TrusSteel - Cold Formed Steel Trusses

1 2 3 4 5 6 7 8 3.07 WIND LOADING Design Responsibility It is the responsibility of the building designer to communicate the wind loading requirements to the truss designer. This includes (but may not be limited to) all of the factors described in the Wind Load Factors list shown in this section. The building code utilized by the local jurisdiction will outline the wind loading requirements for a structure either explicitly or by reference. For instance, the International Building Code (IBC), 2015 edition, references that the American Society of Civil Engineers (ASCE) standard ASCE7-10 be used to determine the wind load applied to a structure. Vertical Loads and Uplift Loads Trusses resist wind loads, which include any loads applied to trusses by the wind when it encounters a structure. When wind encounters a surface of a structure, it creates a load on that surface which must be resisted and transferred. As wind encounters the roof surface of a building, it creates loads on those surfaces that act perpendicular to the surface and can be in either an inward direction or an outward direction. Engineers typically call a load acting inward to the roof surface a downward load from wind. A load acting outward to the roof surface is called an uplift load. The directions of these loads are dependent on geometric factors associated with the building. The magnitudes of these loads are dependent on many factors, including wind speed, wind direction, site geometry, site location, building geometry and building type. Since wind loads act in a direction that is perpendicular to the roof surfaces, a sloped roof surface will have a component of this load that acts in a vertical direction and a component of this load that acts in a horizontal direction. Supporting trusses resist vertical loads, which they eventually transmit down to the building components that support the trusses (walls, girders, etc.). Supporting trusses must also resist uplift loads transmitted from the roof surface. These uplift loads produce uplift reactions at the truss supports that must be resisted. Lateral Loads Since roof structures are typically framed entirely with trusses, it is necessary for trusses to resist the horizontal component of a wind load, often called a lateral load. A truss can resist a lateral load if the truss is attached directly to its supports in a manner that is adequate to transfer this load into the truss support. To do this, the truss support itself must be designed to receive and resist this load and ultimately transfer it down to the building foundation. If the truss-to-support connection does not resist this load adequately, a truss can slide off its supports when a horizontal load is applied. Another way to resist a horizontal load, which is more common in modern building design, is to transmit the load through a diaphragm. Diaphragms are built of structural sheathing that is directly applied to the truss top and/or bottom chords. Common types of structural sheathing are corrugated metal deck (e.g. B-deck) or wood structural panels (e.g. plywood). A diaphragm acts like a beam in that it takes the horizontal load component applied to many trusses and transfers it out to building elements that are able to resist this accumulated horizontal load. A truss that is used to transfer a diaphragm load down to a resisting shear wall is commonly referred to as a "drag truss" as it "drags" the lateral load from the diaphragm to the shear wall. If the building designer intends a truss to be used as a drag truss to transfer lateral loads, it is important that the loads be determined by the building designer and transmitted to the truss designer. Stress Reversal It is important to design a structure and its elements to resist loads for winds coming from all directions. When subjected to wind loads, the internal members of a truss can experience a stress reversal. A stress reversal occurs when a member is subjected to a force that is in the opposite direction as another stress from a different type of load. Cold-Formed Steel (CFS) trusses have performed well when subjected to high wind situations such as hurricanes, down bursts and tornados. Recent hurricane activity in the United States underscores the strong performance of CFS trusses. ENGINEERED BY ALPINE S P E C I F Y I N G / D E S I G N I N G

• Cover