Tension-Only Members in STAAD.Pro: Managing Direct Analysis and PDELTA (STABILITY ANALYSIS)

Tension-Only Members and Direct Analysis in STAAD.Pro

Problem Statement:

When running a Direct Analysis in STAAD.Pro, you may encounter issues if your model contains tension-only members. Warnings like the following are common:

• WARNING: TENSION/COMPRESSION SPECIFIED WITH PDELTA KG OR DIRECT ANALYSIS. SO TENSION/COMPRESSION IS SWITCHED OFF.
• WARNING: DIRECT OR PDELTA KG ANALYSIS REQUESTED. SO TENSION/COMPRESSION ANALYSIS IS NOT ALLOWED.

These warnings indicate that STAAD.Pro ignores Tension/Compression members when conducting Direct Analysis. The question is: Can you run a Direct Analysis with tension-only members?

Solution:

Currently, STAAD.Pro does not support Tension/Compression members in Direct Analysis or PDELTA KG Analysis. When assigned, these attributes are turned off, and the members are treated as Truss Members that can handle both tension and compression. The warnings mentioned above reflect this limitation.

It's important to note that only PDELTA Analysis (an iterative second-order analysis) supports the Member Tension/Compression specification.

Option 1: Using TRUSS Members

You can run a Direct Analysis with these members defined as Truss Members instead of tension-only. After running the analysis, check the axial forces in these members to identify which load cases result in compressive forces. For those cases, use the INACTIVE MEMBER command to deactivate the members and rerun the Direct Analysis. The members can remain active in other cases where they develop tension forces.

Option 2: Using PDELTA Analysis

Alternatively, you can switch to a PDELTA Analysis, which supports Member Tension/Compression assignments. This approach avoids the limitation present in Direct Analysis.

How to Run an Analysis with Inactive Members in Direct Analysis

A common scenario for using inactive members is deactivating braces that are designed to take tension only when they experience compression forces. Start by running a Direct Analysis with these braces defined as Truss Members. Identify which load cases cause compressive forces in these members, then use the INACTIVE MEMBER command to deactivate them for those cases. Rerun the analysis to ensure accurate results. For load cases where the braces develop tension, they can remain active.

An example illustrating the use of INACTIVE MEMBERS in conjunction with Direct Analysis is available. In this example, braces 9 to 14 are designated as tension-only members and are deactivated for load case 1 (gravity load). For load cases 2 and 3 (gravity + lateral loads), the members are selectively deactivated, leaving only those in tension active.

Sample STAAD.Pro File:
Direct_5F00_Analysis_5F00_with_5F00_Inactive_5F00_Members

Reference:
Bentley Community Article