Engineering : Bracing the Truss Support for Applied Lateral LoadsHome » Steel eNotes - Spring 2010 » Engineering : Bracing the Truss Support for Applied Lateral Loads
When a truss support (i.e. wall, beam, etc) is required to be braced, it is often that the truss itself is called upon to provide the bracing. This condition is not a problem for Cold-Formed Steel trusses. Although it is not the truss that is the ultimate element of resistance, the truss plays a role in getting the force to the resisting element. Typically that resisting element is the roof diaphragm. For this Steel Notes article the support will be considered to be a wide flange beam and the diaphragm as the resisting element.
The first step is for the Engineer of Record (EOR) to determine the magnitude and direction of the force that will be applied to the brace as well as the load cases in which this load occurs. Knowing the force applied at the support flange, the truss designer can determine the resulting forces to be applied at the truss location of the brace attachment. Simple trigonometry is used to divide the force into horizontal and vertical components. See Figure 1
Step 2 is to analyze the truss with the applied loads from the brace. The vertical component of the brace force will be resisted by the truss that will then be resisted by the support. The horizontal component will eventually be resisted by the diaphragm.
To evaluate the load path, the truss designer can model the diaphragm by using a vertical roller support placed at the truss peak. See Figure 3.
The two supports shall be modeled as horizontal rollers. Apply the component loads at both brace locations on the truss in the appropriate direction under the appropriate load cases. With the model the brace forces will find their way into the truss and into the resisting element – diaphragm (V-roller).
Typical application is to resist wind loads that find their way into the support beams.
Force at the bottom flange of beam , Fh = 500 lbs
Assume the angle of the brace = 45 degrees
Force in Brace, Fb = 500 / Cos 45 = 707 lbs
Horizontal component applied to truss, Fh = 707 x Cos 45 = 500 lbs
Vertical component applied to truss, Fv = 707 x Sin 45 = 500 lbs
Vertical force applied at support, Fv = 500 lbs
Using this truss analysis the truss will be designed to resist the force from the brace and deliver it to the roof diaphragm that will transfer the load to the load resisting element in the structure. Be sure the brace forces are applied in the correct direction in the appropriate load cases. Appropriate connection details of the brace to the truss and beam should be completed by a qualified designer.
Mike Pellock, P.E.
Vice President - Technical