WHAT IS A ROOF DIAPHRAGM?
Designing a metal building for wind and seismic forces includes calculating the shear forces acting on the walls and designing a method for the transference of those forces to the vertical frame. The roof of a structure contains a component called a roof diaphragm that acts as the transfer vehicle for the forces of the wind and seismic movement against the outer walls.
A roof diaphragm can be constructed of steel, plywood or concrete to create the rectangular plane underlying the roof panels.
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DEFINING A ROOF DIAPHRAGM
A diaphragm is a horizontal component that distributes lateral loads from wind or seismic activity to the shear walls or foundation of a structure. It is often described as performing like a short, deep beam. The diaphragm edge members (also known as chords) perform as flanges and resist bending stresses in the area of the highest load.
Diaphragm edge members can be joists, ledgers, trusses, bond beams, studs or top plates, among other things.
Treating the diaphragm as a beam allows you to calculate the shear wall forces and apply the principles of a beam to the diaphragm, with and without roof openings or irregular roof or building geometry.
The load distribution in a basic rectangle-shaped building is as follows.
The roof diaphragm is oriented horizontally and carries loads to the endwall (shear wall) and then to the foundation.
The side walls carry the load to the roof diaphragm on top and the foundation at the bottom.
The shear walls may also be known as vertical or cantilevered diaphragms. Sheathed elements, such as walls and a roof covered with panels and tied together provide additional strength to the building.
When designed correctly, a roof diaphragm can reduce or eliminate the need for diagonal bracing.
CHALLENGES OF CALCULATING SHEAR
When a roof diaphragm has no openings or obstructions, the calculations for shear are relatively simple. When openings such as skylights, HVAC equipment, chimneys and other interruptions exist in the flat plane of the diaphragm, the calculations develop a higher degree of complexity.
Openings for skylights and other penetrations reduce the stiffness of the diaphragm at the location of each opening. Fastener placement and transfer components must be redesigned to distribute the shear around the openings and across the plane.
Maximum average shear typically exists at the ends of a building.
If there are openings in the diaphragm, the average maximum shear also exists in the deck panels and fasteners to the sides of the penetration.
For symmetrical penetrations spread equally across the diaphragm, the openings closest to either endwall will have the same maximum average shear while the central openings will have a different shear.
To determine shear around each centrally located opening, look at the rectangular deck areas directly above and below the opening.
Sum the forces and moments around the area perimeters to determine total force.
Design the force transfer accordingly.
With asymmetrical openings, you must perform the calculations for individual shear areas.
LOAD TRANSFER
Again, openings and irregular building geometry complicate an already complex process. The stiffness of the components determines the way the load travels across the diaphragm. Fastener type and connections also affect diaphragm stiffness. Load transfer across connections is a critical consideration.
Diaphragm design must balance effective stiffness and load transfer with the economy of materials and the complexity of construction. Shear loads can be collected progressively across the diaphragm using collector elements and drag struts to reduce the amount of shear load transferred to the wall and foundation.
The designer can create pathways for lateral load transfer to the shear walls in several ways.
Create a deck bearing angle on top of the joist ends adjacent to the walls, into the joist seats, the ledger angle and finally to the wall.
Create a deck bearing angle attached to the wall with enough fastenings to transfer the shear to the wall directly from the bearing angle and into the wall.
Install channel members or HSS tubing to the ledge angle between joist seats with the top of the tubing at the same height as the top of the joists. This design would provide deck edge support and a pathway for shear transfer into a higher ledge angle and then to the wall.
Eliminate shear on the joist seats if possible because they typically have limited transfer capacity.
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DETERMINING FASTENER TYPE
Resources are available to assist engineers and designers in determining the correct fastener for a given diaphragm design based on the nominal strengths of a variety of weld and screw combinations. The resulting shear strengths for each fastener design are laid out in tables.
Since the table values are based on nominal shear values, safety and risk factors must be applied before use on a structure.
Multiply table values by the Φ resistance factor to compare forces using Load and Resistance Factor Design (LRFD).
Divide table values by the Ω safety factor to compare forces calculated by Allowable Strength Design (ASD).
Diaphragm design resources may also provide example patterns of typical fastener layouts depending on panel width and geometry.
Look for tables based on the Steel Deck Institute Diaphragm Design Manual. As of this writing, the Fourth Edition released in 2015 is the most recent.
REGIONAL PREFERENCES
There are numerous choices in diaphragm design, but you will find the number narrowed by regional preferences, contractor preferences and experience and cost. Different methods are preferred by various erectors and pre-cast or pre-engineered wall manufacturers.
Steel joist seats are common load transfer vehicles but are limited in rollover shear capacity to about 2.5 kips per service load at 2 ½ inches deep. Alternate methods of load transfer to increase capacity include channel or tube members.
One does not simply place a roof on a building; one must design a diaphragm that transfers shear loads to the walls where it is transferred to the building’s foundation.
The top of a building is exposed to varying degrees of shear load from wind and seismic activity. To complicate matters, many roofs have penetrations, openings and obstructions on the roof that require complex calculations to determine total shear forces and forces in specific areas of the diaphragm field.
Load paths, stiffness variations, fastener types and regional preferences are all critical considerations in diaphragm and roof system design. Resources exist to help designers make fastener determinations, but it is up to the engineer to ensure the diaphragm design meets safety standards for the prevailing conditions and building geometry.
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