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Analyzing Covered Bridges for Live Loads

Analyzing Covered Bridges for Live LoadsCovered Bridge PreservationNational Best Practices ConferenceJune 5-7, 2003

Presented by: Matthew J. Low, P.E. Hoyle, Tanner & Associates, Inc. Manchester, New Hampshire Burlington, Vermont

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OutlineOutline

• Background• Snow Load• Wind Load• Vehicular Load• Pedestrian Load• Load Combinations• Conclusions• Questions

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• 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

BackgroundBackground

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• 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

BackgroundBackground

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• 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 LoadSnow Load

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• Roof snow load may be determined by ASCE 7-02, IBC 2000, etc.

• First determine local ground snow load

Snow LoadSnow Load

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• 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 LoadSnow Load

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• 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 LoadSnow Load

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Wind LoadWind Load

• Covered bridges are susceptible to significant wind pressures

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• 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 LoadWind Load

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• 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 LoadWind Load

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• 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 LoadWind Load

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• Use AASHTO and State DOT Provisions

• Apply H, HS or Lane Load to Produce Maximum Stress

Vehicular LoadVehicular Load

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• Use AASHTO Provisions

• Standard Specifications for Highway Bridges

• Guide Specifications for Design of Pedestrian Bridges

• Typically ranges from 65 PSF to 85 PSF

Pedestrian LoadPedestrian Load

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• Create rational load combinations

• Evaluate snow load at Operating Level

• Evaluate pedestrian load at Operating Level

Load CombinationsLoad Combinations

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• Design/Rating Load Cases

1. DL + Vehicular @ 100%/Inventory Level

2. DL + Vehicular + Pedestrian @ 133%/Operating Level

3. DL + Vehicular + Snow @ 133%/Operating Level

Load CombinationsLoad Combinations

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• Wind Load Combination

• Wind @ 100%

Load CombinationsLoad Combinations

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• AASHTO Specifications do not adequately address covered bridges

• Other codes are available

• ASCE 7-02

• IBC 2000

• NFPA 5000

ConclusionsConclusions

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• 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

ConclusionsConclusions

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• Method allows for preservation, not intended to increase capacity

ConclusionsConclusions

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QuestionsQuestions

Matthew J. Low, P.E. Hoyle, Tanner & Associates, Inc. 150 Dow Street Manchester, New Hampshire (603) 669-5555 mlow@hta-nh.com