Prepared by: ERIC FARM

2 February 1999

Description: This page is intended to guide you in your analysis of timber bridge alternatives. A step by step procedure will be given to work through the TBSR software. References will be given when values are given are taken from outside sources.

Software: This program is distributed by the USFS, Pacific Northwest Region, Structures Group. The contact for this program is:

		Bill Salsig, Engineering Dept.
		USDA Forest Service
		PO Box 3623
		Portland, OR  97208
		Phone: (503) 326-3635

Goal: Design bridge with smallest stringer and deck dimensions that will still output rating ratios greater than one, and which will support the inventory(design), operating(overload) loading required.

In order to get the data input screen, start the application and when prompted hit the enter key and select option 1 (run the TBSR program).

Next you need to name your bridge. This information will be used if you save the bridge data. The UW copy of the program has something going wrong and it won't let you save bridge data. All this means is that the bridge data must re-entered before every analysis.

Once you are at the data input screen you will be prompted for the following information:

See Diagram for Graphic of Terms Below

1. Type of Deck
R - rock or gravel
P - plank
N - nail laminated
G - non interconnected glulam panel
I - interconnected glulam panel

2. Type of Stringer
L - log (measured by diameter)
S - solid sawn timber (measured by width and depth)
G - glulam (measured by width and depth)

3. Roadway Width - measured perpendicular to the centerline from curb to curb

4. Number of Lanes
1 - single lane (18 ft width or less)
2 - double lane (greater than 18 ft width)

5. Number of Stringers (iterative)

6. E of Stringer (modulus of elasticity)
Found in section 5, page 9, column 8 of the USFS Timber Bridge Manual for glulam stringers
Table A-5 of Haygreen/Bowyer for sawn and log stringers

7. Stringer Span (length of bridge from support to support)

8. CL to CL Ext Stringers (distance between centerlines of upstream and downstream stringers)

9. Stringer Width (interative)
Typical dimensions can be found in section 16, pages 7 - 17 of the USFS Timber Bridge Manual

10. Stringer Depth (interative)
Typical dimensions can be found in section 16, pages 7 - 17 of the USFS Timber Bridge Manual

11. Log stringer Diameter (interative)
Given design load and number of stingers, diameter can be found on pages 57 - 64 of the FERIC Handbook

12. Stringer Fb (bending moment)
Found in section 5, page 9, column 3 of the USFS Timber Bridge Manual

13. Stringer Fv (horizontal shear)
Found in section 5, page 9, column 7 of the USFS Timber Bridge Manual

14. Deck Thickness (somewhat interative)
Typical values can be found in section 8-3 longitudinal glulam deck bridges

15. Deck Width (width perpendicular to centerline of decking)
Assume road width plus 2 ft for curbs or rails

16. Deck Skew
Enter zero here

17. Minimum Plank/Panel Width (width of panel measured parallel to centerline)
For glulam panel assume 48 inches (see example in TBSR manual)

18. Decking Fb (bending moment - see NOTE 1)
Found in section 5, page 9, column 9 of the USFS Timber Bridge Manual

19. Decking Fv (horizontal shear - see NOTE 1)
Found in section 5, page 9, column 11 of the USFS Timber Bridge Manual

20. DL/FT of Bridge (dead load per foot of bridge)
Assume 50lbs/cuft for western species

21. Wheel/Track Gage (measured from CL to CL of the two wheel lines)
Assume 6 ft for most applications

Next verify that the Distribution Factors (df) for interior and exterior stringers are nearly equal. If they are not, repeat the data entry step and change the beam dimension, beam spacing, or deck width to get them nearly equal.

Once the df values are nearly equal, specify the design vehicle. The HS20 is used by AASHTO, but if the axle spacing and loadings are known for your vehicle, use the Series of Up to 12 Axle Loads option. This will ask for a description of the vehicle, the number of axles, the load and spacing of each axle.

The next screen will show many values. The ones that are most important are the Inventory and Operating Rating Ratios. Inventory is equivalent to design condition and operating is equivalent to overload condition. These ratios describe the capacity of the bridge. If the ratio is greater than one, the bridge that was inputted can support the given loading. If the ratio is less than one, the bridge will NOT support the loading and must be redesigned.

The other important values on this screen are the GVWs. The Inventory GVW must be greater than the design vehicle for the bridge. The Operating GVW must be greater than the overload vehicle for the bridge.

By manipulating the beam size, configuration, and properties the rating ratios and GVWs can be changed.

NOTE:

1. The bending and shear stresses must be modified to account for water saturation into the beams. For the stringers, it can be assumed that the decking will keep them dry. The decking must be modified. The reduction coefficients can be found in section 5, page 13 of the USFS Timber Bridge Manual. Multiply the bending moment by the value in the Fb column. Multiply the horizontal shear by the value in the Fv column.