Guidelines for Detailing of Reinforcement in Concrete Structures

2015 Guidelines for Detailing of Reinforcement in Concrete Structures EC2 & ACI

CHAPTER ONE
SUMMARY OF CODES AND STANDARDS ON CONSTRUCTION DETAILS

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1.1 SUMMARY OF CODES AND STANDARDS ON CONSTRUCTION DETAILS

1.1.1 PERMISSIBLE MANDREL DIAMETERS FOR BENT BARS EC2 (5) stipulates that:
• the minimum diameter to which a bar may be bent shall be defined as the smallest

diameter at which no bending cracks appear in the bar and which ensures the integrity of

the concrete inside the bend of the bar;
• in order to avoid damage to the reinforcement, the diameter to which the bar is bent
(mandrel diameter) should not be less than ᴓ m,min'.

TABLE T-1.1
MINIMUM MANDREL DIAMETER TO PREVENT DAMAGE

TO REINFORCEMENT (EC2)

a- for bars and wire: Minimum mandrel diameter
Bar diameter
for bends, hooks and loops
ᴓ more or equal than 16 mm 4ᴓ
ᴓ less than 16 mm 7ᴓ

b- for bent welded reinforcement and wire mesh bent after welding :

The mandrel diameter need not be checked to avoid concrete failure if the following
conditions exist:

• the length of the bar anchorage beyond the end of the bend is not over 5 ᴓ;
• the bar is not in an end position (plane of bend close to concrete face) and a cross bar
with a diameter ≥ ᴓ is duly anchored inside the bend;

• the mandrel diameter is at least equal to the recommended values given in Table T-1.1.
Otherwise, the mandrel diameter, ᴓ m,min' must be increased as per Expression:

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TABLE T-1.2

MANDREL DIAMETERS FOR REINFORCING BARS
IN ACCORDANCE WITH TABLE T-1.1(B400 or B 500 steel) (in mm) (EC2)

The same rules are applicable to ties and stirrups.
AR. In cases routinely found in practice, such as depicted in Figure 1-1, the use of 12-mm ᴓ

or larger stirrups leaves the corner unprotected. Consequently, joining two smaller
diameter stirrups is preferable to using 14-mm and especially 16-mm ᴓ elements.

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1.1.2 STANDARD BENDS, HOOKS AND LOOPS EC2
EC2 specifies the bends, hooks and loops depicted in Figure 1-2 for rebar in general. Ties
and stirrups call for special shapes, as specified in EC2, 8.4 and 8.5, and shown in Figure 1-
3.

FIGURE 1-2 ANCHORAGE METHODS OTHER THAN STRAIGHT BARS

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ACI STANDARD HOOKS stipulates that:

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Details and Detailing of Concrete
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1.1.3 COVER REQUIREMENTS BY EC2 AND ACI CODE
The EC2 specifications are summarized below.
(a) General
The concrete cover is the distance between the surface of the reinforcement closest to the
nearest concrete surface (including ties and stirrups and surface reinforcement where
relevant) and the nearest concrete surface. For these intents and purposes, groups of
reinforcing bars cannot be replaced by the equivalent circle: rather, the cover refers to the
actual bars in the group.

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TABLE T-1.5
VALUES OF MINIMUM COVER, Cmin,dur' REQUIREMENTS WITH REGARD

TO DURABILITY FOR REINFORCEMENT STEEL IN ACCORDANCE
WITH EN 10080 (1) (EC2)*

(*) Fire safety may call for higher values.

AR. Except in special cases, covers of under 15 mm should not be used.

Where in situ concrete is placed on other (precast or in situ) concrete elements, the
minimum concrete cover from the reinforcement to the interface may be reduced to a value
meeting the bond requirement only, providing that:

• the concrete strength class is at least C25/30; ( less
• the concrete surface is exposed to an outdoor environment for only a short time

than 28 days);
• the interface is roughened.

For uneven surfaces (e.g. exposed aggregate) the minimum cover should be increased by
at least 5 mm.
AR. If the surface is roughened mechanically, this value should be 20 mm, for mechanical
treatment generates micro cracks in the concrete surface.

(C) Allowance in design for deviation

When calculating the design cover, Cnom' the minimum cover must be increased to allow for
deviations (ΔCdev) by the absolute value of the tolerance for negative deviation. The
recommended allowance is 10 mm.

For concrete poured onto uneven surfaces, the minimum cover should generally be

increased by allowing larger deviations in design. The increase should compensate for the

difference deriving from the unevenness, maintaining a minimum cover of 40 mm for

concrete poured onto prepared ground (including blinding) and 75 mm for concrete poured
directly onto the soil.(*)

The value

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reflects the spacer size.

AR. The 40-mm cover for blinding would appear to be excessive. Where the blinding is
reasonably flat, 25 mm or 1 ᴓ would appear to suffice.

ACI CODE REQUIREMENTS FOR COVER

The ACI318 specifications are summarized below.
The reinforcing for concrete members must be protected from the surrounding
environment; that is, fire and corrosion protection need to be provided. To do this, the
reinforcing is located at certain minimum distances from the surface of the concrete so that
a protective layer of concrete, called cover, is provided. In addition, the cover improves the
bond between the concrete and the steel. In Section 7.7 of the ACI Code, specified cover is
given for reinforcing bars under different conditions. Values are given for reinforced
concrete beams, columns, and slabs; for cast-in-place members; for precast members; for
prestressed members; for members exposed to earth and weather; for members not so
exposed; and so on. The concrete for members that are to be exposed to deicing salts,
brackish water, seawater, or spray from these sources must be especially proportioned to
satisfy the exposure requirements of Chapter 4 of the code.

FIGURE 1-4 Determining Minimum Edge Distance

Nonprestressed cast-in-place concrete members shall have specified concrete cover for
reinforcement at least that given in Table (1.3).

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Precast nonprestressed or prestressed concrete members manufactured under plant
conditions shall have specified concrete cover for reinforcement, ducts, and end fittings at
least that given in Table (1.4).

AC1 DETAILING MANUAL-2004 4
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1.1.4 BAR SPACING BY EC2 AND ACI CODES

Bars must be spaced in such a way that the concrete can be poured and compacted for
satisfactory bonding and strength development. The clear (horizontal and vertical) distance
between individual parallel bars or horizontal layers of parallel bars should not be less than
the larger of (d9 + 5 mm), where d9 is the maximum aggregate size, and 20 mm (Figure 1-5).

Where bars are positioned in separate horizontal layers, the bars in each successive layer
should be vertically aligned with the bars in the layer below. Sufficient space must be left
between the resulting columns of bars for vibrator access and good concrete compaction.

Lapped bars may be allowed to touch one another within the lap length.

FIGURE 1-5
AR. The 20-mm limit for a and b is too narrow to ensure satisfactory concrete casting. For
single layers, a 25-mm space is suggested, and 35 mm for two or more: a should be 2.5
times the diameter of the vibrator needle for bars in any other than the bottom layer in the
beam. Note that the longitudinal ribs on bars usually constitute 0.07 to 0.10 of the diameter
and that bar placement inevitably entails deviations.

ACI CODE REQUIREMENTS FOR BAR SPACING
Minimum spacing of bars. The ACI318 code (7.6) states that the clear distance between
parallel bars cannot be less than 1 in.[4] or less than the nominal bar diameter. If the bars

are placed in more than one layer, those in the upper layers are required to be placed
directly over the ones in the lower layers, and the clear distance between the layers must
be not less than 1 in.

A major purpose of these requirements is to enable the concrete to pass between the
bars. The ACI Code further relates the spacing of the bars to the maximum aggregate sizes
for the same purpose. In the code Section 3.3.2, maximum permissible aggregate sizes are
limited to the smallest of (a) one-fifth of the narrowest distance between side forms, (b)
one-third of slab depths, and (c) three-fourths of the minimum clear spacing between bars.
A reinforcing bar must extend an appreciable length in both directions from its point of
highest stress in order to develop its stress by bonding to the concrete. The shortest

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length in which a bar’s stress can be increased from 0 to fy is called its development
length. If the distance from the end of a bar to a point where it theoretically has a stress
equal to fy is less than its required development length, the bar may very well pull loose
from the concrete. There you will learn that required development lengths for reinforcing
bars vary appreciably with their spacing and their cover. As a result, it is sometimes wise
to use greater cover and larger bar spacing's than the specified minimum values in order to
reduce development lengths. When selecting the actual bar spacing, the designer will
comply with the preceding code requirements and, in addition, will give spacing's and
other dimensions in inches and fractions, not in decimals. The workers in the field are
accustomed to working with fractions and would be confused by a spacing of bars such as
3 at 1.45 in. The designer should always strive for simple spacing's, for such dimensions
will lead to better economy.

TABLE T1-6 MINIMUM BEAM WIDTH [mm] FOR BEAMS WITH INSIDE EXPOSURE ACI
METRIC CODE.

AC1 DETAILING MANUAL-2004 4
Details and Detailing of Concrete
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FIGURE 1- AC1 DETAILING MANUAL-2004 4
Details and Detailing of Concrete
Reinforcement (AC1 3 15-99)

FIGURE 1- BAR ARRANGEMENTS IN BEAM

1.1.5 BUNDLED BARS BY EC2 AND ACI

(a) Bundled bars versus large diameter bars The standard series of large diameter
(ᴓ ≥ 32 mm) reinforcing bars includes two diameters in Europe, 40 mm and 50 mm, and
three in the United States, 11 (ᴓ 35 mm), 14 (ᴓ 44 mm) and 18 (ᴓ 57 mm). While using these

diameters provides for more compact reinforcement, which is a clear advantage, it also

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entails two drawbacks. On the one hand, the substantial load transfers generated call for
carefully designed anchorage. On the other, since such large diameter bars cannot be lap
spliced, construction is more complex and costly. Indeed, even lap splicing, if it were
allowed, would be extremely expensive because of the extra steel needed for the long
overlap lengths that would be required.

The alternative solution is to use bundled bars, which afford the advantages of compact
distribution without the aforementioned drawbacks.

(b) Possibly usable bundles

The EC2 specifications are summarized below.

• As a rule, no more than three bars can be bundled (and their axes must not be in the same
plane).

• In overlap areas and when using compressed bars in vertically cast members in which no
splicing is needed, four bars are required.

• The equivalent diameter (for the ideal bar whose area is the same as the area of the
bundle) must not be over 55 mm.

(c) Equivalent diameters, areas and mechanical strength

The specifications laid down in Tables T-1.6 and T-1.7 are applicable to bars with an
equivalent diameter ᴓn = ᴓ (Nb)0.5, where Nb is the number of bars and ᴓ is the diameter of
each individual bar (Table T-1.8).

TABLE T-1.6.
MAXIMUM BUNDLES GENERAL SPECIFICATIONS

TABLE T-1.7
BUNDLES COMPRESSED BARS IN VERTICALLY CAST MEMBERS

AND OVERLAP AREAS IN GENERAL

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TABLE T-1.8
BUNDLES EQUIVALENT DIAMETERS in mm

( d) Cross-sectional arrangement of bundles

• Distances between bundles or bundles and bars. The provisions of 1.1.4 apply. The
minimum distance must be equal to the equivalent diameter, ᴓn whose values are given in
Table T-1.8. Note that the minimum spacing between bundles is the physical space
between two points on the perimeter of the bar closest to the nearest bar in another bundle.
The space between two bundles should always be large enough to accommodate a vibrator
during concrete casting.

• Cover. The cover must be at least equal to the equivalent diameter, ᴓn' measured as the

distance to the closest bar.

• For anchorage and overlaps in bundled bars, see item 8.9.3 of EC2.

ACI-318-11- Clause 7.6.6.1 to 7.6.6.5 has specified some restrictions. Also see clause 7.7.4
for cover requirement. 7.10.5.1 — All nonprestressed bars shall be enclosed by transverse
ties, at least No. 10 in size for longitudinal bars No. 32 or smaller, and at least No. 13 in size
for No. 36, No. 43, No. 57, and bundled longitudinal bars. Deformed wire or welded wire
reinforcement of equivalent area shall be permitted. Clause 12.4.1 to 12.4.2 for development
length.
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1.1.6 SURFACE REINFORCEMENT BY EC2 AND ACI

For bars with a diameter of over 32 mm, the following rules supplement the specifications
in EC2, 8.4 and 8.7.

When such large diameter bars are used, cracking may be controlled either with surface
reinforcement or by calculating crack widths.

As a general rule, large diameter bars should not be lapped. Exceptions include sections
whose smallest dimension is 1.0 m or where the stress is no greater than 80 per cent of the
design's ultimate strength. In any event, such bars should be lapped with mechanical
devices.

In addition to shear reinforcement, transverse reinforcement should be placed in
anchorage zones with no transverse compression.

(a) Additional reinforcement

For straight anchorage lengths (see Figure 1-5), such additional reinforcement should be at

least as described below.

(i) In the direction parallel to the stressed surface:

(ii) In the direction perpendicular to the stressed surface:

The additional transverse reinforcement should be uniformly distributed in the
anchorage area and bars should not be spaced at more than five times the diameter of the
longitudinal reinforcement.

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For surface reinforcement, (i) and (ii) apply, but the area of the surface reinforcement
should not be less than 0.01 Actext in the direction perpendicular, and 0.02 Actext in the
direction parallel, to the large bars. Actext is the area of the tensile concrete external to the
stirrups (see Figure 1-6).

(b) Additional surface reinforcement
Surface reinforcement may be needed either to control cracking or to ensure adequate
cover resistance to spalling. EC2 addresses this question in Informative Annex J,
summarized below.
Surface reinforcement to resist spalling should be used where the main reinforcement
comprises:
• bars with diameters of over 32 mm;
• bundled bars with an equivalent diameter of over 32 mm.
The surface reinforcement, which should consist of welded-wire mesh or narrow bars,
should be placed outside the stirrups, as shown in Figure 1-6.

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The area of the surface reinforcement Assun should not be less than 0.01 Actext in the
directions parallel and perpendicular to the tension reinforcement in the beam. Where the
reinforcement cover is over 70 mm, similar surface reinforcement should be used, with an
area of 0.005 Actext in each direction for enhanced durability. The longitudinal bars in
surface reinforcement may be regarded as constituting reinforcement to resist any other
action effects whatsoever.

AR. If such additional reinforcement is included, concrete with a suitable slump should be
used and poured and compacted with utmost care.

1.1.7 TIE POINTS FOR REINFORCEMENT BARS BY EC2

The following recommendations are made.

• Slabs and plates. All the intersections between bars around the perimeter of the
reinforcement panel should be tied.

In the rest of the panel, where the bar diameter is 20 mm or less, every second intersection

should be tied. Where the bars are 25 mm or larger, the distance between tied intersections

should not exceed 50 diameters (Figure 1-12) of the thinnest tied bar.

FIGURE 1-

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• Beams. All the corners of the stirrups must be tied to the main reinforcing bars. If welded-
wire fabric reinforcement is used to form the stirrups, the main reinforcing bars at the
corners should be tied at intervals no larger than 50 times the diameter of the main
reinforcing bars.

All the bars not located in the corner of the stirrup should be tied at intervals no larger than

50 times the bar diameter.

Multiple stirrups should be tied together (Figure 1-13).

FIGURE 1-

• Columns. All the ties should be tied to the main reinforcement at the intersections. When
welded-wire fabric cages are used, the vertical wires should be tied to the main
reinforcement at intervals measuring 50 times the bar diameter.
• Walls. The bars are tied at every second intersection.

For the intents and purposes of tying reinforcing bars, precast walls manufactured with the
mid-plane in a horizontal position are regarded to be slabs.

The rules for slabs and plates are applicable to walls cast in situ (Figure 1-14).
• Footings. The horizontal part of the starter bars should be secured at each right-angle
intersection between starter bar and foundation reinforcement. All the ties in footings
should be secured to the vertical part of the starter bars.

AR. The footing assembly should have at least two tie bars.

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FIGURE 1-

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CHAPTER TWO
REINFORCED CONCRETE COLUMN BY EC2 AND ACI CODES

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2 R.C. COLUMNS

A reinforced concrete column is a structural members designed to carry
compressive loads, composed of concrete with an embedded steel frame to provide
reinforcement. For design purposes, the columns are separated into two categories: short
columns and slender columns.

2.1 Detailing rules that conform to BS EN 1992-1-1, Euro code 2:
2.1.1 Design and detailing notes Concrete grade.

Concrete grades less than 28/35 MPa (cylinder strength/cube strength) are not normally
used. Care should be taken to ensure that the design strength of concrete required in a
column does not exceed 1.4 times that in the slab or beam intersecting with it unless
special measures are taken to resist the bursting forces.

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2.1.2 Reinforcement guidelines by BS EN 1992-1-1, Euro code 2

i. Bar diameter:

Recommended minimum bar diameter is 16mm for very small section columns. Minimum
number of bars for rectangular columns is 4. Minimum number of bars for circular columns
is 6 for very small diameter columns, and the minimum of 4 for less than 200mm.

ii. Maximum area of main reinforcement:

Maximum area of reinforcement should not exceed 0.04 Ac unless it can be shown that any
resulting congestion of reinforcement does not hinder the ease of construction. At laps the
maximum area of reinforcement should not exceed 0.08 Ac. Mechanical splices should be
considered where congestion becomes a problem.

iii. Bar spacing: Preferred minimum spacing
• Main bars 75mm (bars 40mm size and greater: 100mm)
• Pairs of bars
• 100mm

When considering the minimum spacing of bars of 32mm size or greater, allowance must
be made for lapping of bars.

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Preferred maximum spacing

• Compression bars 300mm, provided that all main bars in the compression zone are
within 150mm of a restrained bar.

• Tension bars
• 175mm
iv. Links

The size of link should be the greater of a quarter the maximum size of longitudinal
bar and 8mm (for very small diameter columns, less than 200mm, the minimum of 6mm
may apply).Bundled main bars may be represented by a single bar for the purpose of
calculating link size and spacing. This single bar has an equivalent size to give it the same
cross section area as the bundle. An overall enclosing link is required together with
additional restraining links for alternate main bars or bundle of bars. Provided that all other
main bars in the compression zone are within 150mm of a restrained bar no other links are
required .Otherwise additional links should be added to satisfy this requirement. Additional
links are not required for circular columns.

Maximum spacing of links . The least of:

• 20 times the size of the longitudinal bars, or
• The lesser dimension of the column, or
• 400 mm.
The maximum spacing should be reduced by a factor 0.6 in sections within a distance
equal to the larger dimension of the column cross-section above and below a beam or slab.

Where the direction of the longitudinal bars changes (e.g. at laps), the spacing of links
should be calculated. The spacing of links should ensure that there is a link close to the
cranking positions of the main bars. These effects may be ignored if the change in
direction is 1 in 12 or less.

Requirement of links in columns

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v. Moment connection between beam and edge column:

Wherever possible U-bars which can be placed within the depth of beam should be
used. These are fixed in position and concreted with the beam, and thus do not require
precise fixing when the column is being concreted. L-bars which penetrate down into the
column should be used when the distance ‘A’ (see Figure ) is less than the anchorage
length for that bar diameter. These bars must be fixed accurately at the top of the column
lift which is a difficult and unattractive site task. A standard radius to the bend may
normally be used provided a bar of the same size or greater is placed inside the corner
normal to it. A non-standard bend may be required if a corner bar is not present. If so, a
thorough check should be carried out to ensure that the reinforcement fits and will perform
as intended. The critical effective depth may not be obvious, and various locations may
need to be assessed.

Special care should be taken by the Designer and Detailer to make sure that this
reinforcement does not conflict with any beam reinforcement passing through the column
in the other direction.

Connection between beam and edge column

vi. Shear capacity of column:

The maximum tensile reinforcement in the beam or that part required for the moment
connection to the column is also controlled by the shear capacity of the column. Where
there is no edge beam intersecting at approximately the same level as the joint, transverse
column reinforcement should be provided within the depth of the beam (See Figure ). This
may be in the form of links or horizontal U-bar extending into the beam. Unless specified by
the Designer the spacing should be as for the links in the column.

vii. Starter bars:

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It is important to recognize at the design stage the implications of the construction
sequence and the level of foundation on the length of starter bars, e.g.

If the foundation reinforcement is placed at a depth lower than specified the consequent
lap of the first lift of column bars is likely to be too short. For this reason the length of
starter bars from pad footings and pile caps is specified longer than required.

Detailing information:

Design information for detailing should include:

• The section dimensions and its position and orientation relative to particular grid
lines.

• Outline drawings which show clearly what happens to the column above the lift
being considered.

• Kicker height if other than 75mm.
• Concrete grade and aggregate size 20mm.
• Nominal cover to all reinforcement (standard 35mm internal, 40mm external).

Supplementary mesh reinforcement if required.
• A simple sketch of cross-section of column showing the longitudinal reinforcement

in each face of the column, i.e.

1. Number and position of bars.

2. Type of reinforcement and bond characteristics standard (H).

3. diameter of bars.
4. Lap length if other than normal compression laps the linking reinforcement.
5. Type of reinforcement standard (H).
6. Diameter of links, spacing, pattern of links (if special).
• Instructions for lapping of bunched bars if required.
• Special instructions for links within depth of slab or beam.
• If a mechanical or special method of splicing bars is required this must be shown in

a sketch, otherwise the method given in the Model Details will be assumed.
• Special instructions and sketches should be given where services are provided

within the column.
• Details of insertions, e.g. conduit, cable ducting, cladding fixings, etc., should be

given where the placing of reinforcement is affected.

2.1.3 Presentation of working drawings: KUFA UNIVERSITY /FACULTY OF ENGINEERING
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Nominal cover to all reinforcement specified by designer (Normally: Internal 35, External
40).

Figure (3.2): Footing to Column connection

This detail is used where the column is concentric and of the same dimensions as the story
below. Nominal cover to all reinforcement specified by designer (Normally: Internal 35,
External 40)

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Figure (3.3): Column is concentric and of the same dimensions as the story below
This detail applies for stepped or offset columns.

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Figure (3.4): stepped or offset columns

Detail ‘A’ applies when slab depth is not less than: KUFA UNIVERSITY /FACULTY OF ENGINEERING
200 –using ᴓ20 size of column bars
250 –using ᴓ25 size of column bars

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300 –using ᴓ32 size of column bars
Otherwise Detail ‘B’ applies
For single story buildings or where splice bars have been used at the floor below

Figure (3.5): Splice bars details use in columns of single story building, detail A.

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Figure (3.6): Splice bars details use in columns of single story building, detail B.

Top detail

This detail is used for single story buildings and where splice bars have been used at the
floor below.

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Circular columns
Helical binders are used unless circular links are specified by designer.

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2.2 Detailing rules that conform to (ACI 315-99)

According to ACI Code 2.2, a structural element with a ratio of height-to-least lateral
dimension exceeding three used primarily to support compressive loads is defined as
column. Columns support vertical loads from the floor and roof slabs and transfer these
loads to the footings.

Columns usually support compressive loads with or without bending. Depending on the
magnitude of the bending moment and the axial force, column behavior will vary from pure
beam action to pure column action.

Columns are classified as short or long depending on their slenderness ratios. Short
columns usually fail when their materials are overstressed and long columns usually fail
due to buckling which produces secondary moments resulting from the P - D effect.

Columns are classified according to the way they are reinforced into tied and spirally
reinforced columns. Columns are usually reinforced with longitudinal and transverse
reinforcement. When this transverse reinforcement is in the form of ties, the column is
called “tied”. If the transverse reinforcement is in the form of helical hoops, the column is
called “spirally reinforced.” Since failure of columns often cause extensive damage, they
are designed with a higher factor of safety than beams.

3.2.1 Types of Columns

Columns are divided into three types according to the way they are reinforced.

 Tied Columns

A tied column, shown in Figure ( ), is a column in which the longitudinal reinforcement
bars are tied together with separate smaller diameter transverse bars (ties) spaced at some
interval along the column height. These ties help to hold the longitudinal reinforcement
bars in place during construction and ensure stability of these bars against local buckling.
The cross sections of such columns are usually square, rectangular, or circular in shape. A
minimum of four bars is used in rectangular and circular cross sections.

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Tied Columns

 Spirally-Reinforced Columns

They are columns in which the longitudinal bars are arranged in a circle surrounded by a
closely spaced continuous spiral, shown in Figure ( ). These columns are usually circular
or square in shape. A minimum of six bars is used for longitudinal reinforcement.

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2.2.2 Design Considerations by ACI CODE

 Maximum and Minimum Reinforcement Ratios

ACI Code 10.9.1 specifies that a minimum reinforcement ratio of 1 % is to be used in tied or
spirally reinforced columns. This minimum reinforcement is needed to safeguard against
any bending, reduce the effect of shrinkage and creep and enhance ductility of columns.
Maximum reinforcement ratio is limited to 8 % for columns in general to avoid
honeycombing of concrete.

For compression member with a cross section larger than required by consideration of
loading, ACI Code 10.8.4 permits the minimum area of steel reinforcement to be based on
the gross sectional area required by analysis. The reduced sectional area is not to be less
than one half the actual cross sectional dimensions. In regions of high seismic risk, ACI
Code 10.8.4 is not applicable.

 Minimum Number of Reinforcing Bars

ACI Code 10.9.2 specifies a minimum of four bars within rectangular or circular sections; or
one bar in each corner of the cross section for other shapes and a minimum of six bars in
spirally reinforced columns.

 Clear Distance between Reinforcing Bars

ACI Code 7.6.3 and 7.6.4 specify that for tied or spirally reinforced columns, clear distance
between bars, shown in Figure ( ), is not to be less than the larger of 1.50 times bar
diameter or 4 cm. This is done to ensure free flow of concrete among reinforcing bars. The
clear distance limitations also apply to the clear distance between lap spliced bars and
adjacent lap splices since the maximum number of bars occurs at the splices.

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 Concrete Protection Cover:

ACI Code 7.7.1 specifies that for reinforced columns, the clear concrete cover is not to be
taken less than 4 cm for columns not exposed to weather or in contact with ground. It is
essential for protecting the reinforcement from corrosion or fire hazards.

 Minimum Cross Sectional Dimensions:

Minimum sizes for compression members were eliminated to allow wider utilization of
reinforced concrete compression members in smaller size and lightly loaded structures,
such as low-rise residential and light office buildings. When small sections are used, there
is a greater need for careful workmanship. For practical considerations, column
dimensions are taken as multiples of 5 cm.

 Lateral Reinforcement:

Ties are effective in restraining the longitudinal bars from buckling out through the surface
of the column, holding the reinforcement cage together during the construction process,
confining the concrete core and when columns are subjected to horizontal forces, they
serve as shear reinforcement. Spirals, on the other hand, serve in addition to these benefits
in compensating for the strength loss due to spilling of the outside concrete shell at
ultimate column strength.

Ties

According to ACI Code 7.10.5.1, for longitudinal bars 32 mm or smaller, lateral ties 10 mm
in diameter are used. In our country and in some neighboring countries, ties 8 mm in
diameter are used in column construction.

Tests have proven that spacing between ties has no significant effect on ultimate strength
of columns.

ACI Code 7.10.5.2 specifies that vertical spacing of ties is not to exceed the smallest of:

 16times longitudinal bar diameter.
 48times tie diameter.
 Least cross sectional dimension.

ACI Code 7.10.5.3 specifies that ties are arranged in such a way that every corner and
alternate longitudinal bar is to have lateral support provided by the corner of a tie with an

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included angle of not more than 135 degrees. Besides, no longitudinal bar is to be farther
than 15 cm clear on each side along the tie from such a laterally supported bar. When
longitudinal bars are located around the perimeter of a circle, circular ties are used. Figure
( ).a shows a number of tie and spiral arrangements.

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FIG. 9 A (Tie and Spiral Arrangement)

 Spirals:

According to ACI Code 7.10.4.2 spirals not less than 10 mm in diameter are to be used in
cast-in place construction. The clear pitch of the spiral is not to be less than 2.5 cm and not
more than 7.5 cm as dictated by ACI Code 7.10.4.3. The smaller limit is set to ensure flow of
concrete between spiral hoops while the larger limit is set to ensure effective confinement
of concrete core. The diameter of the spiral could be changed to ensure that the spacing
lies within the specified limits.

 Bundled Bars:

For isolated situations requiring heavy concentration of reinforcement, bundles of standard
bar sizes can save space and reduce congestion for placement and compaction of
concrete. Bundling of parallel reinforcing bars in contact is permitted but only if ties
enclose such bundles. According to ACI Code 7.6.6, groups of parallel reinforcing bars
bundled in contact to act as one unit are limited to four in any one bundle, as shown in
Figure (9.b).

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FIG. (9.B): Bundled Bars

3.2.3 Column Reinforcement Details

When column offset are necessary, longitudinal bars may be bent subject to the following
limitations.

1. Slope of the inclined portion of an offset bar with axis of column must not exceed 1
in 6 ,shown in Figure 10.

2. Portion of bar above and below the offset must be parallel to axis of column.
3. Horizontal support at offset bends must be provided by lateral ties, spirals, or parts

of the floor construction. Ties or spirals, if used, shall be placed not more than 15
cm from points of bend. Horizontal support provided must be designed to resist 1.5
times the horizontal component of the computed force in the inclined portion of an
offset bar.
4. Offset bars must be bent before placement in the forms.
5. When a column face is offset 7.5 cm ,or more, longitudinal column bars parallel to
and near the face must not be offset bent. Separate dowels, lap spliced with the
longitudinal bars adjacent to the offset column faces, must be provided as shown in
Figure 11. In some cases, a column might be offset 7.5 cm or more on some faces,
and less than 7.5 cm on the remaining faces, which could possibly result in some
offset bent longitudinal column bars and some separate dowels being used in the
same column.

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3.2.4 Column Lateral Reinforcement

 Ties

In tied reinforced concrete columns, ties must be located at no more than half tie spacing
above the floor or footing and at no more than half a tie spacing below the lowest
horizontal reinforcement in the slab or drop panel above. If beams or brackets frame from
four directions into a column, ties may be terminated not more than 7.5 cm below the
lowest horizontal reinforcement in the shallowest of such beams or brackets, shown in
Figure 12.

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 Spirals

Spiral reinforcement must extend from the top of footing or slab in any story to the level of
the lowest horizontal reinforcement in slabs, drop panels, or beams above. If beams or
brackets do not frame into all sides of the column, ties must extend above the top of the
spiral to the bottom of the slab or drop panel, shown in Figure 13.

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3.3 Details and Detailing of Concrete Columns Reinforcement

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Note: Where column size above is unchanged from below, “upside down” offset bars are
effective in maintaining full moment capacity at end of column. In U.S. practice, this
unusual detail is rare, and should be fully illustrated on structural drawings to avoid
misunderstandings, whenever its use is deemed necessary. For maximum tie spacing, see
table in Supporting Reference Data section.

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Typical seismic-resistant details: columns. 4

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Typical seismic-resistant joint details—Case 1: For regions of high seismic risk. Interior
and spandrel beams narrower than column.

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Typical seismic-resistant joint details—Case 2: For regions of moderate seismic risk.
Interior beam wider than column; spandrel beams narrower than column.

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Typical seismic-resistant joint details—Case 3: For regions of moderate seismic risk.
Interior beam wider than column; spandrel beam is same width as column.

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1. Alternate position of hooks in placing successive sets of ties.
2. Minimum lap shall be 12 in. (300) mm.
3. B indicates bundled bars. Bundles shall not exceed four bars.

4. Elimination of tie for center bar in groups of three limits clear spacing to be 6

in. (150 mm) maximum. Unless otherwise specified, bars should be so
grouped.

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5. Note to Architect/Engineer: Accepted practice requires that design drawings
show all requirements for splicing column verticals, that is, type of splice, lap
length if lapped, location in elevation, and layout in cross section.

6. Note to Detailer: Dowel erection details are required for any design employing
special large vertical bars, bundled vertical bars, staggered splices, or
specially grouped vertical bars as shown

7. Bars must be securely supported to prevent displacement Suring concreting.
8. Tie patterns shown may accommodate additional single bars between tied

groups provided clear spaces between bars do not exceed 6 in. (150) mm.
9. Minimum cover to ties, 11/2 in. (40 mm) for nonprestressed cast-in-place

concrete
10. Spaces between corner bars and interior groups of three and between interior

groups may vary to accommodate average spacing > 6 in. (150) mm.
11. For average spacing < 6 in. (150 mm), one untied bar may be located between

each tied group of three and between a tied group and a corner bar.

Standard column ties applicable for preassembled cages or field erection, special-shaped
columns, and columns with bars in two faces only.

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Typical seismic resistant details: transverse reinforcement in columns.

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