Seismic Provisions For RC Structures ACI318-05
Seismic Provisions For RC Structures ACI318-05
Seismic Provisions For RC Structures ACI318-05
Structures
Provisions of ACI-318-05 chapter 21
must be applied in addition to other
provisions of the ACI code
Limitations on Materials
i. f’c ≥ 3000 psi
iii. Reinforcement shall comply with ASTM A-706. ASTM A-615 Grades 40
and 60 are permitted if they satisfy items iv and v below.
v. fu/fy ≥ 1.25
vi. fyt ≤ 60,000 psi for all transverse reinforcement, including spirals
Flexural Members of Special Moment
Frames
Flexural Design:
24 in 24 in
Beam (14" x 20") Column (18" x 24")
2 - #9
2 - #9 2 - #9 2 - #9
24 ft
4 in
20 in #3 Stirrup
14 in 14 in
Section at Column Face Mid-Span Section
Geometric constraints for beams
• ρmax ≤ 2.5 %
Minimum positive and negative moment
capacity at each section (ACI 318-05, 21.3.2.2)
• Mn+ ≥ 0.5Mn- at column face
bc Ach bc
(shaded Area)
h h
Direction of
earthquake load
bc: Core dimension measured Ach : Area enclosed by perimeter
center-to-center of perimeter tie tie measured out-to-out of tie
Transverse reinforcement
• Transverse reinforcement shall be provided by either single or overlapping
hoops. Crossties of the same bar size and spacing as the hoops shall be
permitted. Each end of the crosstie shall engage a peripheral longitudinal
reinforcing bar. Consecutive crossties shall be alternated end for end along
the longitudinal reinforcement.
Transverse reinforcement
of special moment
hlong D
resisting frames s o h short 12 in D 12 in If beam reinforcement is
continuous over the support:
h short h 20d b ( NWC)
0.4
(ACI 318-05, 21.4.4) c
h h long
s h/4 s' 6 in
h 26d b (LWC)
s’
s 6 (db)long. s' 6(d b )long.
0.01 ρ 0.06
ρ Ast /Ag s s0 but need not be less than 4 in
14 h x
Where; s 0 4 6 in
3
o Consecutive crossties
engaging the same
longitudinal bars have their
90 deg hooks on opposite
6db ≥ 3 in sides of columns
Extension
6db Extension
Vu
Mu
Pu
o h long
o c / 6
hx hx hx
o 18 in hx shall not exceed 14 inches
Design forces
The design shear force, Ve , shall be determined from
consideration of the maximum forces that can be
generated at the faces of the joints at each end of the
member. These joint forces shall be determined using the
maximum probable moment strengths, Mpr, at each end
of the member associated with the range of factored axial
loads, Pu, acting on the member. The member shears
need not exceed those determined from joint strengths
based on Mpr of the transverse members framing into the
joint. In no case shall Ve be less than the factored shear
determined by analysis of the structure.
Transverse reinforcement over the lengths lo ,
identified in 21.4.4.4, shall be proportioned to resist
shear assuming Vc = 0 when both (a) and (b) occur:
T2 = 1.25As fy C1 = T 1
C1 = T1
j j Vj = Ve1- T2 – C1 j j Vj = Ve1 – C1
C2 = T2 T1 = 1.25As fy T1 = 1.25As fy
Ve2 Ve2
Ve3 Ve3
C2 = T2 T1 = 1.25As fy
T1 = 1.25As fy
j j Vj = T1 + C2 – Ve3 j j Vj = T1 – Ve3
T2 = 1.25As fy C1 = T1 C1 = T1
Ve4 Ve4