000 is performed on the load cases.

Figure 4: Load Case definition in SAP2000

Load Combinations Load combinations are groupings of the analysis results of several scaled load cases. Also there are several types of load combinations, depending on how the load cases are to be combined. For example, the ‘Linear Add’ load combination does a simple sum of the forces in each load case per member. In contrast ‘SRSS’ combines the load case forces as the square root of the sum of squares.

Figure 5: Load Combination definition

In Practice A common complaint in STAAD Pro piperack models is bloated load combination definitions. Often one comes across models with hundreds of defined load combinations. The root cause of this is having a separate load combination for each of eight principal directions of the various lateral loads.

In SAP2000 we can alleviate this problem with the following approach:

a. Define a load case for the eight directions for each lateral load. Say we consider wind loads, we’ll have 8 load cases: E, NE, N, NW, W, SW, S, SE.

b. Define a general Wind load combination that combines the 8 load cases as an envelope. The result will have a maximum and minimum member force for each member for the 8 load cases together.

c. Define your design load combinations using the aggregate Wind combination.

Using this approach, a STAAD Pro model with 100+ design load combination can be reproduced in SAP2000 with 20 design load combinations.

P-Delta in SAP2000 There are two methods that can be used in SAP2000 to perform P-Delta analysis:

Non-linear Load Cases The most accurate method of applying P-Delta analysis is to define each load case in the model as a Non-linear load case with P-delta effects.

The drawback of this approach is that non-linear load cases take longer to process, thus for a model with many load cases the analysis time will increase significantly.

Figure 6: Dead Load case, with P-Delta effects defined

Initial Stiffness An alternate approach is to define only one P-Delta load case in the model, with the assumed dead loads applied. Other load cases in the model can then be configured as linear-static load cases, using the stiffness matrix resulting from the P-Delta load case as an initial condition.

Figure 7: P-Delta applied to load case as initial stiffness

Data Input In SAP2000 Unlike STAAD Pro, SAP2000 files are not text-based. This means that you cannot directly edit the SAP2000 file using a text-based editor.

All input in SAP2000 needs to be entered using the graphical interface, or via input tables.

When defining model geometry using the graphical interface defining grid systems is a requirement, as new members and joints can only be placed at grid intersections, or at existing joints.

Units in SAP2000 SAP2000 allows for switching units on the fly. The main SAP2000 window has a dropdown where the input units for the model can be selected. Modifying the value in the dropdown immediately updates all display screens with the newly selected units.

Figure 8: Main input unit selection box

Additionally, most other input windows in SAP, such as the load definition window have their own unit selection dropdowns. These are initially set to the units selected in the main window, but can be changed on the as required.

Figure 9: Input unit selection in the distributed load definition dialog

Wind Loads SAP2000 does not automatically generate wind loads for frame members when using Canadian codes. This means that for open structure wind loads, loads have to be calculated manually, and entered as distributed loads on the frame members.

Viewing Results Upon performing an analysis in SAP2000, reactions, member forces and joint displacements can be viewed for all load cases and combinations.

Figure 10: Selection of analysis results to be displayed

Performing a design check on the structure requires additional steps in SAP2000, once the structural analysis is performed.

1. First, the design preferences, including the design code to be used, must be selected.

Figure 11: Setting design preferences

2. Second, the design combinations for strength check (and optionally displacement check) need to be specified.

Figure 12: Selecting design combinations

3. Finally the design/check of the structure can be performed.

The results screen upon completion of the design check shows member section assignments, and color codes the members based on utilization ratios. It’s important to note that this screen will not indicate failures such as a failed slenderness check.

In order to view all possible failures in the model the appropriate output view must be selected.

Figure 13: Selecting design results for display