Summary of CHAPTER 6 (ACI318-14M) Structural Analysis

SUMMARY OF CH 6 ACI318-14
Structural Analysis
BY: ABDULLAH ABDULRAHMAN KHAIR

Structural Analysis on ACI318-14 By Abdullah Khair
Page | 1
Summary of CHAPTER 6 (ACI318-14M)
Structural Analysis
By: Abdullah Abdulrahman Khair
6.1 this chapter shall apply to methods of analysis, modeling of members and structural
systems, and calculation of load effects.
6.2.3 Methods of analysis permitted by this chapter shall be (a) through (e):
(a) The simplified method for analysis of continuous beams and one-way slabs for
gravity loads
(b) First-order
(c) Elastic second-order in
(d) Inelastic second-order
(e) Finite element
Commentary on 6.2.3
first-order analysis satisfies the equations of equilibrium using the original undeformed
geometry of the structure. When only first-order results are considered, slenderness effects
are not accounted for. Because these effects can be important, 6.6 provides procedures
to calculate both individual member slenderness (Pδ) effects and sidesway (P∆) effects
for the overall structure using the first-order results.
A second-order analysis satisfies the equations of equilibrium using the deformed
geometry of the structure. If the second-order analysis uses nodes along compression
members, the analysis accounts for slenderness effects due to lateral deformations along
individual members, as well as sidesway of the overall structure. If the second-order
analysis uses nodes at the member intersections only, the analysis captures the sidesway
effects for the overall structure but neglects individual member slenderness effects. In this
case, the moment magnifier method (6.6.4) is used to determine individual member
slenderness effects.

Page | 2
What is P-delta effect?
P-delta effect is secondary effect on structure. It is also known as ‘Geometric nonlinearity
effect’. As number of stories increases, P-delta effect becomes more important. If the
change in bending moments and displacements is more than 10%, P-delta effect should
be considered in design.
There are two distinct types of P-delta effects: P-∆ (sometimes referred to as “large P
delta” or “P-large delta”) and P-δ (sometimes referred to as “small P-delta” or “P-small
delta”):
P-∆ has reference to the effects of the vertical loads acting on the laterally displaced
structure. For example, wind or seismic forces, V, cause a horizontal displacement, ∆, of
the structure, while the gravity loads, P, simultaneously act vertically on this displaced
structure. Moments are induced into the structure equal to the total vertical load P times
the structural displacement ∆.
P-δ has reference to the effects of the axial load in an individual member subject to a
deflection (curvature) between its endpoints. For example, column loads, P, due to
gravity, wind, and/or seismic forces act on a column that has a curvature induced by
the connection conditions of supported beams. Moments are induced in the member
proportional to the axial load P times the member deflection δ. Note that axially loaded
beams also experience these effects.

Page | 3
In conventional first order structural analysis, the equilibrium is expressed in terms of the
geometryof the un-deformed structure. In case of linearly elastic structure, relation
between displacement and external force is proportional. In addition, stress-strain
relationship of material is linear. Thus, by definition, this method excludes nonlinearity, but
it generally represents conditions at service loads very well. The first order elastic analysis
is based on following assumptions: -
(1) Material behaves linearly and hence all yielding effect can be ignored.
(2) The member behaves linearly, and the member instability effect such as those caused
by axial compression (these are called P-δ effects), which reduces the member’s flexural
stiffness, can be ignored.
(3) The frame also behaves linearly, and the frame instability effects, such as those
caused by the moments due to horizontal frame deflection and gravity loads acting on
the displaced structure (these are called P-∆ effects), can be ignored.
Though the first–order elastic analysis provides an ‘exact solution’ that satisfies the
requirements of compatibility and equilibrium of the un-deformed structure, it does not
provide any information about the influence of plasticity and stability on the behavior of
the structure. Hence, these influences are normally provided indirectly in member
capacity checks.
A first-order elastic analysis is sufficient for normal framed structures, which are braced
against sway, however, first-order elastic analysis will not yield sufficiently accurate results
for some suspension systems, arches, tall buildings, and structure subjected to early
localized yielding or cracking. All the structure exhibit significant non-linear response just
prior to reaching their limit of resistance because of yielding and buckling effects
associated with axial compressive forces. Hence, this nonlinear behavior is accounted
for by the code formulae (that makes allowance for non-linearity in some empirical or
semi-empirical manner) or by supplementary theoretical or experimental studies. Second
order effects on the frame are accounted by a combination of P-∆ effect, which
corresponds to overall frame, and P-δ effect, which corresponds to individual members
within the frame. Since both of these contribute to the deformation of the frame it is
important to consider their combined effect.

Page | 4

Page | 5
6.3 Modeling assumptions
Relative stiffnesses of members within structural systems shall be based on reasonable
and consistent assumptions.
Ideally, the member stiffnesses EcI and GJ should reflect the degree of cracking and
inelastic action that has occurred along each member before yielding.
To calculate moments and shears caused by gravity loads in columns, beams, and slabs,
it shall be permitted to use a model limited to the members in the level being considered
and the columns above and below that level. It shall be permitted to assume far ends of
columns built integrally with the structure to be fixed.
The analysis model shall consider the effects of variation of member cross-sectional
properties, such as that due to haunches. (Stiffness and fixed-end moment coefficients
for haunched members may be obtained from the Portland Cement Association (1972)).
6.5 Simplified method of analysis for nonprestressed continuous beams and one-way
slabs
It shall be permitted to calculate Mu and Vu due to gravity loads in accordance with this
section for continuous beams and one-way slabs satisfying (a) through (e):
(a) Members are prismatic
(b) Loads are uniformly distributed
(c) L ≤ 3D
(d) There are at least two spans
(e) The longer of two adjacent spans does not exceed the shorter by more than 20
percent

Page | 6
The approximate moments and shears give reasonable values for the stated conditions
if the continuous beams and one-way slabs are part of a frame or continuous
construction. Because the load patterns that produce critical values for moments in
columns of frames differ from those for maximum negative moments in beams, column
moments should be evaluated separately.
Floor or roof level moments shall be resisted by distributing the moment between columns
immediately above and below the given floor in proportion to the relative column
stiffnesses considering conditions of restraint.
6.6 First-order analysis
When using first-order analysis, slenderness effects are calculated using the moment
magnifier approach
Redistribution of moments calculated by an elastic first-order analysis shall be reduction
of moments at sections of maximum negative or maximum positive moment calculated
by elastic theory shall be permitted for any assumed loading arrangement if (a) and (b)
are satisfied:
(a) Flexural members are continuous
(b) εt≥ 0.0075at the section at which moment is reduced
6.6.2 Modeling of members and structural systems
Floor or roof level moments shall be resisted by distributing the moment between columns
immediately above and below the given floor in proportion to the relative column
stiffnesses and considering conditions of restraint.
For frames or continuous construction, consideration shall be given to the effect of floor
and roof load patterns on transfer of moment to exterior and interior columns, and of
eccentric loading due to other causes
It shall be permitted to simplify the analysis model by the assumptions of (a), (b), or both:
(a) Solid slabs or one-way joist systems built integrally with supports, with clear spans not
more than 3 m, shall be permitted to be analyzed as continuous members on knife-edge
supports with spans equal to the clear spans of the member and width of support beams
otherwise neglected.
(b) For frames or continuous construction, it shall be permitted to assume the intersecting
member regions are rigid.

Page | 7
6.6.3 Section properties
Factored load analysis for lateral load analysis, either the stiffnesses presented in 6.6.3.1.1
or 6.6.3.1.2 can be used. These provisions both use values that approximate the stiffness
for reinforced concrete building systems loaded to near or beyond the yield level, and
have been shown to produce reasonable correlation with both experimental and
detailed analytical results (Moehle 1992; Lepage 1998). For earthquake-induced loading,
the use of 6.6.3.1.1 or 6.6.3.1.2 may require a deflection amplification factor to account
for inelastic deformations. In general, for effective section properties, Ec may be defined
as in 19.2.2, A as in Table 6.6.3.1.1(a), and the shear modulus may be taken as 0.4Ec
Moment of inertia and cross-sectional area of members shall be calculated in
accordance with Tables 6.6.3.1.1(a) or 6.6.3.1.1(b), unless a more rigorous analysis is used.
If sustained lateral loads are present, I for columns and walls shall be divided by (1 + βds),
where βds is the ratio of maximum factored sustained shear within a story to the maximum
factored shear in that story associated with the same load combination.
.
For factored lateral load analysis, it shall be permitted to assume I = 0.5Igfor all members
or to calculate I by a more detailed analysis, considering the reduced stiffness of all
members under the loading conditions.

Page | 8
6.6.4 Slenderness effects, moment magnification method
6.6.4.1 Unless 6.2.5 is satisfied, columns and stories in structures shall be designated as
being nonsway or sway. Analysis of columns in nonsway frames or stories shall be in
accordance with 6.6.4.5. Analysis of columns in sway frames or stories shall be in
accordance with 6.6.4.6.
The cross-sectional dimensions of each member used in an analysis shall be within 10
percent of the specified member dimensions in construction documents or the analysis
shall be repeated. If the stiffnesses of Table 6.6.3.1.1(b) are used in an analysis, the
assumed member reinforcement ratio shall also be within 10 percent of the specified
member reinforcement in construction documents.
It shall be permitted to analyze columns and stories in structures as nonsway frames if (a)
or (b) is satisfied:
(a) The increase in column end moments due to second order effects does not exceed
5 percent of the first-order end moments
(b) Qin accordance with 6.6.4.4.1 does not exceed 0.05
6.6.5 Redistribution of moments in continuous flexural members
6.6.5.1Except where approximate values for moments are used in accordance with 6.5,
where moments have been calculated in accordance with 6.8, or where moments in
two-way slabs are determined using pattern loading specified in 6.4.3.3, reduction of
moments at sections of maximum negative or maximum positive moment calculated by
elastic theory shall be permitted for any assumed loading arrangement if (a) and (b) are
satisfied:
(a) Flexural members are continuous
(b) εt≥ 0.0075at the section at which moment is reduced
6.6.5.2 For prestressed members, moments include those due to factored loads and those
due to reactions induced by prestressing.
6.6.5.3 At the section where the moment is reduced, redistribution shall not exceed the
lesser of 1000εt percent and 20 percent.
6.6.5.4 The reduced moment shall be used to calculate redistributed moments at all other
sections within the spans such that static equilibrium is maintained after redistribution of
moments for each loading arrangement.
6.6.5.5 Shears and support reactions shall be calculated in accordance with static
equilibrium considering the redistributed moments for each loading arrangement

Page | 9
6.7 Elastic second-order analysis
General—In elastic second-order analyses, the deformed geometry of the structure is
included in the equations of equilibrium so that P∆ effects are determined. The structure
is assumed to remain elastic, but the effects of cracking and creep are considered by
using a reduced stiffness EI. In contrast, elastic first-order analysis satisfies the equations of
equilibrium using the original undeformed geometry of the structure and estimates P∆
effects by magnifying the column-end sway moments
An elastic second-order analysis shall consider the influence of axial loads, presence of
cracked regions along the length of the member, and effects of load duration. These
considerations are satisfied using the cross-sectional properties

Page | 10
The stiffnesses EI used in an analysis for strength design should represent the stiffnesses of
the members immediately prior to failure. This is particularly true for a second-order
analysis that should predict the lateral deflections at loads approaching ultimate. The EI
values should not be based solely on the moment-curvature relationship for the most
highly loaded section along the length of each member. Instead, they should
correspond to the moment-end rotation relationship for a complete member
Slenderness effects along the length of a column shall be considered. It shall be
permitted to calculate these effects
The cross-sectional dimensions of each member used in an analysis to calculate
slenderness effects shall be within 10 percent of the specified member dimensions in
construction documents or the analysis shall be repeated.
Alternatively, it shall be permitted to calculate immediate deflections using a moment of
inertia of 1.4 times I given in 6.6.3.1, or calculated using a more detailed analysis, but the
value shall not exceed Ig.
6.8 Inelastic second-order analysis
An inelastic second-order analysis shall consider material nonlinearity, member curvature
and lateral drift, duration of loads, shrinkage and creep, and interaction with the
supporting foundation.
An inelastic second-order analysis procedure shall have been shown to result in
prediction of strength in substantial agreement with results of comprehensive tests of
statically indeterminate reinforced concrete structures.
The cross-sectional dimensions of each member used in an analysis to calculate
slenderness effects shall be within 10 percent of the specified member dimensions in
construction documents or the analysis shall be repeated.
Redistribution of moments calculated by an inelastic second-order analysis shall not be
permitted.
6.9 Acceptability of finite element analysis
his section was introduced in the 2014 Code to explicitly recognize a widely used analysis
method.
The finite element model shall be appropriate for its intended purpose.
The licensed design professional should ensure that an appropriate analysis model is used
for the particular problem of interest. This includes selection of computer software
program, element type, model mesh, and other modeling assumptions.

Page | 11
For inelastic analysis, a separate analysis shall be performed for each factored load
combination.
For inelastic finite element analysis, the rules of linear superposition do not apply. To
determine the ultimate member inelastic response, for example, it is not correct to
analyze for service loads and subsequently combine the results linearly using load factors.
A separate inelastic analysis should be performed for each factored load combination.
The licensed design professional shall confirm that the results are appropriate for the
purposes of the analysis.
The cross-sectional dimensions of each member used in an analysis shall be within 10
percent of the specified member dimensions in construction documents or the analysis
shall be repeated.
Redistribution of moments calculated by an inelastic analysis shall not be permitted.

Summary of CHAPTER 6 (ACI318-14M) Structural Analysis

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to Summary of CHAPTER 6 (ACI318-14M) Structural Analysis

Similar to Summary of CHAPTER 6 (ACI318-14M) Structural Analysis (20)

Recently uploaded

Recently uploaded (20)

Summary of CHAPTER 6 (ACI318-14M) Structural Analysis