ACI Seminar by Tabsh - ALHOSN University Seminar by Tabsh.pdf · Flat slab with drop panels.This...

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Dr. Sami W. Tabsh, P.E.OTAK International

Abu Dhabi, [email protected]

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Outline

1. Introduction2. Gravity Loads on Buildings3. Lateral Loads on Buildings4. Load path5. Gravity Load Resisting Systems6. Lateral Load Resisting Systems7. Summary

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1. IntroductionThe 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. IntroductionThe important design considerations are:

1. ArchitecturalInternal layout to meet functional requirementsAesthetic qualities

2. StructuralStrength (ultimate loads, P-Δ effect, ductility)Serviceability (excessive cracking, deflections, vibrations)

3. ServicesPlumbing, lift, ventilation & power

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1. IntroductionFor 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. IntroductionLoads 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.

A. 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 BuildingsBasic 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

Sand/cement screed

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2. Gravity Loads on BuildingsBasic area weights:Hollow block (CMU) walls (including plaster):

150 mm thick 2.7 kN/m2



Equiv. unif. weight of partitions:Light 1.5 kN/m2

Average 2.5 kN/m2

Hollow block walls

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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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B. Live loads are short duration forces 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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2. Gravity Loads on BuildingsLive 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)

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Assembly areas- Fixed seating- Moveable seats

1Flat Roofs

52.5

Office buildings- Lobbies- Offices

5 for ground floor (4 for other)2

Hotels and multi-family residential housing- Public rooms & corridors- Private rooms

2Residential (1 and 2 family dwellings)

Uniform Load (kN/m2)Occupancy/Use

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

1. Elevator machinery 100% 2. Light machinery (motor-driven) 20% 3. Power-driven units 50%4. Hangers for floors or balconies 33%

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3. Lateral Loads on BuildingsThe major lateral loads on building structures are wind and earthquake loads.Wind load on structures is affected by:

Wind speed and gust effectHeight and stiffness of buildingCross-sectional shape of buildingSurrounding topography and terrainPresence of openings in the building envelope

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

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3. Lateral Loads on BuildingsASCE7-05 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 factorKd = directionality factor

I = importance factorV = Basic wind velocity (m/s)

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3. Lateral Loads on BuildingsThe 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.

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3. Lateral Loads on BuildingsThe Wind Directionality Factor, Kd, accounts for uncertainties in the direction of the wind on the structure.

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3. Lateral Loads on BuildingsThe Importance Factor, I, accounts for the use of the facility and its design life.

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3. Lateral Loads on BuildingsThe Exposure Category is based on ground surface roughness that is found from topography, vegetation, and construction.

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3. Lateral Loads on BuildingsA Velocity Pressure Exposure coefficient Kz or Kh, is determined based on the exposure category.

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3. Lateral Loads on BuildingsWind 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 BuildingsThe 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).

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3. Lateral Loads on BuildingsInternal 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 BuildingsExternal pressure coefficients, Cp, define the distribution of the wind load around the envelope. They depend on the building’s surface location and plan dimension.

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3. Lateral Loads on BuildingsFinally, 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 BuildingsEarthquake load on structures is affected by several factors:

Earthquake intensityGeotechnical data at building siteMass of the buildingStiffness of the buildingCross-sectional shape of buildingHeight of the building

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3. Lateral Loads on Buildings

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

1. Determine building occupancy category and corresponding importance factor (I)

2. Determine basic ground motion parameters (SS & S1)

3. Obtain the site classification (A to F) and site coefficient adjustment factors (Fa & Fv)

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3. Lateral Loads on Buildings4. Get design ground motion parameters (SDS

& SD1) and seismic design category (A to D)

5. Select structural system and system parameters (R, Cd & Ωo)

6. Examine the system for configuration irregularities

7. Determine the lateral force analysis procedure

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3. Lateral Loads on BuildingsThe Importance Factor, I, accounts for the use of the facility and its design life.

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3. Lateral Loads on Buildings• Use seismic hazard maps to get designground motions (IBC 2006)

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

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3. Lateral Loads on BuildingsEvaluate Seismic Design Category (SDC) based on soil data:

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3. Lateral Loads on Buildings

Determine whether the building has horizontal or vertical structural irregularities.

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3. Lateral Loads on BuildingsDetermine structural system parametersResponse modification (R), system over-strength (Ωo) and deflection amplification (Cd) factors.

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3. Lateral Loads on BuildingsEquivalent Lateral Force Procedure

Determine Base Shear: SV C W=where

Equivalent Lateral Force ProcedureDetermine Base Shear: SV C W=Determine Base Shear: SV C W=wherewhere

Fx

hxw1

w2

wx

F1

F2

Fnwn

w3

i=1i=2

i=x

i=n

i=3

i=n-1 wn-1

Fx

hxw1

w2

wx

F1

F2

Fnwn

w3

i=1i=2

i=x

i=n

i=3

i=n-1 wn-1

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4. Load PathLoads 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

2nd Floor

1st Floor

Footing

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4. Load Path

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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.

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5. Gravity Load Resisting Systems

2-way slab1-way slab

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

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5. Gravity Load Resisting SystemsOne-way systems are floor or roof panels in which the load spans in one direction only, between parallel supports.

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

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5. Gravity Load Resisting SystemsExamples of gravity load resisting floor systems include:

1. Flat plate 2. Flat slab (with drop panels and/or

column capitals) 3. Two-way slab4. One-way slab on beams5. One-way ribbed system 6. Two-way waffle system

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

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5. Gravity Load Resisting SystemsTwo-way slabs are floor panels supported along all four sides by drop beams.

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

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5. Gravity Load Resisting SystemsOne-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 SystemsTwo-way joist (or waffle) slab. This floor has joists in both directions. It is the strongest and will have the least deflection.

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Flat Plate Flat Slab

1-way Slab

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Ribbed slab (void) Ribbed slab (Hourdi) Waffle Slab

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Minimum thickness requirements for 2-way slabs (ACI 318-08)Longer Clear Span (ft)

Min

imum

Thi

ckne

ss (

in)

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Several factors affect the selection of one structural floor system for gravity loads over another:

I- Economy of constructionII- ServiceabilityIII- Load carrying abilityIV- Economy of materialV- Architectural considerations

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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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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). 58

5. Gravity Load Resisting SystemsIII- Load carrying ability:1. Flat plates: spans < 6 m (residential & light

commercial) 2. Flat slabs: spans 6-8 m3. 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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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.

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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:

1. Primary collection (load transfers from walls/cladding to diaphragms)

2. Horizontal distribution (load transfers from diaphragms to vertical members), and

3. Vertical transportation (load transfers from vertical members to foundations)

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Wind3

2 1

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There are three main lateral load resisting structural systems for low and medium risebuildings. They are:1. Braced frames, 2. Shear walls, and3. Rigid framesA combination of the above 3 systems may also be used in medium rise buildings.

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1. 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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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 Systems2. 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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Shear wall buildings are very stiff structures against lateral loads.They are often used on up to 30-40 stories.

Vertical analogy as cantilever beams

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6. Lateral Load Resisting Systems3. 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.

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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 SystemsFor high-rise buildings, the lateral load resisting system is complex, and may consist of one of the followings:

1. Framed tube 2. Trussed tube3. Tube-in-tube4. Bundled tube

These systems were first introduced by Fazlur Khan in the 1960s while he was with Skidmore, Owings and Merrill in Chicago.

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Trussed TubeFramed Tube Tube-in-Tube Bundled Tube72


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7. Summary1. The structural engineer and architect should

collaborate early to satisfy the conflicting requirements of selecting a structural system.

2. The factors that affect the selection of a floor system for gravity loads are economy of construction, serviceability, load carrying ability, and architectural considerations.

3. 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.

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