B5.1 General
Seismic design requires that components of the structure be connected or tied together in such a manner that they behave as a unit. Diaphragms and their connections are important structural elements for creating this interconnection and contribute to seismic force-resisting system performance in the following ways:
- (a) Connect the distributed mass of the building to the vertical elements of the seismic force-resisting system (braced frames, moment frames, or shear walls)
- (b) Interconnect the vertical elements of the seismic force-resisting system, thus completing the system for resistance to building torsion
- (c) Provide lateral stability to columns and beams including nonseismic force-resisting system columns and beams
- (d) Provide out-of-plane support for walls and cladding
The elements that make up a diaphragm are generally already present in a building to carry other loads, such as gravity loads.
For recommendations on the design of diaphragms, see Sabelli et al. (2011).
In order for the seismic systems defined in the Provisions to provide ductility, the system must have capacity to deliver forces to the frames corresponding to the frame strength. For this reason, ASCE/SEI 7 requires collectors to be designed for the overstrength seismic load in seismic design categories C through F.
Where the diaphragm transfers lateral forces associated with horizontal offsets in the seismic force-resisting system (i.e., offset between vertical frames above and below), the diaphragm acts as part of the load path for the seismic force-resisting system (SFRS). The diaphragm is designed for the capacity-limited seismic forces associated with the transfer to ensure the diaphragm does not fail before the ductile mechanism in the SFRS can form. The effect of transfer forces on diaphragm shear, collector forces, and additional demand on chords needs to be considered.
B5.2 Truss Diaphragms
In some structure types, a horizontal truss is used in lieu of a steel deck or composite diaphragm. In such cases, there is typically an orthogonal grid of beams with diaphragm-shear deformations resisted by members that are diagonal in plan.
ASCE/SEI 7 does not provide prescriptive direction on how to consider horizontal truss diaphragms. Although there is a school of thought that diagonal and cross brace members could be allowed to buckle or hinge as a source of additional energy absorption, the Provisions require that these elements be designed for the overstrength seismic load in accordance with the capacity-limited design approach of the Provisions, unless the exception of this section is met.
An exception to the requirement in this section is allowed for a three-dimensional system with ordinary seismic force-resisting systems [ordinary moment frames (OMF) and ordinary concentrically braced frames (OCBF)] or cantilevered column systems [ordinary cantilevered column systems (OCCS) and special cantilevered column systems (SCCS)] in which the horizontal truss is treated similarly to an OCBF. In this analogy, the diagonals in the horizontal truss are similar to braces, beams parallel to the diaphragm span (chords) are similar to braced frame columns, and beams perpendicular to the diaphragm span are similar to braced frame beams.
These three-dimensional systems are not expected to be subjected to large inelastic demands due to the relatively small factors assigned to the OMF, OCBF, OCCS, and SCCS systems. These design provisions are intended to encourage some ductility in the horizontal truss by citing specific provisions for OCBF. Two considerations in the design of a horizontal truss using this exception are providing a continuous load path and discouraging brittle behavior.
While a braced frame acts as a vertical cantilever from the foundation, a horizontal truss acts as a beam between supports. Horizontal trusses should span from one braced frame (or moment frame) to another and engage braced frames (or moment frames) oriented in different directions. Members and connections transferring load from one part of the horizontal truss to another or from the horizontal truss to the braced frame (or moment frame) are designed as collectors and collector connections. A three-dimensional computer model is recommended to capture forces associated with the three-dimensional load path and force components at the interface of the horizontal and vertical planes.
Another issue in the design of a three-dimensional system with horizontal trusses is consideration of brace buckling. A moderate amount of ductility is expected during buckling of the horizontal truss diagonals because these Provisions require that the truss diagonals satisfy the requirements for moderately ductile sections. As stated in Sections F1.5 and F1.6, these Provisions also require that the beams, beam connections, and brace connections be designed for overstrength seismic loads, thus encouraging brace buckling before other limit states. These Provisions prohibit K-braced frames (see Sections F1.4b, F2.4b, and F4.4b) because they can produce large moments on chords and collectors after brace buckling. Although not required, it may also be desirable to consider the directions of the diagonals in the horizontal truss and orient diagonals in opposing directions to retain strength after brace buckling.