Civil Engineering Reference
In-Depth Information
vertically discontinuous wall at grid lines 2 and 3. Shear wall SW3 at grid line 2 sup-
ports the roof diaphragms that span from grid lines 1 and 2 and from grid lines 2 to 3.
Since the shear wall at grid line 2 does not continue to the foundation, the second-floor
diaphragm must span from grid line 1 to 3. The lateral force from SW3 must be trans-
ferred through the second-floor diaphragm into shear walls SW2 and SW5. The loading
of the second-floor diaphragm is shown in the figure, which consists of the uniform
load tributary to the second floor plus the concentrated force from shear wall SW3.
Shear wall SW5 is a high-demand wall that receives the reaction of the second-floor
diaphragm plus the tributary load from the roof diaphragm, and it must be carefully
designed due to the complex load paths within the wall line.
Whenever horizontal or vertical irregularities exist, elements supporting discon-
tinuous walls or frames must comply with ASCE 7-05 Section 12.3.3.3, which states the
following:
Columns, beams, trusses, or slabs supporting discontinuous walls or frames of structures hav-
ing a horizontal irregularity type 4 as listed in Table 12.3-1 or vertical irregularity type 4 in
Table 12.3-2 shall have the design strength to resist the maximum axial force that can be devel-
oped in accordance with the load combinations with over-strength factor of Section 12.4.3.2.
The connections of such discontinuous elements to the supporting members shall be adequate
to transmit the forces for which the discontinuous elements were required to be designed.
This requirement applies to all structures that have type 4 vertical and horizontal
irregularities designed in seismic design categories (SDC) B through F. The purpose
of the special load combinations is to protect the gravity load carrying system and to
prevent the collapse of the elements supporting the discontinuous walls or frames,
which would shift the lateral force of that element to other parts of the structure. The
ASCE 7-05 Commentary notes that some elements are incapable of safely resisting
ground-shaking demands through inelastic behavior. To ensure safety, these ele-
ments must be designed with sufficient strength to remain elastic. There is still some
debate within the engineering community over which elements should be included
in the requirements of ASCE 7-05 Section 12.3.3.3. The Structural Engineers Associa-
tion of California (SEAOC) noted in volume 2
4
of its
Structural Seismic Design Manual
that its Seismology Committee is proposing a code change to include only compres-
sion elements.
Controlling basic seismic load combinations (see ASCE 7-05 Sections 12.4.3 and
12.4.3.1 for special notes and exceptions:
Basic load combinations for strength design with overstrength factor:
E
m
=
E
mh
+
E
v
for LC 5
ASCE 7-05 Eq. 12.4-5
for strength design or LC5 and LC6 for allowable stress design
E
m
=
E
mh
-
E
v
for LC 7
ASCE 7-05 Eq. 12.4-6
for strength design or LC8 for allowable stress design
where
E
mh
= Ω
o
Q
E
and
E
v
= 0.2
S
DS
D.
Strength design, seismic, ASCE 7-05 Section 12.4.3.2
5. (1.2 + 0.2
S
DS
)
D
+ Ω
o
Q
E
+
L
+ 0.2
S
7. (0.9 - 0.2
S
DS
)
D
+ Ω
o
Q
E
+ 1.6
H
Search WWH ::
Custom Search