Story Plastic Mechanism Models
In the presented DBD method, the story mechanisms are presumed to occur due to the yielding of the lateral resisting system. Therefore, the failure mechanism of the stories that depends on the structural geometry, yield strains and lateral load resisting system may be determined. In the case of a capacity – designed structure, the lateral mechanism can be predicted with good approximation. For steel braced frames considered in this study, the mechanisms are assumed to form by yielding of braces in concentric systems and by flexural yielding of link beams in eccentric systems. In these models one column lift is assumed for each story, which means that no rotational column plastic hinge is produced.
For concentric X bracing systems as shown in fig. 2-a the lateral story displacement St can be written as a function of story shear V, brace span Ls, brace length Lb, brace sectional area Ab and elastic section modulus E as,
E. Ab L
displacement Syi in a concentric braced frame can obtained as,
s = VjA g = Lg yitf. Ab L L.
which does not depend on the brace sectional area. Similarly, for the Chevron bracing shown in fig. 2- b, the same equations may be obtained as follows,
2Vt A E. Ab L
It is again clear that for Chevron bracing the displacement shape in yielding case of the structure is independent of the brace sectional properties. Thus for the concentric systems, an equal span and story height result in a linear deformed shape of the structure over the height at yield condition.
For eccentric bracing systems (fig. 2-c), the lateral displacement depends on plastic rotation capacity of the link beam, Qp, and can be expressed through the following equations,
Sy^H, .SinQQ) – li.(1 – Cos(Qp-Q)) (17)
where Q is defined as a function of section plastic moment M and plastic stiffness Kp as,
Q Mp v. h,
p Kp 2Kp
For small length link beams, the shear mechanisms are formed while in moderate length link beams moment plastic hinges are generally formed. The latter case has been considered here. The value of вis generally related to the connection and link beam details.
Figure 2: Structural models for (a) X bracing, (b) chevron bracing and (c) eccentric bracing systems