Guidelines To Restoring Structural Integrity of Covered Bridge Members

The purpose of these guidelines is to present the steps necessary for decision makers to identify effective rehabilitation techniques for restoring the structural integrity of covered bridge members. The intent is to retain the maximum amount of historic fabric while ensuring public safety and minimizing future maintenance requirements. To make informed repair decisions about existing covered bridges, it is important to (1) start with a basic understanding of the type of bridge, (2) know the current condition, (3) be informed of what to consider when conducting an engineering analysis, and (4) be aware of various repair options that can meet the long-term goals for the bridge.

 decision makers (selectmen, county commissioners, city planners, preservation officers, etc.) that have responsibility for repairing and maintaining existing covered bridges

Executive Summary............................................................2

Acknowledgments ..............................................................4

Chaper 1:

Introduction........................................................5

Purpose of the Guidelines ...............................................5

Need for Repairs to Covered Bridges..............................5

Truss Types.....................................................................5

The Secretary of the Interior’s Standards for the Treatment of Historic Properties..............................6

Where do You Start?.......................................................8

References.......................................................................8

Chapter 2:

Condition Assessment.......................................9

Mechanisms of Deterioration..........................................9

What to Look For..........................................................11

Where to Look...............................................................12

What Tools to Use.........................................................12

Challenges when Assessing Condition..........................16

References.....................................................................18

Chapter 3:

Engineering Analysis......................................21

Construction Details......................................................22

Timber Joinery and Connections...................................23

Considerations for Engineering Analysis of Timber Joinery and Connections........................................24

Considerations for Engineering Analysis of a Burr Arch ...............................................................25

Chapter 4:

Supporting the Bridge for Repairs ..................28

Cribbing, Shoring, Staging, and Rigging ......................28

Other Considerations.....................................................31

References.....................................................................32

Chapter 5:

Repairs............................................................ 34

Rehabilitating the Bridge Using Traditional Timber Repairs................................................................... 34

Strength Enhancement .................................................. 38

Visual Grading to Determine Strength of Bridge Members.................................................... 38

Increasing Load Capacity by Augmenting or Reinforcing Existing Members.............................. 40

Increasing Load Capacity Using Laminated Timber .... 41

Increasing Load Capacity by Reinforcing with FiberReinforced Polymers ............................................. 42

Increasing Load Capacity through Post-Tensioning ..... 43

Increasing Clearance..................................................... 44

Reducing Risk caused by Overload .............................. 44

Increasing Durability .................................................... 45

References .................................................................... 46

Chapter 6:

Maintenance.................................................... 47

Cleaning........................................................................ 47

Vegetation and Moisture Management......................... 49 

Siding and Roofing ....................................................... 49

Fire Protection .............................................................. 51

Remedial Treatments to Retard Decay and Insect Attack................. 52

Connections and Fasteners............................................ 53

References .................................................................... 54

Chapter 7:

Bibliography ................................................... 55