Civil Engineering Reference
In-Depth Information
Table 2-6 Reduction Multiplier (RM) for Live Loads, Building #2
Interior Columns Edge Columns Corner Columns End Shear Wall Units
Story K
LL
A
T
(ft
2
) RM
LL
A
T
(ft
2
) RM
LL
A
T
(ft
2
) M
LL
A
T
(ft
2
) RM
5 (roof) * * * *
4 1920 0.59 960 0.73 480 0.94 1440 0.65
3 3840 0.49 1920 0.59 960 0.73 2880 0.53
2 5760 0.45 2880 0.53 1440 0.65 4320 0.48
1 7680 0.42 3840 0.49 1920 0.59 5760 0.48
*
No reduction permitted for roof live load (ASCE 4.8.2); the roof should not be included in the influence areas of the
floors below.
For the interior columns, the reduced live load is L = 0.42L
o
at the first story (A
T
= 4 bay areas
4 stories =
20
24
4
4 = 7680 sq ft). The two-way slab may be designed with an RM = 0.94 (A
T
= 480 sq ft for one
bay area). Shear strength around the interior columns is designed for an RM = 0.59 (A
T
= 1920 sq ft for 4 bay
areas), and around an edge column for an RM = 0.73 (A
T
= 960 sq ft for 2 bay areas). Spandrel beams could
be designed for an RM = 0.94 (one bay area). If the floor system were a two-way slab with beams between
columns, the interior beams would qualify for an RM = 0.73 (2 bay areas).
2.2.4
Factored Loads
The strength design method using factored loads to proportion members is used exclusively in this topic.
The design (service) loads must be increased by specified load factors (ACI 9.2), and factored loads must be
combined in load combinations depending on the types of loads being considered. The method requires that the
design strength of a member at any section should equal or exceed the required strength calculated by the code-
specified factored load combinations. In general,
Design Strength
≥
Required Strength
or
Strength Reduction Factor (
φ
)
Nominal Strength
≥
Load Factor
Service Load Effects
All structural members and sections must be proportioned to meet the above criterion under the most critical
load combination for all possible load effects (flexure, axial load, shear, etc.).
For structures subjected to dead, live, wind, and earthquake loads only, the ACI applicable load combinations
are summarized in Table 2-7:
The strength reduction factors are listed below:
Tension-controlled sections*
0.90
Compression-controlled sections*
Members with spiral reinforcement conforming to 10.9.3
0.75
Other reinforced members
0.65
Shear and torsion
0.75
Bearing on concrete
0.65
* For definition of tension controlled, compression controlled sections and strength reduction for transition sections, see Chapters 3 and 5.
Search WWH ::
Custom Search