CE 72.32 (January 2016 Semester) Lecture 5 - Preliminary Design and Sizing

Dr. Naveed Anwar
Executive Director, AIT Consulting
Affiliated Faculty, Structural Engineering
Director, ACECOMS
Design of Tall Buildings
Hybrid Learning System

Dr. Naveed Anwar
Director, ACECOMS
Lecture 5: Preliminary Design
and Sizing
Design of Tall Buildings

Design of Tall Building: Hybrid Learning System, Dr. Naveed Anwar 3
• The structural design of a tall building involves conceptual design, approximate
analysis, preliminary design and optimization, followed by the detailed and final
design.
• Codes and standards are used effectively to match limiting stresses,
displacements and accelerations.
• Risk analysis with safety and reliability, is often included in arriving at suitable
factors of safety in sliding and overturning.
• The initial selection of a structural system involves architectural, mechanical and
electrical requirements.
• Different floor systems are studied, in combination with 3 to 4 lateral systems,
with consequent structural schemes, almost 15 of them, for various
combinations between gravity and lateral.
• Preliminary design and optimization of various schemes follows in an iterative
fashion by satisfying drift and acceleration limits.
Preliminary Design

• Often simple software systems are used in this stage, such as frame (for
moment frames) and shear-flexure cantilever beam (for shear wall-frame
buildings), and cantilever box beam (for framed tubes) models.
• Methods developed by Fazlur Khan and Sbarounis (1964) and Heidebrecht and
Bryan Stafford Smith (1973) are used for shear wall-truss frame interaction while
the method by Coull (1974) is used for framed tubes.
• Goldberg (1975) also has approximate methods of analysis for tall buildings
composed of frames, shear wall-frames and framed tubes.
• Displacements and member forces are obtained, and corresponding
components are designed at different levels.
Simplified Software Models

• The major quantity of interest in arriving at the cost of a structural system is its
unit weight, in lbs/sft or in kg/sqm. In other words, the weight is directly
associated with the overall efficiency of the system in carrying gravity and lateral
loads.
• The stiffness of the system is also associated with weight.
• An ideal structural system could be the one in which the steel required to carry
the gravity loads alone could carry the wind loads.
• Optimization could be such that the wind could be carried by keeping stresses
within the difference between allowable stresses for gravity plus wind and
stresses due to gravity alone, usually a one third increase.
• However, this is not always possible as height to width ratios may not allow this
design to be achieved. Some premium for wind is often required.
Optimum Structural Systems

Structural Optimization

• The efficiency of the structural system is often determined by its height to width
ratio.
• The larger width for any height usually means larger stiffness. This implies larger
bay widths and larger lever arm for flange frames in framed tubes.
• The optimum height to width ratio should be between 5 and 7.
• In shear truss-frame buildings, the width of the truss should be less than about
12 relative to its height.
Height to Width Ratios

• The span length of girders often determines the steel quantity for the floor
framing.
• Smaller spans for exterior frames will lead to more efficient framed tube systems.
Span Dimension of Girders

• The proportions of members of the frame play a leading role in efficiency, with
deeper members being more effective in resisting drift.
• Deeper members also affect mechanical-architectural cost and increased floor
heights. The design optimization should include these costs. Larger column
widths and deeper spandrel may lead to more efficient framed tubes.
• The orientation of the wider columns should be along the plane of the frame.
Column spacing could be arranged in such a way that all gravity steel can
effectively carry wind with very little increase in weight for girders.
• Floor framing should be so arranged that most beams frame directly into
columns. Thus, gravity loads could be directly carried without extra girders.
Member Sizes of a Frame

• The floor framing is usually about 20% of the structure weight. It is useful to
optimize this subsystem before hand.
• Span to depth ratios, spacing of beams, slab thickness, composite design, and
openings for mechanical ductwork, should be carefully considered in floor system
design for efficiency.
• Span to depth ratios for floor framing are usually good at 20 to 24. This is the
minimum depth for strength and stiffness. Open web trusses could be used for
long spans.
• Composite action between trusses and slabs should be developed by shear
connection. Two way grid systems are often avoided as fabrication costs are
higher. However, in concrete design, they are used if repeating formwork is used.
• The widest possible spacing of beams and largest spans for slabs should be used.
The composite floor systems also have larger stiffness and diaphragm stiffness for
the floors.
• Solid slabs are better than slabs with cellular openings because the diaphragm
stiffness is increased.
Floor Framing Design

• This is an important consideration for framed tube system in extremely tall
buildings.
• This effect should be minimized by using deep spandrels and wide columns and
smaller spacing between columns.
• Transfer beams are used at lower levels to carry less number of openings. The
stiffness between column and girder should be balanced. Sometimes, deeper built
up I-shaped beams are used to increase stiffness.
• Field welding should be minimized by using 3 story sub-assemblies of column-
girder trees with field bolted at points of inflection as these reduce erection costs.
• High strength steel is not often beneficial because fabrication costs are high for
these. Reduction in total number of pieces to be assembled will result in cost
savings.
Shear Lag Effects

Simplified Modeling of Buildings

Design of Tall Building: Hybrid Learning System, Dr. Naveed Anwar
• Components not meeting the acceptance criteria for primary components are
designated as secondary and are removed from the mathematical model.
• Only primary components are modeled.
• The force-displacement characteristics of components are bilinear and the
degrading portion of the backbone curve is not explicitly modeled.
Simplified Modeling
13

• Stick Models
• Sandwich Beam Models
• Cantilever Tube Models
• Shear-Flexure Interaction Models
• Reduced Order Continuum Models
Various Proposed Simplified Models

Approximate Analysis and
Design Procedures

• Methods based on simplified loading
• Methods based on simplified structural model
• Combination of above
– Linear and Nonlinear Static Procedures
– Pushover Methods for Lateral Load Analysis
Simplified Analysis Procedures

Cost-Sensitive Design
18

The Cost Issues
What is the Project Cost?
How are Project Cost and Design related?
What factors affect the Cost?
How can the Cost be reduced?
How are Performance and Cost interrelated?
19

What is the Cost of a Project?
• Cost may include:
– Financial Cost (loan, interest, etc.)
– Planning and Design Cost
– Direct Construction Cost
– Maintenance Cost
– Incidental Cost
– Liquidated Cost (lost profit, etc.)
– Opportunistic Cost
– Environmental Cost
– Emotional Cost
– Non-determinist Resources
Cost is the
“Consumption of Resources”
20

Construction Cost of a Building Project
Planning and Design Cost 2-4%
• Architectural Design 70%
• Structural Design 20%
• Building Services Design 10%
Direct Construction Cost 90-95%
• Site Development 5-10%
• Structural Shell 25-40%
• Architectural Finishes 40-60%
• Building Services 5-15%
Supervision and Management Cost 1-3%
21

Cost of Concrete Building Structural Shell
Foundations
25-40%
Excavation and
Preparation
10-20%
Structural
Components
90-80%
Concrete
50-60%
Steel
40-50%
Super Structure
60-75%
Columns, Shear Walls,
etc.
15-25%
Concrete
30-40%
Steel
40-50%
Formwork
20-30%
Floor Slab System
75-85%
Concrete
30-40%
Steel
30-40%
Formwork
25-35
22

• How are Cost and Design related?
– Economical Design
– Least Cost Design
– Low Cost Design
– Optimum Design
– Optimized Design
– Cost Sensitive Design
– Cost Effective Design
– Cost Conscious Design
Cost and Design
24

Optimization
• There is a need to define what to optimize and what are the parameters that
can be changed.
• Optimizing one or two items may “un-optimize” the others
• Optimizing everything is a “Holy Grail”
(...and “Holy Grail” doesn't exist)
25

• Optimization
– When objective function has the
least value
– Single Variable, Multi-Objective
– Multi Variable, Single Objective
– Multi Variable, Multi Objective
• Least Cost Design
– When objective function is the
Cost and the least cost is achieved
– Cost needs to be defined
– Variables and their constraints
need to be defined
Optimum Design
26

Levels of Optimization
Levels of Optimization
Micro-Micro Level
One part of a
component, “Steel”
Micro Level
One Component,
“Column”
Local
One part or aspect
Global
Entire Problem,
Project
Universal
Entire System
27

• The minimum possible material for a building of any number of stories cannot be
less than that required for gravity loads alone.
• If there were no lateral loads such as wind or earthquake, any high-rise building
could be designed primarily for gravity loads. Such a design would not impose any
premium for height. Since there is no way to circumvent the gravity loads resulting
from dead and live loads.
• However, for high-rises, the required resistance to overturning moment combined
with the necessity of limiting lateral deflections among other requirements will
almost always require additional material over and above that required for gravity
load alone.
Concept of “Premium for Height”

• Lateral loads begin to show
dominance at about 50 stories
and become increasingly
important with greater height.
• Above 50 stories, lateral bracing
often makes the difference
between an economical and an
expensive solution.
• The objective is to arrive at a
bracing system that keeps the
additional material required for
lateral loads to a minimum.
Concept of “Premium for Height”

• In concrete construction, major factors responsible for reducing reinforcement
and concrete quantities are:
– New framing techniques, such as, skip joist construction in which every
other joist is eliminated have caught on with a consequent reduction in the
weight of floor framing.
– Increased use of mechanical couplers for transferring compression and
tensile forces.
– Use of welded cage for column ties, beam stirrups, etc., resulting in the
reduction of reinforcement.
– Use of high-strength concrete; 6,000 psi (41,370 kPa) to 10,000 psi (98,950
kPa) strengths are quite common.
– Use of lightweight aggregate typically reduces 10–20 psf (479–958 Pa) in
the dead load of the structure. The resulting savings in mild steel
reinforcement is approximately 10%–15%.
Factors for Reduction in
Weight of Structural Frame

– Most fire codes do not require as great a thickness of slabs when structural
lightweight concrete is used. Typically a thickness of at least 1/2 in. (12.5
mm) of concrete can be taken off from floor slabs without reducing the fire
rating.
– Use of 75 ksi (517 MPa) steel reinforcement.
• Because of the factors noted above, a typical glass curtain wall skyscraper
weighs 8–9 lb/ft3 (1.25–1.41 kN/m3) as compared to 15 lb/ft3 (2.35 kN/m3) for
buildings built in the 1940s.
Factors for Reduction in
Weight of Structural Frame (cont.)

Typical Steel Quantity Requirements
vs. The Building’s Height
32

Cost and Performance
33

Cost vs. Time
PerformanceCost
Time
Scope
• Time, Cost and Performance need to
be balanced
• Time can be a “Cost”
• How to reduce construction time:
– Automation
– Reducing the number of
components
– Prefabrication
– Innovative structural systems
– Innovative construction process
and management
34

The general belief
Increased Performance → Increased
Cost
Increased performance can be
achieved for the same cost
Reduced cost for same performance
Improved/efficient designs
May not always be true
Easy Needs knowledge, innovation,
better tools, better
technology, critical thinking,
out-of-the-box solutions
35

A reinforced concrete
wall section
(a)
(b)
(c)
(a)
(b)
(a)
(c)
P
M
P
M
36

Dr. Naveed Anwar
Director, ACECOMS
Thank You

CE 72.32 (January 2016 Semester) Lecture 5 - Preliminary Design and Sizing

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to CE 72.32 (January 2016 Semester) Lecture 5 - Preliminary Design and Sizing

Similar to CE 72.32 (January 2016 Semester) Lecture 5 - Preliminary Design and Sizing (20)

More from Fawad Najam

More from Fawad Najam (7)

Recently uploaded

Recently uploaded (20)

CE 72.32 (January 2016 Semester) Lecture 5 - Preliminary Design and Sizing