SEISMIC DESIGN OF RC FRAMES

The content

1- Types of Structural Framing System

2- Structural Framing System

3- Design of special Moment Resisting frames

4- Flexural Members of Special Moment Frames

5-Special Moment Frame Members Subjected to

Bending and Axial Load

6- Joints of Special Moment Frames

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1- Types of Structural Framing System

ACI 318-05 has three design and performance levels:

1. Ordinary Moment Resisting Frames :

2. Intermediate Moment Resisting Frames

3. Special Moment Resisting Frames

These systems Correspond to low, moderate

and high seismic risk levels.

2- Structural Framing System

OMRF structures are expected to perform within the elastic range of deformations when subjected to seismic excitations.

IMRF & SMRF Buildings in moderate to high seismic risk regions are often designed for earthquake forces that are less than those corresponding to elastic response at anticipated earthquake intensities.

Lateral force resisting systems for these buildings may have to dissipate earthquake induced energy through significant inelasticity in their critical regions

3- Design of special Moment Resisting frames

3- Design of special Moment Resisting frames

4- Flexural Members of Special

Moment Frames

The content

4-1- Flexural Design

4-2- Shear design

4-3- Detailing Requirements

4-4- Computing Mpr

4-1- Flexural Design

Members designed to resist primarily flexure (Pu ≤ Agf’c/10) are subject to additional design and detailing considerations for improved seismic performance. These requirements consist of:

4-2- Shear Design

- Seismic design shear Ve in plastic hinge regions is associated with maximum inelastic moments that can develop at the ends of members when the longitudinal tension reinforcement is in the strain hardening range (assumed to develop 1.25 fy)

This moment level is labeled as probable flexural strength, Mpr

(4) Seismic design shear in beams and columns of special frames

4-3- Detailing Requirements

1. Geometric constraints 2. Minimum positive and negative moment

capacities along member length 3. Confinement of critical regions of

elements for improved deformability, 4. Promotion of ductile flexural response 5. Prevention of premature shear failure

(1)

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(2)Details of transverse reinforcement for beams of special frames

Internal forces in a reinforced concrete section at probable moment resistance

4-4- Computing Mpr

probable flexural strength, Mpr

Once Mpr is obtained, the seismic design shear can be computed from the equilibrium of forces shown:

5-Special Moment Frame

Members Subjected to Bending

and Axial Load

The content

5-1- Flexural Design

5-2- Shear design

5-3- Strong-Column Weak-Beam concept

5-4- Confinement Reinforcement

5-1- Flexural Design

Members designed to resist earthquake forces while subjected to factored axial compressive force of Pu > Agf’c/10 are designed following the requirements of Sec. 21.4 of ACI 318-05

Columns that fall in this category are designed using the interaction diagrams. Minimum and maximum reinforcement

respectively. %, 6% and 1ratios of

reduction in the maximum limit of reinforcement ratio %2The

limit specified for ordinary building columns is %8from the intended to reduce the congestion of reinforcement that may occur in seismic resistant construction

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5-2- Shear Design

Once the seismic design shear force is computed, the plastic hinge regions at the ends of the column ( ) will be designed for Ve. In the design, however, the shear resistance provided by concrete, Vc will be neglected (Vc = 0) if both of the following conditions are met:

i) Ve ≥ 50% of the maximum shear strength required within due to the factored column shear force determined by structural analysis.

ii) Pu (including earthquake effects) < Ag f’c / 20

5-2- Shear Design

Seismic design shear in columns is computed as shear force associated with the development of probable moment strength (Mpr) at column ends when the associated factored axial force, Pu is acting on the column.

These moments are computed with reinforcement strengths in tension equal to 1.25 fy, reflecting the contribution of longitudinal column reinforcement in the strain hardening range

A conservative approach for estimating column Mpr for shear calculations is to use nominal moment capacity at balanced section, since this would be the maximum moment capacity for the column.

5-3- Strong-Column Weak-Beam Concept

The strong-column weak-beam concept is enforced in the ACI Code through Sect. 21.4.2.2, which states that the flexural strength of columns should be 6/5 of that of the adjoining beams, as indicated below:

• Σ Mnc is the sum of nominal flexural strengths of the columns framing into the joint, computed at the faces of the joint under factored axial forces such that they give the lowest flexural strength

5-3- Strong-Column Weak-Beam Concept

Σ Mnb is the sum of the nominal flexural strengths of the beams framing into the joint, computed at the faces of the joint. For negative moment capacity calculations, the slab reinforcement in the effective slab width, as defined in Sec. 8.10 of ACI 318-05

5-4- Confinement Reinforcement

The total cross-sectional area of rectangular hoop reinforcement, Ash, shall not be less than required by Eq. (21-3) and (21-4).

6-Joints of Special Moment Frames

The content 6-1- Joint Shear Strength 6-2- Joint Shear Strength-effective joint area 6-3- Joint shear, Vx-x in an interior beam-column joint 6-4- Joint shear, Vx-x in exterior beam- column joint 6-5- Joint Reinforcement

6-1- Joint Shear Strength

The joint shear produces diagonal tension and compression reversals which may be critical for premature diagonal tension or compression failures, unless properly reinforced.

The joint shear may especially be critical in edge and corner joints, which are not confined by the adjoining beams on all four faces.

A member that frames into a joint face is considered to provide confinement to the joint if at least ¾ of the face of the joint is covered by the framing member

•Vn of the joint shall not be taken as greater than the

values specified below:

F=0.85 Vu ≤ FVn ,

6-1- Joint Shear Strength

6-2- Joint Shear Strength-effective joint area

6-3- Joint shear, Vx-x in an interior beam-column joint

6-4- Joint shear, Vx-x in exterior beam-column joint

The column confinement reinforcement provided at the ends of columns should continue into the beam-column

not confined by the framing beams on if the joint is joint the previous section., as described in all four faces

For interior joints, with attached beams externally confining the joint on all four faces, the spacing of joint reinforcement can be relaxed to 6 in.

6-5- Joint Reinforcement

SEISMIC DESIGN OF RC FRAMES

The content

1- Types of Structural Framing System

2- Members of Intermediate Moment Frames

3- Joint of Intermediate Frames

4- Members not Designed as Part of the

Lateral-Force-Resisting System

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1- Types of Structural Framing System

ACI 318-05 has three design and performance levels:

1. Ordinary Moment Resisting Frames :

2. Intermediate Moment Resisting Frames

3. Special Moment Resisting Frames

These systems Correspond to low, moderate

and high seismic risk levels.

2- Members of Intermediate

Moment Frames

The content

2-1- Flexural Design

2-2- Shear design

2- Members of Intermediate Moment Frames

2-1- Flexural Design

• Members of intermediate moment frames located in regions of moderate seismicity and are designed to resist primarily flexure (Pu ≤ Agf’c/10), will meet the beam design requirements shown below:

• Members subjected to higher axial loads will be designed as columns following the requirements for columns outlined in the next slide.

• The transverse reinforcement in beam-column joints of intermediate moment frames will conform to Sec. 11.11.2 of ACI 318-05.

first hoop

located at s0/2

2- Members of Intermediate Moment Frames

2-2- Shear Design • The shear strength φVn of members of

intermediate moment frames will be at least equal to the shear force associated with the development of nominal capacities of members at their ends while also subjected to the effects of factored gravity loads

• Also, the shear strength should not be lower than the maximum shear obtained from the design load combinations where the earthquake loading is assumed to be twice the magnitude prescribed by the governing code

(11)Seismic design shear in beams and columns of special frames

3- Joint of Intermediate Frames

Sec. 11.11.2 of ACI 318-05

• Except for connections not part of a primary seismic load-resisting system that are restrained on four sides by beams or slabs of approximately equal depth, connections shall have lateral reinforcement not less than that required by Eq. (11-13) within the column for a depth not less than that of the deepest connection of framing elements to the columns.

Joint of Intermediate Frames

4- Members not Designed as Part of the Lateral-Force-Resisting System

Members of structures located in regions of high seismic risk, but not forming part of the lateral force resisting system, must be investigated for sufficient deformability during seismic response. These members, although not designed to resist seismic forces will deform along with the seismic lateral force resisting system. Therefore, they should have adequate strength and deformability to allow the development of design displacement δu, as per the requirements of Sec. 21.11 of ACI 318-05.

SEISMIC DESIGN COUPLING BEAMS

The content

1- Coupled wall geometry and target yield

mechanism

2- Forces in coupled shear wall

3- Coupling beam requirements

4- Coupling Beam flexural design -ACI315-05

5- Coupling Beam shear design -ACI315-05

6- Coupling beam design example

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1- Coupled wall geometry and target yield mechanism

2- Forces in coupled shear wall

Case-1

3- Coupling beam requirements

Case 2

Case 3

4- Coupling Beam flexural design

ACI315-05

The content

4-1- Coupled shear wall flexural design

4-2- Rectangular spandrel beam design,

flexural reinforcing

4-1- Coupled shear wall flexural design

4-2- Rectangular spandrel beam design, flexural reinforcing

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maxa a Tension Reinforcing only

5- Coupling Beam shear design

ACI315-05

The content

5-1- Rectangular spandrel beam design,

shear reinforcing

5-2- Rectangular spandrel beam design,

shear reinforcing

Seismic spandrel only

5-1- Rectangular spandrel beam design, shear reinforcing

'0.17c c s spandrel

V f t d

': 0.83n c s spandrel

if V f t d Failure reported yes

No

': 0.66s c s spandrel

if V f t d Failure reported No

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': / 0.83n u c s spandrel

if V V f t d Failure reported

yes

No

5-2- Rectangular spandrel beam design, shear reinforcing Seismic spandrel only

'0.33u c s spandrel

V f t dDiagonal reinforcing is reported when:

0.75s

6- Coupling beam design example

0.75s