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Dr. Sami W. Tabsh, P.E. OTAK International Abu Dhabi, UAE [email protected]

Introduction Gravity Loads on Buildings Lateral Loads on Buildings Load path Gravity Load Resisting Systems Lateral Load Resisting Systems Summary

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1. Introduction

The building design team consists of: (1) Owner, (2) Architect, (3) Structural Engineer, and (4) Services Engineers (Mechanical, Electrical & Plumbing). Team should collaborate EARLY to agree on a form of structure to satisfy the conflicting requirements. The structural system of the building depends on the architectural requirements.

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1. Introduction

The important design considerations are: 1. Architectural 2. Structural

Internal layout to meet functional requirements Aesthetic qualities Strength (ultimate loads, P- effect, ductility) Serviceability (excessive cracking, deflections, vibrations) Plumbing, lift, ventilation & power

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3. Services

1. Introduction

For a safe and economical design, the design process should give an optimum solution. The criteria for design can be: - Minimum cost - Minimum weight - Minimum construction time - Minimum labor Best solution is probably a combination of the above.

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1. Introduction

Loads on buildings are specified by the ASCE 7-05 Standard:

Minimum Design Loads for Buildings and Other Structures,

which is the basis behind the 2006 IBC code. This standard addresses dead load, live load, flood, snow, wind, rain, ice, earthquakes and load combinations.

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2. Gravity Loads on Buildings

The major gravity loads on building structures are dead and live loads. Dead loads are fixed-position gravity loads (i.e. long-term stationary forces). They consist of the weight of all materials of construction incorporated into the building including architectural, structural, and MEP items. Dead load also includes the weight of any fixed equipments.

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2. Gravity Loads on Buildings

Basic volumetric weights:

Concrete 24 kN/m3 Sand/cement screed 20 kN/m3 Steel 77 kN/m3 Glass 25 kN/m3 Wood 5-6 kN/m3 soil 12-19 kN/m3 Asphalt 20 kN/m3

A.

Sand/cement screed

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2. Gravity Loads on Buildings

Basic area weights: Hollow block (CMU) walls (including plaster):

150 mm thick 200 mm thick 300 mm thick Light Average 2.7 kN/m2 3.2 kN/m2 4.2 kN/m2 1.5 kN/m2 2.5 kN/m2

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2. Gravity Loads on Buildings

Typical calculation of superimposed dead load on floor (without partition loads): Screed (75mm-thick) 1.5 kN/m2 Finishes (tiling + grout) 1.0 kN/m2 False ceiling & services 0.5 kN/m2 Total 3.0 kN/m2

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Hollow block walls

Equiv. unif. weight of partitions:

2. Gravity Loads on Buildings

B.

2. Gravity Loads on Buildings

Live load has two components: (1) Sustained, which is less uncertain and acts over a long period (e.g. furniture) (2) Transient, which is more uncertain and acts over a short period (e.g. people)

which change in location and magnitude during the life of the structure. They include the weight of people, furniture and movable partitions. They are based upon intended use or occupancy of the building (e.g. residential versus office).

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Live loads are short duration forces

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2. Gravity Loads on Buildings

2. Gravity Loads on Buildings

Occupancy/Use Residential (1 and 2 family dwellings) Hotels and multi-family residential housing - Public rooms & corridors - Private rooms Office buildings - Lobbies - Offices Assembly areas - Fixed seating - Moveable seats Flat Roofs Uniform Load (kN/m2) 2 5 for ground floor (4 for other) 2 5 2.5 3 5 1

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2. Gravity Loads on Buildings

Live loads include adequate allowance for ordinary impact conditions. Weight of machinery and moving loads shall be increased for impact by:

1. 2. 3. 4.

3. Lateral Loads on Buildings

The major lateral loads on building structures are wind and earthquake loads. Wind load on structures is affected by:

Wind speed and gust effect Height and stiffness of building Cross-sectional shape of building Surrounding topography and terrain Presence of openings in the building envelope

Elevator machinery Light machinery (motor-driven) Power-driven units Hangers for floors or balconies

100% 20% 50% 33%

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3. Lateral Loads on Buildings

In open structures, internal pressure can combine with external pressure on the roof, side and leeward walls, causing larger forces on the members.

3. Lateral Loads on Buildings

ASCE7-05 specifies the wind velocity pressure qz (N/m2) at height z (m) as: qz = 0.613 Kz Kzt Kd V 2 I where Kz = exposure coefficient Kzt = topographic factor Kd = directionality factor V = Basic wind velocity (m/s) I = importance factor

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3. Lateral Loads on Buildings

The Basic Wind Speed, V, is associated with an annual probability of 0.02, adjusted for equivalence to a 3-s gust wind speed at 10 m above ground in exposure Category C.

3. Lateral Loads on Buildings

The Wind Directionality Factor, Kd, accounts for uncertainties in the direction of the wind on the structure.

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3. Lateral Loads on Buildings

The Importance Factor, I, accounts for the use of the facility and its design life.

3. Lateral Loads on Buildings

The Exposure Category is based on ground surface roughness that is found from topography, vegetation, and construction.

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3. Lateral Loads on Buildings

A Velocity Pressure Exposure coefficient Kz or Kh, is determined based on the exposure category.

3. Lateral Loads on Buildings

Wind speed-up effects at abrupt changes in the topography are accounted for in the Topographic Factor, Kzt.

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3. Lateral Loads on Buildings

The Gust Effect Factor, G or Gf, accounts for the turbulence effect of the wind on the structure and is affected by the stiffness of the structure (rigid versus flexible).

3. Lateral Loads on Buildings

Internal pressure coefficients, GCpi , shall be determined from Fig. 6-5 based on the building enclosure classifications (enclosed, partially enclosed, or open).

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3. Lateral Loads on Buildings

External pressure coefficients, Cp, define the distribution of the wind load around the envelope. They depend on the building's surface location and plan dimension.

3. Lateral Loads on Buildings

Finally, the design wind pressures, p, for buildings are determined by:

p = q G Cp - qi (GCpi)

where

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3. Lateral Loads on Buildings

Earthquake load on structures is affected by several factors: Earthquake intensity Geotechnical data at building site Mass of the building Stiffness of the building Cross-sectional shape of building Height of the building

3. Lateral Loads on Buildings

Analysis of buildings subjected to earthquakes in ASCE7-05 requires:

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2.

3.

Determine building occupancy category and corresponding importance factor (I) Determine basic ground motion parameters (SS & S1) Obtain the site classification (A to F) and site coefficient adjustment factors (Fa & Fv)

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3. Lateral Loads on Buildings

4.

3. Lateral Loads on Buildings

The Importance Factor, I, accounts for the use of the facility and its design life.

5.

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Get design ground motion parameters (SDS & SD1) and seismic design category (A to D) Select structural system and system parameters (R, Cd & o) Examine the system for configuration irregularities Determine the lateral force analysis procedure 31

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3. Lateral Loads on Buildings

· Use seismic hazard maps to get design ground motions (IBC 2006)

3. Lateral Loads on Buildings

Evaluate Seismic Design Category (SDC) based on soil data:

Equivalent to: UBC 97, Zone=I (Z=0.075)

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3. Lateral Loads on Buildings

Determine whether the building has horizontal or vertical structural irregularities.

3. Lateral Loads on Buildings

Determine structural system parameters

Response modification (R), system over-strength (o) and deflection amplification (Cd) factors.

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3. Lateral Loads on Buildings

Equivalent Lateral Force Procedure

Determine Base Shear: where

4. Load Path

Loads acting on a building follow a path through the structure and must be resisted by the ground. Loads accumulate as they are routed through key connections in a building. Member connections are critical links in a load path. Failed connections may cause collapse.

Roof

V = CSW

Fn Fx F2 F1

i=n i=n-1 i=x i=3 i=2 i=1

wn wn-1 wx w3 w2 w1 hx

2nd Floor

1st Floor

Footing

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4. Load Path

4. Load Path

Load path is a chain. It is only as strong as its weakest link. The roof/floors, beams, girders, columns, shear walls, bracing members, foundation and connections are links in the chain.

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4. Load Path

The load path must be continuous and complete for all possible loads on the structure.

5. Gravity Load Resisting Systems

Structural behavior of gravity load resisting systems can be mainly classified as either 1way or 2-way slab.

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1-way slab

2-way slab

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5. Gravity Load Resisting Systems

One-way systems are floor or roof panels in which the load spans in one direction only, between parallel supports.

5. Gravity Load Resisting Systems

Two-way slabs are floor panels supported along all four sides. Significant bending occurs in both span directions, hence 1dimensional flexure theory does not apply.

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5. Gravity Load Resisting Systems

Examples of gravity load resisting floor systems include: 1. Flat plate 2. Flat slab (with drop panels and/or column capitals) 3. Two-way slab 4. One-way slab on beams 5. One-way ribbed system 6. Two-way waffle system

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5. Gravity Load Resisting Systems

Flat plate system. There are no beams between the columns. Instead, the floor is heavily reinforced in both directions. Edge beams may be used on the perimeter.

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5. Gravity Load Resisting Systems

Flat slab with drop panels. This system consists of a flat plate with column capitals to provide shear resistance around the columns.

5. Gravity Load Resisting Systems

Two-way slabs are floor panels supported along all four sides by drop beams.

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5. Gravity Load Resisting Systems

One-way slab on beams. The floor loads are transferred to parallel beams, which are then transferred to the columns.

5. Gravity Load Resisting Systems

One-way ribbed slab. The ribs act like small beams between a thin slab. They are created with removable forms or with permanent hollow concrete masonry units.

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5. Gravity Load Resisting Systems

Two-way joist (or waffle) slab. This floor has joists in both directions. It is the strongest and will have the least deflection.

5. Gravity Load Resisting Systems

Flat Plate

Flat Slab

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1-way Slab

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5. Gravity Load Resisting Systems

5. Gravity Load Resisting Systems

Minimum Thickness (in)

Ribbed slab (void)

Ribbed slab (Hourdi)

Waffle Slab

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Longer Clear Span (ft)

Minimum thickness requirements for 2-way slabs (ACI 318-08)

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5. Gravity Load Resisting Systems

Several factors affect the selection of one structural floor system for gravity loads over another: I- Economy of construction II- Serviceability III- Load carrying ability IV- Economy of material V- Architectural considerations

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5. Gravity Load Resisting Systems

I- Economy of construction: 1. Flat plates generally have the simplest formwork and least labor costs. 2. Two-way slabs require forming drop beams; which is labor intensive. 3. May be governed by local customs of builders. 4. Waffle slabs use standardized prefab forms.

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5. Gravity Load Resisting Systems

II- Serviceability: 1. Deflections are most difficult to control in beamless slabs. 2. Deflections can be controlled somewhat by adding capitals and/or beams. 3. Two-way slabs with beams are most suitable for minimizing deflections. 4. Negative moment cracking may be a problem in flat plates (near columns).

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5. Gravity Load Resisting Systems

III- Load carrying ability: 1. Flat plates: spans < 6 m (residential & light commercial) 2. Flat slabs: spans 6-8 m 3. Two-way slabs: spans 6-9 m 4. Waffle slabs: spans > 9 m 5. Beamless slabs are at a disadvantage if lateral loads are being carried by frames.

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5. Gravity Load Resisting Systems

IV- Economy of material: 1. Ribbed slabs and waffle slabs require less steel than other systems. 2. Voids in ribbed and waffle slabs decrease the amount of concrete required and significantly reduce the weight.

5. Gravity Load Resisting Systems

V- Architectural considerations: 1. Presence of capitals and beams may be objectionable to the architect. 2. Flat plate construction can minimize story height in areas with height restrictions. 3. Beamless slabs provide flexible column arrangements and partition locations

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6. Lateral Load Resisting Systems

In buildings, lateral load (say wind) is transferred to the foundation in 3 stages: Primary collection (load transfers from walls/cladding to diaphragms) Horizontal distribution (load transfers from diaphragms to vertical members), and Vertical transportation (load transfers from vertical members to foundations)

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6. Lateral Load Resisting Systems

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Wind

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6. Lateral Load Resisting Systems

There are three main lateral load resisting structural systems for low and medium rise buildings. They are: 1. Braced frames, 2. Shear walls, and 3. Rigid frames A combination of the above 3 systems may also be used in medium rise buildings.

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6. Lateral Load Resisting Systems

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Braced Frames: Such structures consist of a frame strengthened with diagonal bracing members. The columns and beams carry the gravity load, while the bracing carries the lateral load.

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6. Lateral Load Resisting Systems

Braced frames are mostly used in steel buildings since the diagonal bracing has to resist tension. Bracing generally takes the form of steel rolled sections, circular bar sections, or tubes.

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6. Lateral Load Resisting Systems

Shear Walls: They were first used in 1940. They act like deep cantilevered beams supported at the ground. They can resist both gravity (load bearing) and lateral loads, transmitted to them by the floors.

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6. Lateral Load Resisting Systems

Shear wall buildings are very stiff structures against lateral loads. They are often used on up to 3040 stories.

3.

6. Lateral Load Resisting Systems

Rigid Frames: Sometimes referred to as moment-resisting frames. They are composed of reinforced concrete portal frames, with the lateral load mainly resisted by flexure.

Vertical analogy as cantilever beams

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6. Lateral Load Resisting Systems

Rigid frames resist lateral loads through beams and columns. They tend to have large drift (lateral deflection). They are mainly used in low/medium-rise buildings (up to 20 stories).

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6. Lateral Load Resisting Systems

For high-rise buildings, the lateral load resisting system is complex, and may consist of one of the followings: 1. Framed tube 2. Trussed tube 3. Tube-in-tube 4. Bundled tube These systems were first introduced by Fazlur Khan in the 1960s while he was with Skidmore, Owings and Merrill in Chicago. 70

6. Lateral Load Resisting Systems

6. Lateral Load Resisting Systems

Framed Tube

Trussed Tube

Tube-in-Tube

Bundled Tube

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7. Summary

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The structural engineer and architect should collaborate early to satisfy the conflicting requirements of selecting a structural system. The factors that affect the selection of a floor system for gravity loads are economy of construction, serviceability, load carrying ability, and architectural considerations. Rigid frames, shear walls and braced frames are effective lateral load resisting systems for medium rise buildings. For high-rise buildings, the lateral load resisting system may consist of a framed tube or its derivations. 73

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