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Find reactions and bar forces for truss

Jonathan Ochshorn

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Directions: Enter values for loads and their positions on any nodes of the truss, using any consistent units. To eliminate a load, set its magnitude to zero. Refer to the diagrams for variable names and node numbering schemes. For example, use ft for distances; use either lb or kip units for loads; the reactions will be in either lb or kips, depending on your choice. Vertical loads are assumed to act in a downward direction, with vertical reactions acting in an upward direction. Upward-acting loads should be entered as negative numbers; a negative vertical reaction indicates that its direction is actually downward-acting rather than upward-acting. Similarly, horizontal loads are assumed to act from left to right, while the horizontal reaction, Ha, is also assumed to act from left to right, as shown in Fig. 1.

The truss geometry may be varied by specifying the overall length, height, and number of panels. Reactions may be placed at any node, as long as they do not align vertically. Note that nodes are numbered from left to right, with odd numbers along the top chord and even numbers along the bottom chord. A maximum of 8 truss "panels" may be used. Four types of trusses may be selected: Howe, Pratt, Warren, and Warren with verticals.

To find a bar force, enter the node numbers of the bar under "Bar force calculation." Units correspond to those used for the loads, with positive values indicating tension and negative values indicating compression.

Press "update" button.

More detailed explanations and examples can be found in my text.

truss types and numbering schemes
Fig. 1. Truss types, node numbering, and assumed positive directions for loads and reactions
 

            Bar force calculation
  Overall   Number Ra/Ha Rb Select bar between:  
Truss type length Height of panels node no. node no. node no. node no. Bar force
 
Node vertical load horiz. load    
 
 
 
 
 
 
   
 
   
 
 
 
 
 
 
 
 
   
 
Reaction Ra    
Reaction Rb    
Reaction Ha    
 
Checks:    
Errors:  
Howe-flat checks:  
Ra node OK  
Rb node OK  
bar force nodes OK  
even number of panels  
reaction x-coord differ  
height > 0  
overall length>0  
   
Pratt-flat checks:  
Ra node OK  
Rb node OK  
bar force nodes OK  
even number of panels  
reaction x-coord differ  
height > 0  
overall length>0  
   
Warren checks:  
Ra node OK  
Rb node OK  
bar force nodes OK  
more than 1 panel  
reaction x-coord differ  
height > 0  
overall length>0  
 
Warren-vert checks:  
Ra node OK  
Rb node OK  
bar force nodes OK  
even number of panels  
reaction x-coord differ  
height > 0  
overall length>0