Eccentric braced frames... ... ... ... ... .para. 7-3c(5)
Steel moment resisting frames... ... ... para. 7-5f.
(1) Function. Floors and roofs, acting as diaphragms, are the horizontal resisting elements in a
structure. Diaphragms are subject to lateral forces due to their own weight plus the tributary weight of
walls connected to them. The diaphragms distribute the lateral forces to the vertical elements: the shear
walls or frames, which resist the lateral forces and transfer them to lower levels of the building and finally
to the ground.
If floors or roofs cannot be made strong enough, their diaphragm function can be
accomplished by horizontal bracing. In an industrial building, horizontal bracing can be the only resisting
element. Where there is a horizontal offset between resisting vertical elements above and below, the
diaphragm transfers lateral forces between the elements.
(2) Diaphragms. Usually the roof and floors of the structure perform the function of distributing
lateral forces to the vertical-resisting elements (such as walls and frames).
These elements, called
diaphragms, make use of their inherent strength and rigidity, supplemented, when needed, by chords and
collectors. A diaphragm is analogous to a plate girder laid in a horizontal plane (or inclined plane, in the
case of a roof). The floor or roof deck functions as the girder web, resisting shear; the joists or beams
function as web stiffeners; and the chords (peripheral beams or integral reinforcement) function as flanges,
resisting flexural stresses (Figure 7-46). A diaphragm may be constructed of any material of which a
structural floor or roof is made. Some materials, such as cast-in-place reinforced concrete and structural
steel, have well-established properties and present no problems for diaphragm design once the loading and
reaction system is known. Other materials, such as wood sheathing and metal deck, have properties that
are well-established for vertical loads, but not so well established for lateral loads. For these materials, tests
have been required to demonstrate their ability to resist lateral forces. Moreover, where a diaphragm is
made up of units such as sheets of plywood or metal deck, or precast concrete units, the characteristics of
the diaphragm are, to a large degree, dependent upon the connections that join one unit to another and to
the supporting members.
(3) Horizontal bracing. A horizontal bracing system may also be used as a diaphragm to transfer the
horizontal forces to the vertical-resisting elements. A horizontal bracing system may be of any approved
material. A common system that is not recommended is the rod or angle tension-only bracing used in older