📺Partially Fixed Column Bases at SLS

Posted on September 3rd, 2019 in Analysis

Summary

The sources detail a method for reducing horizontal deflections in sway frames by employing partial fixity at the column bases, specifically differentiating between ultimate and serviceability limit states.

Initial Problem and Traditional Solutions

The initial design model exhibited significant horizontal deflections under wind loading. Key serviceability deflections were recorded as 10 millimetres (dead plus wind serviceability) and 6 millimetres (dead plus live plus wind serviceability). Deflection from live-only loads was negligible.

To reduce these deflections, two conventional options exist: either stiffening the structure by putting in bigger beams and columns, or introducing fixed bases. However, fixed bases present challenges related to foundation design.

The Partial Fixity Technique

An alternative "trick" is to use a design rule that allows the column bases to be pinned for the ultimate limit state while being partially fixed under serviceability. This approach is justified because connections are more likely to act robustly under serviceability conditions.

Implementation Steps

The process involves initially changing the static supports from pinned to fully fixed. Then, the restraint on the bottom of the columns is intentionally freed up to different degrees for different loading cases using the partial fixity feature found under member loading.

1. Defining Partial Fixity Groups: Two partial fixity groups were created:

   • N0: The mnemonic for 0 fixity (meaning pinned).
   • N2: The mnemonic for 20% fixity.
2. Applying Fixity to Members: The partial fixity attributes (N0 and N2) were applied to the bottom of the columns. While the fixity was technically activated in all three axes, the rotation around the MZ axis of the member would be the critical focus in reality for a plane frame.

3. Activating Groups Globally: To efficiently manage the restraints across multiple loading cases, the factor for these groups was applied globally.

   • For all ultimate loading cases, the N0 load factor was set to one, ensuring the columns act as pinned (0 fixity).
   • For all service loading cases, the N2 load factor was set to one, meaning the columns act as 20% fixed.

Results and Design Implications

Following the analysis with the partial fixity settings applied:

• Deflection Reduction: For the service cases, the horizontal deflections were reduced. The deflection under one service case was pulled down to 5.8 mm, achieving a general reduction to around 5 mm or 6 mm. This was considered a "nice reduction" from the original values .
• Structural Behaviour: The 20% fixity causes the member's curvature to show a crossing over, demonstrating that it is not totally fixed.
• Ultimate Limit State Checks: The change did not affect the steel design (which is based on the ultimate limit state) because the ultimate cases were modeled as pinned (N0). There might be a slight improvement on column checks. Since the ultimate cases were modeled as pinned, there are no moments present in the ultimate analysis.
• Foundation Checks: The pad foundations showed a failure due to slight overstressing in loading case 6 (dead + live + wind + notional). However, the analysis was set to ignore service moments. This means that the influence of the partial fixities (noted as "10% fixities" in the source, although 20% was set up) applied during service loading cases are ignored in the foundation design check. The foundations are still treated as purely axially loaded, ignoring service moments and horizontal shear.

Overall, this partial fixity approach provides an improvement to the design by effectively managing horizontal deflections while maintaining the simplified, axially loaded foundation assumptions for the ultimate limit state.

Summary with Timestamps

The following table summarises the process and outcomes detailed in the sources regarding the technique used to reduce horizontal deflections in sway frames by applying partial fixity.