Magazine

NDT Verifies Garage Barrier Safety

Armin B. Mehrabi and Malcolm K. Lim

When you own or manage a parking facility, you need to know that it's safe and structurally sound. Timely identification of potential problems prevents serious harm or massive repairs. When you can gain that knowledge without damaging the structure or needlessly interrupting your operation, so much the better. Nondestructive testing (NDT) techniques for assessing condition of parking facilities can aid in this goal.
Construction elements such as floor slabs, beams, and columns can be evaluated with little or no disturbance either to the structures or their occupants. Just as noninvasive, medical diagnostic sounding techniques offer obvious advantages in patient care, NDT can provide clear benefits in structural engineering and construction applications.
Most recently, an advanced technique for evaluation of cables on long-span bridges was added to the diagnostic tools offered on parking structure assessment. The technique verified safety of the barrier cables installed to restrain vehicles from crossing the edge of the parking deck.
Vehicle barrier
CTL worked with Markham, IL-based Castle Construction Corp. on a recent project at the Chicago Police Department's 16th District parking garage. One of the barrier cables at the parking garage had broken upon being hit by a car, and Walker Parking Consultants, the parking garage designer, recommended that Castle determine whether the cables had been tensioned under a safe force range. CTL welcomed the opportunity to adapt one of its NDT techniques to a different type of structural
component.
Originally developed in 1997 to evaluate cable-stayed bridges using a laser vibrometer, the technique adapts most any kind of precision vibration sensor to measure cable forces. It allows the operator to stand at a distance, monitor a cable with the laser beam or sensor, and record the cable vibration. After all of the desired cables are targeted, the cable vibration frequencies are input into computer-based algorithms, and forces can be calculated within 2 percent of their actual values. The resulting force distribution can be used to make much more refined judgments about the condition, aging, reliability and maintenance of cable-supported structures than were previously possible. The innovation was so practical and effective that it quickly became the NDT tool of choice for cable-supported bridge evaluation nationwide.
The builder of the 16th District garage was confronted with the prospect of making multiple barrier force verifications using hydraulic rams to disassemble the barrier cables, a process which would have consumed a week or more. Instead, the builder chose to use NDT, permitting all measurements to be made in two hours. Measurement of forces in a series of randomly selected barrier cables of the police garage showed that the forces were within a safe range of design specified values and reassured the owner of the safe use of the structure.
Techniques
NDT techniques and their benefits encompass:
* Impulse radar. The impulse radar technique uses high-frequency electromagnetic energy to determine the presence and location of steel reinforcement, prestressing steel, conduit and other embedded objects. Precise layout of embedments is important in preparation for parking garage maintenance and upgrades.
* Impulse response. Impulse response testing uses a low-strain impact to send a stress wave through the tested element. It can be used to detect delamination around steel reinforcement, low-density concrete (honeycombing), and voids or cracking in concrete elements.
* Corrosion rate measurements. Corrosion rate measurement is a rapid, nondestructive field test method for measuring the corrosion rate of reinforcing steel in concrete structures. Based on the test, the future loss of the cross-section of the reinforcing bars due to corrosion at given conditions can be defined, and remaining service life of facilities can be estimated.
* Impact-echo. The impact-echo method employs transient stress waves and their reflection to detect, locate, and classify flaws within hardened concrete. It can be effective in evaluating the thickness of a concrete member, locating voids or areas of poor consolidation, and detecting the degree and effectiveness of grouting in bonded post-tensioning ducts.
Working with experienced NDT consultants will help ensure that the most reliable, accurate and cost-effective methods are brought to bear on your particular situation.

Dr. Armin B. Mehrabi is a Senior Principal Engineer in CTL's Structural Laboratory Division. Malcolm K. Lim is Principal Engineer in CTL's NDT Group. They can be reached at
(800) 522-2CTL

Article Abstract from June, 2003




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