Technical_Report_STEEL_FRAMED_BUILDING-Seismic_Analysis.pdf

1.0 Design Parameters
1.1 Ground motion and system parameters. For this Design Project the parameters are as
follows.
Building location : IRAQ - Mosul
Occupancy Risk Category : II
Importance Factor Ie : 1.0
Mapped Spectral Parameters :
SS = 1.73
S1 = 0.93
Site Class : C
Site Coefficients ;
Fa = 1.0
Fv = 1.5
Spectral Response Coefficients ;
SDS = 1.153
SD1 = 0.93
Seismic Design Category; D
Lateral System ;
Moment Resisting frame - intermediate steel moment frame
Braced Frame System - ordinary steel concentrically braced frame
Response Modification Coefficients ;
R = 4.5
R = 3.25
Seismic Response Coefficients Cs ; 0.05
Design Base Shear ; As calculated per FEM program
Analysis Procedure ; Model Analysis Procedure
Note that Standard (chapter 12 Seismic design Requirement for Building Structures) permits an
ordinary steel moment frame for buildings that do not exceed one story and 60 feet tall with a roof
dead load not exceeding 20 psf. Intermediate steel moment frames with stiffened bolted end plates
and ordinary steel concentrically braced frames are used in this project.

North-south (N-S) Z-direction:
Moment-resisting frame system = intermediate steel moment frame
(Standard Table 12.2-1)
R = 4.5
0 = 3
Cd = 4
East-west (E-W) X-direction:
Braced frame system = ordinary steel concentrically braced frame
(Standard Table 12.2-1)
R = 3.25
0 = 2
Cd = 3.25
1.2.0 Loads
Roof live load (L), snow = 25 psf
Roof dead load (D) = 15 psf
Wind load ( X- direction) = Per ASCE-7-2010
Wind load ( Z- direction ) = Per ASCE-7-2010
Seismic load ( X- direction) = Per IBC-2012
Seismic load ( Z- direction ) = Per IBC-2012
Weight of Side Wall panels = 25 psf
Note:
Roof dead load includes roofing, insulation, metal roof deck, purlins, mechanical and electrical
equipment, and the self-weight of that portion of the main frames that is tributary to the roof under
lateral load.
1.3.0 Materials
Concrete for footings: fc' = 2.5 ksi
Slabs-on-grade: fc' = 4.5 ksi
Reinforcing bars: ASTM A615, Grade 60
Structural steel (European IPE, HEA, UPE sections): S355 ≈ ASTM A36
Plates (except continuity plates): ASTM A36
Bolts: ASTM A325
Continuity Plates: ASTM A572, Grade 50

2.0 Structural Design Criteria
2.1.0 Building configuration.
Because there is o mezzanine or levels at any end, vertical weight irregularities are considered to
apply (Standard Sec. 12.3.2.2). However, the upper level is a roof and the Standard exempts roofs
from weight irregularities. There also are no plan irregularities in this building (Standard Sec.
12.3.2.1).
2.2.0 Redundancy.
In the N-S direction, the moment frames do not meet the requirements of Standard
Section 12.3.4.2b since the frames are only one bay long. Thus, Standard Section 12.3.4.2a must
be checked.
By inspection, the critical frames are at gridline 1-1. The effect of loss of a connection at either of
these frames—one at a time—is evaluated to determine whether the system has sufficient
redundancy. A copy of the three-dimensional model is made using FEM Package program
STAAD.Pro v21 2017, with the moment frame beam at Gridline A pinned. The structure is
checked to make sure that an extreme torsional irregularity (Standard Table 12.3-1) does not occur
by comparing the maximum drift to 1.4 times the average drift:
Δ Δ
1.4( ) Δ
2
K A
A


5.20 >> 2.5 inches
where:
∆A = maximum displacement at knee along Gridline A, in.
∆K = maximum displacement at knee along gridline K, in.
The maximum drift is less than 1.4 times the average. Thus, the structure does not have an extreme
torsional irregularity when a frame loses moment resistance.
Additionally, the structure must be checked in the N-S direction to ensure that the loss of moment
resistance at Beam A has not resulted in more than a 33 percent reduction in story strength. This
can be checked using elastic methods (based on first yield) as shown below, or using strength
methods. The original model is run with the N-S load combinations to determine the member with
the highest demand-capacity ratio. This demand-capacity ratio, along with the applied base shear,
is used to calculate the base shear at first yield:
Thus, the loss of resistance at both ends of a single beam only results in a 6 percent reduction in
story strength. The moment frames can be assigned a value of = 1.3.

In the E-W direction, the OCBF system meets the prescriptive requirements of Standard
Section 12.3.4.2a. As a result, no further calculations are needed and this system can be assigned
a value of = 1.3.
2.3.0 Orthogonal load effects.
A combination of 100 percent seismic forces in one direction plus 30 percent seismic forces in the
orthogonal direction must be applied to the columns of this structure in Seismic Design Category
D (Standard Sec. 12.5.4). The Standard requires this in conditions in which the interaction of
orthogonal ground motions is likely to have a significant effect. In this case, the columns that are
shared by orthogonal frames require consideration of simultaneous accelerations in the orthogonal
building axes.
2.4.0 Structural component load effects.
The effect of seismic load (Standard Sec. 12.4.2 page 86 ) is:
LOAD COMBINATION No. 24
L + 0.2 S + ρ EQz + (1.2 + 0.2 SDs ) D = L + 1.3 EQz + 1.431 D
LOAD COMBINATION No. 25
1.6 H + ρ EQz + (0.9 - 0.2 SDs ) D = 1.6 Wz + 1.3 EQz – 0.67 D
2.5.0 Drift limits.
For a building assigned to Risk Category II, the allowable story drift (Standard Table 12.12-1)
is:
 a = 0.025hsx in the E-W direction
 a/ = 0.025hsx/1.0 in the N-S direction
At the roof ridge, hsx = 15 ft. and = 4.5 in > 2.5 in (checked by Staad program)
At the knee (column-roof intersection), hsx = 14.3 ft. and a = 4.3 in.
Note
In Standard Table 12.12-1 permits unlimited drift for single-story buildings with interior walls,
partitions, etc., that have been designed to accommodate the story drifts. The main frame of the
building can be considered to be a one-story building for this purpose, given that there are no
interior partitions except below the mezzanine.

2.6.0 Seismic weight.
The weights that contribute to seismic forces are:
Roof D = (0.015)(40)(80) = 48 kips
Panels at sides = (2)(0.025)(14.5)(80)/2 = 29 kips
Panels at ends = (2)(0.025)(14.5)(40)/2 = 58 kips
Main frames = 0.045 kips / ft ≈ 22 kips
Seismic weight = 157 kips
The weight associated with the main frames accounts for only the main columns, because the
weight associated with the remainder of the main frames is included in the roof dead load above.
The computed seismic weight is based on the assumption that the wall panels offer no shear
resistance for the structure. Additionally, snow load does not need to be included in the seismic
weight per Standard Section 12.7.2 because it does not exceed 30 psf.
3.0Analysis
Base shear will be determined using an ELF analysis BY FEM Package program STAAD.Pro
v21 2017.
3.1.0 Roof deck diaphragm.
In the E-W direction, Torsion is not significant, so a simple approximation is to take half the force
to each side and divide by the length of the building.
In the N-S direction, the shear is highest just west of gridline 1-1 due to the higher stiffness of the
frames. A three-dimensional model or a rigid-diaphragm analysis is required to determine the
diaphragm reactions at each frame, from which the diaphragm shears are determined.
From the FEM analysis (Figure 3.1.1 and 3.1.2 ), fames 1-1 & 7-7 together resist 60% of the roof
shear, while only 35% of the roof is east of line 2-6. Thus approximately 45% of the roof shear
must be resisted by the diaphragm immediately west of line I.

Figure (3.1.1)
FFF
Figure (3.1.2)

Technical_Report_STEEL_FRAMED_BUILDING-Seismic_Analysis.pdf

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