General requirements (beams seismic: ACI 318)

Maximum allowed factored axial force

Flexural elements with high axial loading values under any load combination are handled in the seismic design and detailing as compressive members.

If SFRS Type = Ordinary Moment Frame, then no axial compression limit applies.

If SFRS Type = Intermediate Moment Frame or SFRS Type = Special Moment Frame


P_max	=	A_g * f’_c / 10
where
P_max	=	Maximum allowed compression value on the member
A_g	=	Gross area of the concrete section
f’_c	=	specified compressive strength of concrete

The check passes if;


P_u	≥	P_max
where
P_u	=	Maximum factored compressive axial force anywhere in the span considering all load combinations

Maximum allowed effective depth

The maximum allowed effective depth of a beam part of a Special Moment Frame is proportional to its clear span length to limit the overloading of the adjacent joints and columns.

If SFRS Type = Special Moment Frame


d	≤	d_max
where
d	=	Distance between the extreme compression fiber and the longitudinal tension reinforcement centroid
d_max	=	Maximum allowed distance between the extreme compression fiber and the longitudinal tension reinforcement centroid
d_max	=	0.25 * l_n
where
l_n	=	Length of the clear span measured from face-to-face of supports

Minimum allowed width

Beams part of Special Moment Frames in buildings subjected to earthquake effects have a minimum width limit for their web.

If SFRS Type = Special Moment Frame


b_w	≥	b_w,min
where
b_w	=	Beam web width
b_w,min	=	Minimum allowed beam web width
	=	MAX( 0.3 * h, 250 mm)	Metric units
	=	MAX( 0.3 * h, 10 in)	US units
where
h	=	Overall depth of the concrete section

Maximum allowed width

Despite not being advisable, beams in Special Moment Frames are allowed to be wider than the supporting columns up to a fixed limit.

Note: The maximum lateral extension of a beam on each side of the joining column is beyond scope in the current release of Tekla Structural Designer.