Truss end reactions

[StrEng007]

Say you have a single-story masonry structure with bond beams and wood trusses with an overhang. For lateral wind, most engineers would agree the wall is braced at the top by the trusses that bear over them (axial load into the end of the truss), which will "push" that load into the diaphragm. In this situation, each end of the truss is assumed to be "pinned" to the wall below due to wind acting in either direction.

Trusses are typically designed to accommodate some horizontal displacement at one end. When this displacement is restricted (as mentioned above), a large horizontal thrusting reaction may develop on each side.

I don't see many engineers accounting for these horizontal reactions, yet we're requiring that trusses be pinned on each end. Are most engineers ignoring this?

 

[engineering_patrol]

@BAretired, correct if you have vertical rollers at supports, but things change if you have hinges (x & y axis supports)

 

[BAretired]

@BAretired, correct if you have vertical rollers at supports, but things change if you have hinges (x & y axis supports)

If you have a pin at one end and horizontal roller at the other, the pin takes the applied load, namely 10# horizontal and the vertical reaction at each end is zero. What things change?

EDIT: If you have a pin at each support, no member is stressed, including the bottom chord.

 

[BAretired]

If the truss were to be solved using a frame program where all joints are rigid, I believe KootK's statement would be correct, namely:

3) Any applied compression will make it's way into the top chord as well as the bottom chord. And the top chord is better braced.

Even if all joints are hinged, members other than the bottom chord might feel some stress. The difference lies in the simplified assumptions of the traditional method of solving trusses wherein axial deformations are ignored. I don't know offhand how significant that difference would be. For some trusses, it may be very significant.

BA

 

[StrEng007]

With a 10# load applied at each end, all members have 0 stress except the bottom chord which has 10# force in every panel.

Do you think the diaphragm system will impart loads to the top chord of the truss? In your example, we're assuming the load can travel directly to the bottom chord. However, with plywood sheathing, isn't the plane of the diaphragm where the force will travel to (unless you detail your ceiling to take the load).

At each panel edge that is fastened to the top chord, the fasteners will have a varying x & y component of force (based on roof pitch) that comes from the in-plane shear within the diaphragm. Previous post on that discussion.

 

[StrEng007]

2) An unbalanced set of wall loads in which the imbalance alone will make its way into whatever diaphragm system

What do you mean by an unbalanced set of wall loads?

 

[KootK]

What do you mean by an unbalanced set of wall loads?

SUM(Fx) <> 0 for loads applied to the truss by the walls.

 

[BAretired]

Do you think the diaphragm system will impart loads to the top chord of the truss? In your example, we're assuming the load can travel directly to the bottom chord. However, with plywood sheathing, isn't the plane of the diaphragm where the force will travel to (unless you detail your ceiling to take the load).

Of course the roof deck will impart wind loads to the top chord of the truss. Wind pressure is normal to the surface of roof or wall. Some of the wind load will travel to the truss reaction points; some of the load will travel along the diaphragm and be resisted by shear walls.

BA

 

[KootK]

@BAretired: I'm convinced, thanks for showing me the error of my ways.

 

[BAretired]

@KootK: You're welcome. It's a rare event.

BA