17TH STREET BRIDGE CANOPY FAILURE INVESTIGATION Atlanta, Georgia Project Background 17th Street Bridge 3-span trapezoidal, variable depth steel box girders with cast-in-place concrete deck 830 ft. long, 137 ft. wide Connects Midtown Atlanta to Atlantic Station Spans over 20 traffic lanes (Interstate 75/85 Downtown Connector) Carries 6 lanes of traffic and features pedestrian sidewalks on north and south side Construction completed in 2004 Engineer of Record URS Corporation General Contractor C.W. Matthews Contracting Co., Inc. Overall view of 17th Street Bridge Canopy-Fence Structure Attached to south parapet wall Structural support frames Galvanized steel Built-up column and cantilever beam assemblies 64 total along south side (with 5 additional column assemblies) Spaced at 10'-6" on center Architectural shade screen Stainless steel tube shapes bolted to frames Perforated 14 gauge stainless steel sheets Canopy fence along south parapet of bridge Canopy-Fence Structure Section through bridge looking east Elevation view of canopy fence Canopy-Fence Structure Undulating appearance due to varying support frame geometries Pitch of cantilever arms varied Type A Type B Type C Type D Overall size and geometry of individual elements unchanged Canopy frame geometries Canopy-Fence Structure Epoxy adhesive anchors 7/8" diameter 4 anchors per frame Anchored to outside (south) face of concrete parapet through flange in column assembly No specification for anchor material or adhesive system No specification for hole diameter or embedment depth Sufficient embedment to develop tension service load of 4,000 lb. per anchor Adhesive anchor Canopy Failure August 13, 2011 at 11:20 PM Approximately 7 years after construction 190 ft. long section of canopy-fence detached from south parapet and fell onto roadway below 19 canopy support frames along east end fell No injuries and only minor vehicle damage reported GDOT immediately removed remaining canopy structure on south side Canopy frames on roadway below Canopy Failure Field Investigation Visual assessment Concrete deck Parapet Anchor holes Canopy frames Materials sampling for laboratory study In-situ load testing of anchor rods Field investigation Visual Assessment Concrete Deck Distress marks and scrapes on south sidewalk Indicate inward rotation and impact of frames during failure Land surveyor contracted to survey locations and extents of distress on bridge deck Distress marks at Location 54 Distress marks at steel pedestrian perch Visual Assessment Concrete Deck Distress marks at Location 55 Distress marks at Location 56 Visual Assessment Concrete Deck Visual Assessment Parapet Spalls at anchor holes at Location 58 Spalls at top of parapet at Location 66 Visual Assessment Anchor Holes Within failure area, anchors pulled out Exception: lower 2 rods at Location 67 shear failure of rods Inspected using optical borescope and videoscope Average depth of embedment = 8" Anchor holes cored Diameter of holes = 1-1/8" Holes spacing as shown in design drawings Borescope inspection Videoscope inspection Visual Assessment Anchor Holes Epoxy observations Generally bonded to substrate concrete Frequent voids/air pockets, especially in back 1.5" of hole Varied in color Opaque dark gray Opaque light gray Translucent brown Wet epoxy components extracted at 3 locations Wet epoxy component extracted from inside hole 56B Visual Assessment Anchor Holes Videoscope Observations Visual Assessment Anchor Holes Anchors outside failure area were torch cut during removal 168 torch-cut rods inspected 112 observed with measurable withdrawal (pull-out) Minimum 1/16" Maximum 1-3/4" Scorch marks indicate torch cut between column and parapet at some locations Anchor withdrawal away from failure Torch-cut anchor away from failure Visual Assessment Canopy Frames Observed in medians below bridge and later at off-site storage yard 19 frames in failure area Types of distress observed: Bent and missing connection plates Failed/bent/twisted plates in column/cantilever assemblies Bent anchor rods Miscellaneous paint scrapes Field-modified holes for anchor rods Missing connection plates on cantilever Field-modified hole for anchor rod Visual Assessment Canopy Frames Other observations Top Left: Black paint marks on Frame 52 Top Right: Bent anchor rods Bottom Left: Welded plates for eccentric anchorage on Frame 51 Bottom Right: Yellow paint on bottom of cantilever tube assembly Materials Sampling Wet epoxy materials from holes 6 concrete cores from south face of parapet (at anchor hole locations) 4 concrete cores from north face of parapet 3 for compressive strength testing 1 for petrographic study Coring on south face of parapet In-Situ Load Testing Direct tension testing Performed on 5 existing anchors away from failure area Tested rods showed no evidence of prior withdrawal Only 1 of 5 rods failed below capacity of test equipment (~15,000 to 20,000 lb.) Load testing of anchor rod In-Situ Load Testing Results of in-situ load tests Anchor Maximum Applied Load Number (lb.) 13A 10,436 33A 17,634 33B 21,094 34A 20,374 41A 15,619 Anchor tested to failure Load test stopped prior to failure Design 4,000 lb. service load Anchor 13A tested to failure Data collection unit for load testing Laboratory Investigation Petrographic studies of concrete Common normal weight concrete Moderately week paste-aggregate bond Compressive strength testing of concrete Average compressive strength = 6,787 psi Assessment of adhesive material Bisphenol A/Glycidyl Ether-based epoxy Poor mixing Poor proportioning Incomplete filling of holes Core 55d split for visual assessment Laboratory Observations Core 50d split showing incomplete filling of anchor hole Laboratory Observations Various anchor rods collected from debris Analysis Estimate actual tensile loads on anchors during service Estimate of typical frame self-weight = 2,075 lb. Estimate of tributary weight of shade canopy for single frame = 550 lb. Total = 2,625 lb. Design service = 4,000 lb. Center of gravity located approximately 2'-6" inboard of south face of parapet Direct tension on lower anchors under gravity loading 2'-6" Lower anchor bolts in tension Elevation view of canopy fence Analysis Estimate of tensile reaction at single bottom anchor due to overturning moment (self-weight only) for various assumed bottom anchor withdrawals (frame rotations). Tension @ Bottom Anchors Service Load, P (lb) anchor (in) Tensile reaction at bottom anchor, Tbot (lb) 1600 1500 2,625 0 1,053 1400 2,625 0.5 2,625 1.0 2,625 1.5 1,107 1,161 1,216 T (lb) 1300 1200 2,625 2.0 1,268 1100 2,625 3.0 2,625 4.0 1,376 1,483 1000 0 1 2 3 4 anchor (in) Collapse Collapse Collapse Collapse Collapse Collapse Collapse Collapse Collapse Sequence Canopy support frames experienced significant inward rotation as lower anchors partially withdrew due to sustained tensile loading and creep. At time of failure, lower anchors at Frames 55 and 56 were first to fully release. Visual Assessment Concrete Deck Collapse Sequence Canopy support frames experienced significant inward rotation as lower anchors partially withdrew due to sustained tensile loading and creep. At time of failure, lower anchors at Frames 55 and 56 were first to fully release. Frames west of 55 and east of 56 fell in opposite directions due to restraint from eccentrically supported Frame 51. Upper anchors were subjected to bending and increased tensile demand as frames rotated and eventually pulled free. Collapse Sequence Frames pulled adjacent frames as they rotated inward. Once free of parapet, frames began to translate (slide) back over the parapet to the south. Moving away from point of initiation, frames experienced increased translation as they were pulled south by adjacent frames while rotating. Collapse arrested at Frame 48 to west Rate of loading Quality of mixing of epoxy adhesive Conclusion Primary causes of canopy failure related to epoxy anchor adhesive: Poor resistance to long-term creep Sustained tensile loading Secondary causes Disproportionate mixing of adhesive components Incomplete mixing of adhesive components Inclusion of air voids High temperature in service Conclusion Over time, the anchors connecting the supporting frames withdrew as the epoxy adhesive deformed and failed under a sustained tensile load that was substantially lower than the design service load. Poor performance of epoxy anchors was similar to that observed by NTSB on collapse of ceiling panels in I-90 tunnel in Boston, July 2006. Conclusion Following collapse in Boston, design community has become more aware of poor performance of adhesive anchors under sustained load. 17th Street Bridge was designed and constructed approximately 2 years before the effect of sustained loading on adhesive anchors was identified by NTSB on the failure in Boston. Recommendations If GDOT elects to re-install the canopy structure on the south parapet, the use of a positive connection is recommended to secure the frames to the parapet wall. If GDOT elects to leave the fence system on the north parapet in place, the existing adhesive anchors should be modified to use a positive connection to the parapet.