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Page last updated
February 28, 2005

User Advisory

Overview

The ALS has seismic anchoring requirements for user equipment which are intended to minimize hazards to people and equipment in the event of a serious earthquake.

The guidelines in this document will help users design their equipment to meet ALS seismic requirements, thus avoiding the need for modifications at the ALS. This document does not replace Berkeley Lab and ALS official procedures and policies concerning seismic safety.

Policy

All experimental equipment installed at the ALS must be designed to resist earthquakes of magnitude 7.0 Richter on the Hayward fault which runs near LBNL and those of magnitude 8.3 Richter on the San Andreas fault, and be in compliance with the seismic-safety criteria specified in the Health and Safety Manual, PUB 3000, Chapter 23.

All equipment at the ALS will be reviewed before use to check whether the equipment is in compliance with Berkeley Lab and ALS seismic-safety requirements.

Guidelines

Violent earth motions in both the vertical and horizontal directions are to be expected during earthquakes. In critical designs, computer simulations are used to explore equipment vibration resonances relative to a design basis earthquake frequency spectrum. However, this detailed analysis is not necessary for most beamline and experiment equipment used at the ALS.

The Health and Safety Manual states that "Proper anchorage is the key to earthquake safety." In simple terms, this means: Bolt it to the floor. Although some designers may imagine that freedom of movement via rollers or resilient mountings might protect delicate equipment from seismic accelerations, experience shows that this is not the case. In many instances, freedom of movement will amplify ground motion, increasing the risk of damage and injury.

Horizontal and Vertical Accelerations

Designing equipment to withstand horizontal accelerations is the primary consideration in meeting seismic-safety requirements. A simple guideline which may be used to meet ALS requirements is: "Design and mount all hardware to withstand a 0.7 g horizontal acceleration." (A "g" represents the acceleration of gravity. A 0.7 g horizontal acceleration generates a horizontal force equal to 70% of the weight of the restrained object.)

This 0.7 g guideline eliminates the need for resonance simulations in most cases. As a side benefit, designing for 0.7 g horizontal loading provides a more rigid structure, with higher natural frequencies of vibration. In choosing structural elements for 0.7 g horizontal accelerations, materials must be stressed below their expected yield points, with a limit of 50% of yield stress at welded joints. A helpful concept is to imagine the completed system rotated 90° and bolted to a wall instead of a floor. If the device could be cantilevered in this fashion without damage or parts coming loose, it will be earthquake resistant when it is bolted to the floor.

It is up to the designer to provide adequate spacing of anchor bolts to withstand the toppling moment created by a 0.7 g horizontal force acting on the assembled equipment's center of mass. Provisions for six anchor bolts are recommended for stands and pedestals, since obstructions embedded in the ALS concrete floor sometimes prevent the use of some bolt holes. ALS technicians will provide flush, "female," threaded floor anchors. Detailed information about anchors and floor obstructions is available in LSME Note 745, "Concrete Floor Anchor Applications."

Special structural provisions for vertical accelerations are seldom necessary, since the structure and anchoring needed to withstand horizontal accelerations usually provides more than adequate strength in the vertical dimension. However, loose components which merely "sit" on their mountings must be avoided, since objects will be thrown upward in a severe earthquake.

Figure 1. Example of typical anchor–bolt application.

Alignment and Vibrations

Many ALS experiments have demanding alignment and vibration isolation requirements, and the need for rigid bolting of the equipment to the floor for earthquake reasons would seem to conflict with these requirements. One solution is using the ALS "six strut system" for support of position-sensitive equipment in the beamline. This support system allows secure bolting to the floor and precise alignment in all six degrees of freedom, and can usually be made rigid enough to avoid natural frequencies of vibration below 20 Hz. In practice, this solves almost all vibration difficulties. For advice and help in implementing the system, contact ALS engineer William Thur (Phone: 510-486-5689, Fax: 510-486-4102, Email: thur@lbl.gov).

Soft, resilient suspension systems for extreme vibration isolation will not meet ALS seismic requirements unless close-fitting restraint "bumpers" are provided for all degrees of freedom. Several mm of working clearance is acceptable, and the rigid bumper surfaces should be faced with an elastic material such as neoprene to absorb and distribute impact loads.

Electronic Racks

Electronics racks at the ALS are not exempt from seismic requirements. Racks which will be in place for longer than four weeks must be bolted to the floor, with the heaviest components placed as low in the rack as possible. Single-width racks can often be anchored with a single anchor bolt that is centered through a base plate in the bottom of the rack. Some slack should be left in cables to allow for differential motion during an earthquake. Low electronics carts which are moved frequently must have casters with locking wheels.

More Information

For more information on ALS seismic safety requirements, contact ALS engineer William Thur (Phone: 510-486-5689, Fax: 510-486-4102, Email: thur@lbl.gov).