Which one of the following is applicable in case of analysis of portal bracing?
- APortal bracings are provided at one end of the truss girder bridge in the plane of end posts
- BIn addition to the lateral force due to wind, the bracings are also designed to carry a lateral shear equal to 1¼ percent of the total compressive force in two end postsCorrect
- CThe maximum wind load of loaded span is taken into consideration
- DPortal bracings are generally provided in more than one plane, located anywhere except in the central transverse plane of end posts
Solution
Concept
Design of portal bracing of a truss bridge
Step-by-step solution
1. Portal bracing is provided at both ends of a through truss bridge, in the plane of the end posts. 2. It transfers the lateral wind load on the top chord to the bearings. 3. The bracing is designed for the wind shear plus an additional lateral shear taken as a small percentage of the total compressive force in the two end posts.
Explanation
Besides wind, portal bracing is designed for a lateral shear equal to a percentage of the end-post compression, to allow for lateral instability of the posts.
Why the other options are wrong
Portal bracing is provided at both ends (not one end) and in the plane of the end posts (not "anywhere except the central plane"). The choice of wind load case also involves the loaded and unloaded conditions.
Shortcut
Portal bracing: wind shear + lateral shear from end-post compression.
Common mistake
Thinking it is provided at one end only.
Correct answer: B) In addition to the lateral force due to wind, the bracings are also designed to carry a lateral shear equal to 1¼ percent of the total compressive force in two end posts