1. ANALYZING THOSE INTRIGUING TOWN LATTICE TRUSSES by Phillip C. Pierce, P.E. Consulting Engineer South Kortright, New York

2. 2 Basics of Town lattice b Plank versus timber - this discussion is aimed at plank lattice b Repetitive web elements - typically at about 4’-0” spacing and a little steeper than 45 degrees b Chords comprised of two layers on each side of the lattice (i.e., a total of 6 planes of elements) b Chord elements are not spliced, but terminated

3. 3 Basics of Town lattice continued  Typically two levels of chords at bottom - sometimes only one level at top b Intersections traditionally joined by wooden pegs (“trunnels”) b End posts

4. 4 Structural Behavior b Highly indeterminate b No single bearing point b Bottom chord subject to bending plus axial

5. 5 Methods of Analysis b Simplified - plate girder analogy b Refined - any number of means and software

6. 6 Simplified Analysis b Key is selection of span length b Chord forces are usually more predictable b Lattice forces much less predictable

7. 7 Refined Analysis b Element support conditions critical b Joint stiffnesses critical b Mountains of output

10. 10 Means of Verification of Analysis b Deflection is usually easiest - in theory b Extremely small deflections b Example - 20 tons => less than half an inch

15. 15 Strain Gauges? b Variability of timber properties make direct use of strains suspect b Can be useful in relative relationships - especially at chord terminations

18. 18 Allowable Stresses? b Who knows?!!!! b Standard approach - NDS

19. 19 Trunnel Allowables? b Who knows?!!!! b Some tests have been performed

20. 20 Overstresses b Expect them b Acceptance dependent on experience and judgement

21. 21 More Research Needed b More on joints b More on timber allowables for historic bridges

22. 22 Analysis Needed at All? b Is it helpful? b Yes, but - highly determinate on judgement