See Photo:  Structural Modal Testing - ANCO MK-18 Eccentric Mass Vibrator
 

EARLY 20TH CENTURY MARVEL

The lower Aswan dam, constructed across the Nile in 1898-1902 under the direction of the famed British engineer, Sir Benjamin Baker, was hailed as an engineering marvel at the beginning of the twentieth century. The two-kilometer long gravity dam, originally 20 meters in heights, was raised twice, to 25 meters in 1912 and 34 meters in 1933, as illustrated in the accompanying figures. Construction consists of granite block and mortar walls, plus filling of the core with "Herculean Concrete" - rubble stones and mortar deposited, unmixed, by hand, and accounting for 40% of the volume of the dam. The construction was performed by a work force of 11,000 men. The lower dam was used to control the Nile flooding for the 1,000 kilometers between Aswan, Cairo, and the Mediterranean Sea, and revolutionized Egyptian irrigation and agriculture. The lower dam was also used for power production until it was replaced by the upper Aswan earth fill dam, built five kilometers upstream by the Soviets during 1960-1971. The upper dam impounds Lake Nassar and currently provides approximately 40% of Egypt’s electric power requirements. The lower dam continues to be important for tail race control at the upper dam, as a vital Nile road traffic crossing, and because the city of Aswan (with a population of approximately 200,000) lies directly below the lower dam.

SEISMIC SAFETY STUDIES

Over the past thirty years, grouting of the lower dam has been required to reduce leakage. Because of the required grouting and the location of the city of Aswan below the dam, the Egyptian High Aswan Dam Authority contracted with HARZA Engineers of Chicago to perform a complete evaluation of the dam, including its seismic adequacy. HARZA conducted extensive coring to determine dam integrity and to assess material properties for use in a dynamic finite element model. Coring provided local but not global (or average) material properties. The variability of the unusual and undocumented construction materials suggested that dynamic vibration tests be used to measure global dynamic properties of the dam for use in verifying the computer finite element model. HARZA contracted with ANCO to perform field eccentric mass vibration tests on three section of the lower Aswan dam and lock.

ANCO used its MK-18 eccentric mass vibrator to excite the dam with a sinusoidal force up to ten tons over the range of 1 Hz to 25 Hz. Acceleration responses on the order of .001 g to .0001 g were recorded and analyzed by spectral techniques to reveal a first transverse mode of dam vibration with a resonant frequency of 8 Hz. This information, along with the measured mode shape and dynamic stiffness, was used by HARZA to refine the assumed density and stiffness of the dam material properties in the finite element model used to predict seismic response. Using this verified model, HARZA was able to reduce the level of conservatism required in its analysis, which ultimately indicated that the Aswan dam had sufficient seismic capacity (resistance to uplift and sliding).

FIELD TESTING OF CIVIL STRUCTURES

ANCO has used field testing to verify the dynamic properties of a variety of civil structures, including dams, bridges, offshore oil platforms, power plant boiler structures, turbine foundations, office buildings, stacks, historical office structures, and nuclear power plant containment buildings. Testing uses portable equipment and can be performed in a matter of days. The identified dynamic properties, namely damping, resonant frequency, mode shape, and dynamic stiffness, allow the for the validation and modification of dynamic models, so as to reduce conservatism, demonstrate increased safety margins, detect changes due to structural degradation, suggest optimal modeling approaches, and advance the state-of-the-art in structural modeling.

Combined testing and analytical efforts can significantly reduce the overall cost of dynamic modeling of critical and unusual structures. These efforts often reveal margins that reduce or eliminate the need for costly strengthening or other structural modifications.