Vibration Testing of Joist Supported Footbridges

Abstract

This paper presents a portion of extensive experimental and analytical studies on two joist supported laboratory footbridges to determine the effect of bottom chord extensions on their static and dynamic properties. Finite element computer models were created to simulate and compare the results of stiffness and vibrations tests for different bottom chord extension configurations. The philosophy was to test the footbridges and update the finite element model, trying to minimize the discrepancies between the test results and the FE model for better response prediction. The stiffness test results indicate that bottom chord extensions help reduce deflections; placing them before the concrete placement is observed to be a better solution than reinstalling them to a joist supported system with a cured concrete slab. The vibration test results indicate that bottom chord extensions increase the natural frequencies, but they do not necessarily decrease the peak acceleration levels. INTRODUCTION Advances in building construction technology have led to the use of lightweight and high strength materials, which have resulted in larger bays and longer spans in floor systems. Also, the trend towards a “paperless” office decreased the damping and the amount of actual live load on the building floors. As a result, floor vibrations have become an area of serviceability concern. The AISC/CISC Design Guide 11Floor Vibrations Due to Human Activity (Murray et al. 1997) is available for designers to determine acceptable acceleration levels and minimize excessive floor vibrations. Open web steel joist systems are popular in steel building construction; however, vibrations of joist supported floor systems, especially the effect of extended bottom chords, are not very well understood. A single span (7 ft by 30 ft) and a three span (7 ft by 90 ft) joist supported laboratory footbridges (Figures 1 and 2) were constructed at the Virginia Tech Structures and Materials Research Laboratory, Blacksburg, Virginia, using 1.5VL deck (depth=1.5 in.) and a 4.5 in. normal weight concrete slab (total slab depth= 6.0 in.) supported on two parallel lines of 30K7x30 ft span, joists at 4 ft on center. The joists are supported on concrete bearing walls on outer supports and built-up cross sections at interior