Design Example 1
Concrete Diaphragm Design—Four-Story Building
Overview
This example illustrates the design of concrete diaphragms, chords and collec­tors for a four-story office
building with vertical and horizontal irregularities in accordance with the provisions of the 2012 In­
ternational Building Code (2012 IBC) for Seismic Design Category (SDC) B and SDC D. The gravity loadresisting system consists of concrete beams and girders on the interior and exte­rior, which are supported
by concrete columns. Lateral loads are distributed to the vertical elements through a 61/2-inch-thick nor­mal
weight concrete slab at each of the roof and floor levels. Resis­tance to lateral loads is provided by 12-inchthick concrete shear walls in both directions. Typical floor plans and sec­tions of the structure are shown in
Figures 1-2 through 1-8. A three-dimen­sional view of the structure is shown in Figure 1-4.
The following steps provide a detailed analysis of some of the important seis­mic requirements for the
diaphragm, chord and collector design in accordance with the 2012 IBC. Semi-rigid diaphragm is used to
represent the floor and roof diaphragm in the lateral analysis procedure. ETABS is used to analyze the three
dimensional model of the example building. Shell elements are used to model the diaphragm.
This example is not a complete building design. Many aspects have not been included and only select steps
of the seismic design have been illus­trated. The following irregularities are covered in this example:
a. Horizontal Irregularity Type 4
b. Vertical Irregularity Type 4
Outline
This example will illustrate the following parts of the design process:
1. Determination of Diaphragm Demands for SDC B
2. Considerations for Horizontal and Vertical Irregularities
3. Determination of Diaphragm Shear at Second Level for SDC B for North-South Direction
4. Design of Diaphragm at Second Level for SDC B
5. Design of Collector at Third Level for SDC B for East-West Direction
6. Brief Discussion of Seismic Design Categories C through F
7. Determination of Diaphragm Demands for SDC D
8. Design of Diaphragm at Second Level for SDC D
9. Design of Collector at Third Level for SDC D
Guide to the Design of Common Irregularities in Buildings 1
Design Example 1 n Concrete Diaphragm Design—Four-Story Building
Given Information
Site data:
Site Class D (stiff soil), by default
Building data:
The example building is Risk Category II in accordance with Table 1.5-1 of ASCE 7-10.
ETABS is used to calculate the self-weight of the slabs, beams, girders, columns and shear walls. All
member sizes assumed in the analysis are shown in Figures 1-2 and 1-3. The following are the modeling
as­sumptions incorporated into the computer analysis:
• The concrete shear walls and concrete slabs were assigned a stiffness modifier of 0.35 to model
cracked section properties (ACI 10.10.4.1).
• All building columns were assigned a stiffness modifier of 0.0001 to eliminate lateral force
resistance.
• All nodes at the foundation level for columns were assigned pinned supports.
• All nodes at the foundation level for shear walls were assigned pinned supports.
• Self-weight of slabs, beams, columns and walls are automatically calculated by the program.
• Static analysis was carried out to design the example building.
• Live load of 100 psf is used for floors.
• Additional dead loads:
Floor mass (seismic):
Ext. Cladding 10 psf
Partitions10 psf
MEP 4 psf
Flooring 2 psf
Misc. 3 psf
29 psf
Floor loads (gravity):
Ext. cladding
10 psf
MEP 4 psf
Flooring
2 psf
Partitions15 psf
Misc. 3 psf
34 psf
Roof mass (seismic):
Ext. cladding
5 psf
Partitions 5 psf
MEP 4 psf
Roofing 3 psf
Mechanical
10 psf
Misc. 3 psf
30 psf
2 Guide to the Design of Common Irregularities in Buildings
Design Example 1 n Concrete Diaphragm Design—Four-Story Building
Roof loads (gravity): Ext. cladding
5 psf
MEP
4 psf
Roofing
3 psf
Mechanical 10 psf
Misc.
3 psf
25 psf
Notes:
1. The exterior cladding weight (mass) was accounted for by including it as part of floor and roof
mass. This is conservative and a more accurate representation would be to apply these loads as
line loads applied to the perimeter beams of the analysis model.
2. Per ASCE Section 12.7.2, 10 psf is included for partition load.
3. The typical story height is 15 feet.
The following are the total seismic weights tributary to each floor level, as calculated by the computer
program:
Wroof = 3, 524 k
W4th floor = 3, 720 k
W3rd floor = 3, 720 k
W2nd floor = 3, 720 k
ΣW
= 14, 684 k
Note: Seismic weights are equal for both the north-south and east-west directions.
The following material properties are assumed for this example:
f
'c = 4,000 psi
f y = 60, 000 psi
Typical slab: 61/2-inch-thick with #5 @ 12 inch o.c., top and bottom in the east-west direction. Temperature
and shrinkage reinforcement: #4 @ 12 inch o.c. in the north-south direction. See Figure 1-1 below.
Typical concrete framing member sizes are as follows:
Interior beams
21 × 24
Perimeter beams
21 × 24
Interior girders
24 × 28
Exterior columns
21 × 21
Interior columns
24 × 24
Guide to the Design of Common Irregularities in Buildings 3
Design Example 1 n Concrete Diaphragm Design—Four-Story Building
TYPICAL FLOOR SLAB
6½″
¾″ CLR
TYP
#5 @ 12″ OC TOP AND BOTTOM
TYP SLAB REINFORCEMENT
#4 @ 12″ OC TEMPERATURE &
SHRINKAGE REINFORCEMENT
Figure 1‑1. Typical concrete slab section
A
B
30′-0″
COLLECTOR
30′-0″
4
30′-0″
C
30′-0″
D
30′-0″
E
A
15′-0″
TYP
30′-0″
F
21″×21″ EXTERIOR
COLUMN, TYP
G
30′-0″
COLLECTOR
Fig 1-9
3-#6 ADDED
CHORD BAR
AT SDC B;
6-#7 ADDED
CHORD BARS
AT SDC D
24″×24″ INTERIOR
COLUMN, TYP
COLLECTOR
30′-0″
3
SHEAR
WALL
BELOW
SHEAR
WALL
BELOW
30′-0″
2
COLLECTOR
24″×28″
INTERIOR
GIRDER, TYP
21″×24″
INTERIOR
BEAM, TYP
COLLECTOR
N
1
12″ THICK
SHEARWALL, TYP
COLLECTOR
8½″ THICK
CONCRETE SLAB,
WITH #5 @ 12″
TOP & BOTTOM, TYP
21″×24″
PERIMETER
BEAM, TYP
SLAB EDGE,
TYP
Figure 1-2. 2nd level floor plan
4 Guide to the Design of Common Irregularities in Buildings
COLLECTOR
3-#6 ADDED
CHORD BAR
AT SDC B;
6-#7 ADDED
CHORD BARS
AT SDC D
Design Example 1 n Concrete Diaphragm Design—Four-Story Building
A
30′-0″
B
30′-0″
C
D
COLLECTOR
15′-0″
TYP
COLLECTOR
4
30′-0″
E
30′-0″
B
F
30′-0″
21″×21″ EXTERIOR
COLUMN, TYP
G
30′-0″
COLLECTOR
Fig 1-10
30′-0″
SHEAR
WALL
BELOW
30′-0″
3
24″×24″ INTERIOR
COLUMN, TYP
30′-0″
2
24″×28″
INTERIOR
GIRDER, TYP
COLLECTOR
B
21″×24″
INTERIOR
BEAM, TYP
N
SHEAR
WALL
BELOW
Fig 1-10
CHORD
REINFORCEMENT
TYP
1
12″ THICK
SHEARWALL, TYP
21″×24″ PERIMETER
BEAM, TYP
SLAB EDGE, TYP
6½″ THICK CONCRETE
COLLECTOR SLAB, WITH #5 @ 12″
TOP & BOTTOM, TYP
COLLECTOR
Figure 1-3A. 3rd level floor plan
A
30′-0″
B
COLLECTOR
4
30′-0″
15′-0″
TYP
C
30′-0″
D
30′-0″
COLLECTOR
E
30′-0″
F
21″×21″ EXTERIOR
COLUMN, TYP
G
30′-0″
COLLECTOR
30′-0″
CHORD
REINFORCEMENT
TYP
30′-0″
3
24″×24″ INTERIOR
COLUMN, TYP
30′-0″
2
21″×24″
INTERIOR
BEAM, TYP
24″×28″
INTERIOR
GIRDER, TYP
COLLECTOR
N
1
12″ THICK
SHEARWALL, TYP
6½″ THICK CONCRETE
COLLECTOR SLAB, WITH #5 @ 12″
TOP & BOTTOM, TYP
21″×24″ PERIMETER
BEAM, TYP
SLAB EDGE, TYP
COLLECTOR
Figure 1-3B. 4th level floor plan
Guide to the Design of Common Irregularities in Buildings 5
Design Example 1 n Concrete Diaphragm Design—Four-Story Building
4
A
OUT OF PLANE OFFSET
(TYPE 4 HOR. IRREG.)
1
2
3
IN-PLANE DISCONTINUITY
(TYPE 4 VERTICAL IRREG.)
Figure 1-4. Three-dimensional view of the structure
A
B
C
D
E
F
G
ROOF
4TH FLOOR
3RD FLOOR
2ND FLOOR
BASE
Figure 1-5. Typical E-W elevation (gridline 1)
6 Guide to the Design of Common Irregularities in Buildings
Design Example 1 n Concrete Diaphragm Design—Four-Story Building
A
B
C
D
E
F
G
ROOF
4TH FLOOR
3RD FLOOR
2ND FLOOR
BASE
Figure 1-6. Typical E-W elevation (gridline 4)
1
2
3
4
1
2
3
4
B
B
B
B
A
A
A
A
ROOF
ROOF
4TH FLOOR
4TH FLOOR
3RD FLOOR
3RD FLOOR
2ND FLOOR
2ND FLOOR
BASE
BASE
Figure 1-7. Typical N-S elevation (gridline B/A)
Guide to the Design of Common Irregularities in Buildings 7