Modular Tuned Mass Damper Units for the Spring Mountain Road Pedestrian Bridges

Modern pedestrian bridges tend to be long and slender in form, usually leading to a structural design with relatively low frequency primary modes of vibration. This type of structure can be excited to a nearly resonant response by various types of synchronized crowd activities and added damping is often required to prevent excessive structural motions and loadings. The three Spring Mountain Road pedestrian bridges use unique Modular Tuned Mass Dampers to provide the desired added damping. INTRODUCTION The three Spring Mountain Road pedestrian bridges are located in Las Vegas, Nevada and were constructed for the Department of Public Works of Clark County, Nevada in 2005. The site is on the famous Las Vegas “Strip,” and new construction in the immediate area was expected to substantially increase pedestrian traffic across the intersections associated with each bridge. Total daily traffic on the bridges was expected to be more than 150,000 people. Two mega resorts were being constructed at the site; the Wynn Las Vegas ($2.5 billion) and the PalazzoVenetian ($1.7 billion). Existing attractions at the site are the Treasure Island Hotel and Casino (2,700+ rooms/suites), and the Fashion Show Mall (200 stores, 2 million sq. ft.). Figure 1 is an aerial view of the site. The 157 ft. span West Bridge goes from Treasure Island to the Fashion Show Mall. The 143 ft. span North Bridge goes from Fashion Show Mall to Wynn Las Vegas, and the 165 ft. span East Bridge from Wynn Las Vegas to the Palazzo-Venetian. The three bridges feature an extremely slender steel girder structural section, only 54 inches in total depth, with concrete towers at each end of the main span. The resulting design produced calculated first and second mode vertical frequencies ranging from 1.48 Hz to 1.9 Hz for the three bridges. In comparison, the Millennium pedestrian bridge in London, England had frequencies in the 0.5 Hz to 1.1 Hz range, and when originally opened exhibited unacceptable high deflections when excited by crowds of pedestrians, as reported by Taylor [1]. Although the Spring Mountain Road pedestrian bridges had appreciably higher frequencies, these still are well within the bandwidth generated by human walking motions. The Fashion Show Mall has music playing much of time, with the potential for groups of people dancing on the bridges. In addition, an exuberant (or rowdy) crowd might be expected to repeatedly jump while on the bridge, and this motion also could be generated in the frequency bandwidth of interest. The major concern was the so-called “synchronized foot-fall” effect documented on the Millennium Bridge by Dallard [2] whereby crowds of only 50-100 people would generate biodynamic feedback between the bridge and people. When this occurs, seemingly unrelated persons walking randomly on the bridge will synchronize their motions within a short period of time, thus causing an uncontrolled resonant response in the structure. In the fully resonant state, the amplification factor for a simple single degree of freedom (SDOF) oscillator is: Amplification factor ζ 2 1 X X