Structural Considerations for Trusses

Transcription

1 Introduction Structural Considerations for Trusses L0 L1 L3 L4 L5 L6 L7 Pratt Truss 1 Truss Types Three types of trusses in Alberta: Pony trusses Through trusses Deck trusses Traffic loads are transferred to the trusses by a floor system. Truss Panel Point Floor System Deck supported by Stringers Wind Bracing Floor Beams supported by Trusses at Truss Panel Points Stringers supported by Floor Beams 2 3 1

2 Pony Truss Through Truss Upper Wind Bracing System Portal Brace Wind Bracing Floor Beams Batter Posts 4 5 Portal Brace & Batter Posts Deck Truss Wind Load Portal Brace Batter Posts Floor Beam Lower Wind Bracing System 6 7 2

3 Deck Truss Wind Bracing Truss Configurations The way truss members carry loads depends on the configuration of the truss. Most trusses are either Pratt or Warren trusses. Diagonal Bracing Upper Wind Bracing System Main Truss Members Lower Wind Bracing System 8 9 Pratt Truss Warren Truss Crossed Diagonals Top Chord Top Chord U7 Diagonal Bottom Chord Bottom Chord Diagonal L

4 Truss Curvature Top Chord Bottom Chord Tension U3 being held up by the left support being pulled down by the loads to the right L

5 U3 being pulled down by the loads to the left being held up by the right support L U3 U7 being held up by the left support being pulled down by the loads to the right L8 L

6 U3 U7 being held up by the left support being pulled down by the loads to the right L8 L being pulled down by the loads to the left U3 L3 being held up by the right support Summary Diagonals that slope away from the nearest support as they approach the bottom chord are primarily tension members. Some diagonals near the center of the truss may go into compression. Diagonals that slope towards the nearest support as they approach the bottom chord are primarily compression members. Crossed diagonals are tension members

7 Slope away from nearest support approaching BC Summary Diagonals Slope towards nearest support approaching BC Crossed Loads in s Tension Compression Tension Loads in s Loads in s Top chord in compression Top chord in compression Bottom chord in tension Bottom chord in tension

8 Loads in s Loads in s U1 L1 Bottom chord in tension Loads in s Loads in s U1 L1 Bottom chord in tension

9 Loads in s Loads in s U7 Top chord in compression L Summary Loads in s s connected to diagonals at both ends will be in compression if the diagonals are in tension and in tension if the diagonals are in compression. s connected to diagonals at the top chord only will be in tension for pony and through trusses and will be unloaded for deck trusses. s connected to diagonals at the bottom chord only will be in compression for deck trusses and will be unloaded for pony and through trusses. Diagonals at both ends Diagonals in tension Summary Loads in s Diagonals in compression Connected to Diagonals at top chord For pony & through trusses For deck truss Diagonals at bottom chord For pony & through trusses For deck truss Compression Tension Tension Zero Zero Compression

10 Summary Loads in TC, BC & BP Top chords are in compression. Bottom chords are in tension. Single Angle Steel Sections Double Angles Batter posts are in compression. Four Angles Channel Double Channel W-Section Tension Member Compression Member Misalignment of Member

11 Compression Member Compression Member Bent Member Rippling of Plate Compression Member Compression Member Bracing Rippling of Plate

12 Class Exercise Class Exercise U0 U7 U L8 U7 U L8 L Truss Loads L0 U L1 L3 L4 L5 L6 L7 L8 Questions??