§15.[F3] Eccentrically Braced Frames (EBF)

The following frame configurations are considered valid EBF frames by the RAM Structural System.

Note: If any other lateral member (beam or column or brace) is supported by a link in any of the above configurations, the frame will not be considered a valid EBF frame.

§15.2 [F3.5b] Links

§15.2a – The maximum axial force in the link is taken as the maximum axial load from all the standard provision load combinations.

§15.2fb – The axial force (Pu (LRFD), Pa (ASD)) in the link is the same as that calculated for §15.2e.

§15.2c – The link rotation angle is taken as the largest calculated link rotation from all seismic load cases. The program uses the approximate link rotation angle calculation described in the commentary. The formulae provided in the commentary are modified as follows to account for unsymmetrical geometry. Note that the frame displacement (Delta) is calculated as the net difference in horizontal displacement of the column top node relative to the column bottom node, projected into the plane of the original frame. The story ht (h) is calculated as the node to node distance from the beam-column joint down the column to the next braced level. The link length (e) is the clear length (from face of column if at a support).

EBF beams are required to span between two steel, lateral columns.

Δ is calculated from the end furthest away from the link.

θ_{p} = \frac{Δ}{h}

γ = \frac{θ_{p} (L - e)}{e}

Configuration 1

γ_{1} = θ_{p 1} + \frac{θ_{p 2} b + θ_{p 1} a}{e}

γ_{2} = θ_{p 2} + \frac{θ_{p 2} b + θ_{p 1} a}{e}

Configuration 2

γ_{1} = \frac{(θ_{p 1} + θ_{p 2})}{2} \frac{(e_{1} + a)}{e_{1}}

γ_{2} = \frac{(θ_{p 1} + θ_{p 2})}{2} \frac{(e_{2} + b)}{e_{2}}

Configuration 3

The calculated angle is then multiplied by the Cd value provided by the engineer in the Codes dialog box, to produce the plastic link rotation angle. This magnified rotation angle is then compared to the code prescribed limits.

The calculated plastic link rotation angle in conservatively not reduced by the elastic link rotation angle.

For the central EBF configuration, where two different rotation angles are calculated, the larger angle is reported.

Note: No additional consideration is given to the calculations for sloped beams, that is, the link displacement is assumed to be perpendicular to the link. Also, no consideration is given for rigid body translation and rotation that may exist in an EBF frame in the upper stories of a tall structure.

§15.3 [F3.5b(4)] Link Stiffeners

Link Stiffeners are detailed in the output with a schematic diagram (not-to-scale) similar to that shown inf the following figure. In the output the table below the schematic provides details on the stiffener numbers and dimensions. The table contains the following columns.

Setting	Description
Link	link number on the beam (1 or 2)
e	the length of the link
#	number of equal spaces for the intermediate stiffeners
X	spacing of the first stiffener
WxT End	width and thickness of the end stiffeners
WxT Int	width and thickness of the intermediate stiffeners
Int. Stiff Sides	the number of sides of the beam web that the intermediate stiffeners are required on

§15.4 [F3.6c] Link-To-Column Connections

§15.4a – Link-to-column connection design should be based on test results and are not designed in RAM Frame.

§15.4b – If the beam-to-column connection is reinforced so as to preclude yielding over a length of the link the engineer is responsible for determining the applicability of the RAM Frame results. This situation is not currently considered in the EBF design performed by RAM Frame.

§15.6 [F3.6e] Diagonal Brace and Beam Outside of Link

The design of Eccentric Braced Frames are sensitive to the geometric configuration of the braces (and link length). In particular the ability to get the beam outside the link to meet code specification may require a change in the geometry of the brace rather than an increase or decrease in beam size. The engineer is referred to the AISC website for references (Steel Tips) regarding efficient EBF brace configurations.

§15.6a – To perform the diagonal brace capacity provision the link beam at the top of the brace is located and the link capacity (1.25 Ry Vn) is calculated. The analyzed load case that produces the largest shear in the identified link is also determined. The brace is then designed for a load combination that consists of a single term (load factor x load case). The combination factor is taken as the ratio of the link capacity to the maximum link shear force. The load case in the combination is that which produced the maximum link shear force. This load combination should thus produce the desired shear force (equal in magnitude to the link capacity) in the link. For example, if a link capacity is 100 kip, and the maximum shear in the link is 40 kip from load case E1, then the generated load combination is 100/40 E1 = 2.5 E1 (both positive and negative combinations are considered). All the above-mentioned parameters are shown on the report.

Note: These calculated load factors could be fairly large in magnitude if the beam used for the link is overly conservatively sized or if the geometry of the braces is such that the link experiences more of a bending rather than a shear failure.

§15.6b – The design of the beam outside the link is performed in a similar fashion to §15.6a. That is, the link(s) on a beam are identified and load combinations are generated based on the ratio of the link shear to link capacities. For each link on a beam there are two load combinations created (as described in §15.6a). These combinations are then used to determine if the beam (outside of the link) is adequate to resist the forces that would ensure link yielding before beam failure. Note: The code stipulates that the beam capacity (outside of the link) can be increased by Ry. In RAM Frame the program implements this provision by reducing the demand (load factor in the combination) by Ry. All the above mentioned parameters are shown on the report.

§15.6d – The brace connection forces are taken as the maximum forces at the top of the brace, generated by the load combination(s) created according to §15.6a.

The design of a column according to load combinations A4 and A5 is not performed as part of the seismic provisions, but rather as a part of the code check performed in standard provision mode. In seismic mode RAM Frame identifies all beams that frame into an EBF column and determines if they are EBF beams. If so, the links are identified and, similar to §15.6a, load combinations are generated that would result in the appropriate force being generated in the links and the column in consideration. In the event that there are non-EBF beams supported on the column the user will be given a warning. The warning is to let the user consider the fact that there may be a loading condition that would result in the non-EBF frame beam causing controlling loads on the column, prior to the EBF link beams yielding. This is most likely to be a consideration when the non-EBF beam frames into the column perpendicular to the EBF beams.