National Best Practices Conference Covered Bridge Preservation Analyzing Covered Bridges for Live Loads Covered Bridge Preservation National Best Practices Conference June 5-7, 2003 Presented by: Matthew J. Low, P.E. Hoyle, Tanner & Associates, Inc. Manchester, New Hampshire Burlington, Vermont

National Best Practices Conference Covered Bridge Preservation Outline Background Snow Load Wind Load Vehicular Load Pedestrian Load Load Combinations Conclusions Questions

National Best Practices Conference Covered Bridge Preservation Majority of covered bridge construction from 1820 to 1910 prior to modern vehicle loads AASHTO Specifications introduced in 1931 Specifications include little guidance for application of live loads to covered bridges, specifically: –Snow Loads –Wind Loads Background

National Best Practices Conference Covered Bridge Preservation Consider use of other codes Other codes are available: IBC 2000 NFPA 5000 ASCE 7-02 Minimum Design Loads for Building and Other Structures State/City Building Codes Covered bridge load combinations must be developed Background

National Best Practices Conference Covered Bridge Preservation Snow load is ignored for conventional bridges per AASHTO 25 states have covered bridges and experience snow fall Covered bridges act as “unheated structures” Snow Load

National Best Practices Conference Covered Bridge Preservation Roof snow load may be determined by ASCE 7-02, IBC 2000, etc. First determine local ground snow load Snow Load

National Best Practices Conference Covered Bridge Preservation Steps to determine roof snow load by ASCE 7-02 Ps = (Cs)(0.7)(Ce)(Ct)(I)(Pg) Ps = Sloped Roof Snow Load Cs = Cold Roof Slope Factor Ce = Exposure Factor Ct = Thermal Factor I = Importance Factor Pg = Ground Snow Load Snow Load

National Best Practices Conference Covered Bridge Preservation Example with 6:12 Pitch Roof Ce = 1.0 (Table 7-2) Ct = 1.2 for unheated structure (Table 7-3) Cs = 0.8 +/- (Figure 7-2) I = 1.0 (Category I) Pg = 40 PSF (Burlington, VT) Therefore, Ps = 26.9 PSF This value is hardly insignificant Snow Load

National Best Practices Conference Covered Bridge Preservation Wind Load Covered bridges are susceptible to significant wind pressures

National Best Practices Conference Covered Bridge Preservation Wind load in AASHTO very conservative Not developed for Covered Bridges AASHTO: 100 mph base wind velocity Wind pressure = 75 PSF for Truss Bridges Wind pressure = 50 PSF for Girder Bridges Wind Load

National Best Practices Conference Covered Bridge Preservation ASCE 7-02, Method I = Simple Diaphragm Buildings Ps = (  I) (Ps30) = Adjustment Factor I = Importance Factor Ps30 = Wind Pressure for Exposure B, h = 30 feet and I = 1.0 Wind Load

National Best Practices Conference Covered Bridge Preservation Example With Exposure B, Height = 30 Feet, 6:12 Pitch Roof I = 0.87 (Table 6-1, Table 1-1) = 1.0 Basic wind speed = 100 MPH Therefore, According to ASCE 7-02, Figure 6-2 Ps Wall = 0.87 * 14.4 PSF = 12.5 PSF Ps Roof = 0.87 * 3.3 PSF = 2.9 PSF Significantly less than AASHTO values Wind Load

National Best Practices Conference Covered Bridge Preservation Use AASHTO and State DOT Provisions Apply H, HS or Lane Load to Produce Maximum Stress Vehicular Load

National Best Practices Conference Covered Bridge Preservation Use AASHTO Provisions Standard Specifications for Highway Bridges Guide Specifications for Design of Pedestrian Bridges Typically ranges from 65 PSF to 85 PSF Pedestrian Load

National Best Practices Conference Covered Bridge Preservation Create rational load combinations Evaluate snow load at Operating Level Evaluate pedestrian load at Operating Level Load Combinations

National Best Practices Conference Covered Bridge Preservation Design/Rating Load Cases 1.DL + 100%/Inventory Level 2.DL + Vehicular + 133%/Operating Level 3.DL + Vehicular + 133%/Operating Level Load Combinations

National Best Practices Conference Covered Bridge Preservation Wind Load Combination 100% Load Combinations

National Best Practices Conference Covered Bridge Preservation AASHTO Specifications do not adequately address covered bridges Other codes are available ASCE 7-02 IBC 2000 NFPA 5000 Conclusions

National Best Practices Conference Covered Bridge Preservation Snow load and wind loads determined by ASCE 7-02 Load cases based on probability of occurrence and experience Design and rating combinations proposed for DL, vehicular, pedestrian, snow loads Wind loads analyzed separately Conclusions

National Best Practices Conference Covered Bridge Preservation Method allows for preservation, not intended to increase capacity Conclusions

National Best Practices Conference Covered Bridge Preservation Questions Matthew J. Low, P.E. Hoyle, Tanner & Associates, Inc. 150 Dow Street Manchester, New Hampshire (603)