Cabinet Overturning: Inadequate Anchorage and High CG

Case classification

Illustrative case study. This scenario is constructed from common failure patterns observed in post-earthquake assessments of tall, top-heavy cabinets.

Scenario description

A tall control cabinet (2.2 m height, 0.6 m width, 0.8 m depth) weighing 600 kg had a transformer mounted at the top, raising the centre of gravity to 1.6 m above the floor. The cabinet was anchored with four bolts at the base corners. During a seismic event, the cabinet overturned, pivoting about the rear edge of the base and falling away from its mounting position. The anchorage failed in tension on the front bolts and the cabinet separated from the floor.

Analysis of the failure

The overturning moment from the seismic load was: M_ot = F * h_cg = (m * Sa) * h_cg = (600 * 3.43) * 1.6 = 3293 N*m. The stabilising moment from self-weight was: M_st = W * (d/2) = (600 * 9.81) * 0.4 = 2354 N*m. The overturning moment exceeded the stabilising moment, producing a net uplift that the front anchors could not resist. The tension per front anchor was: T = (M_ot - M_st) / (2 * d) = (3293 - 2354) / (2 * 0.8) = 587 N per anchor, plus the shear per anchor from the lateral force. The anchors were sized for shear only, with no tension capacity considered.

Root causes

• The centre of gravity was high (1.6 m) due to the transformer at the top, producing a large overturning moment
• The anchorage was designed for shear only, with no consideration of tension from overturning
• The cabinet depth (0.8 m) was small relative to the CG height, giving a small stabilising moment
• The vertical seismic acceleration was not considered — it would reduce the stabilising moment further
• The cabinet was tall and narrow, making it inherently susceptible to overturning

Lessons learned

• The centre of gravity height must be considered in the overturning check — a high CG dramatically increases the overturning moment
• Anchorage must be designed for both tension and shear — shear-only anchorage is inadequate for tall cabinets
• The stabilising moment depends on the cabinet width or depth — narrow cabinets are more susceptible to overturning
• Vertical seismic acceleration reduces the stabilising moment and must be included in the overturning check
• Tall, narrow cabinets should be assessed for overturning even if the anchorage appears adequate for shear

Remediation

The cabinet was re-installed with tension-capable anchors (adhesive anchors with sufficient embedment) at all four corners. The transformer was relocated to the bottom of the cabinet where possible, lowering the CG. The cabinet was also braced to an adjacent wall at the top to provide a secondary load path for the lateral force. The overturning check was re-performed with the revised CG and the vertical seismic acceleration included.