Transcript Structural Engineering - Civil-Engineering

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STRUCTURAL ENGINEERING
Structure of a Building
The primary function of
a building structure is
to support and transmit
the loads and forces to
the ground.
“Tracing the Loads”
or
“Chasing the Loads”
Characteristics of a Structure
Stability – needed to maintain shape.
The structure is dependent upon
balanced forces and equilibrium
Strength - ability of the structure to
withstand the applied forces, usually
includes a “factor of safety”
Economic Value – includes choices
made about the design, materials, and
function of the structure
Structural Elements
Structural elements in the building
consist of:
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Stringers or Beams
Girders
Columns
Footings
Connections
Steps in Structural Design
1.
2.
3.
4.
5.
6.
7.
8.
Planning – what function will the structure serve
Preliminary structural configuration and layout
Establishing the loads to be carried
Preliminary sizing of members
Analysis of structural members
Evaluate and compare the preliminary design
Redesign or repeat the above steps as this is an
iterative process
Designing and detailing the structural components
Forces and Loads
Design Loads
Dead Loads (DL) – fixed loads
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
building materials or components and the weight of structural
components
Given load of building, which is either calculated or is known
Live Loads (LL) – transient and moving loads
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Occupancy loads and furnishing loads, building usage varies
Snow loads
Construction loads
Live Load maybe variable during structures lifetime
Building codes specify Live Loads for floor and roof loadings
Design Loads (continued)
Wind Load (WL) –

Depends on Height and
location of structure
(Exposure categories)
Resulting loads yields:
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Lateral load on walls
Downward and upward
pressure on roofs
Overturning of the structure
Design Loads (continued)
Earthquake Loads (EQ)
 Seismic load based on
building mass , type and
configuration.
Epicenter
 Vertical and lateral forces
(dynamic)
 Building codes can simplify
loading
Hypocenter
Seismic
Forces at Base
of Building
Design Loads and “Factor of Safety”
Structural Design contains a “factor of safety.”
In order to accomplish this, Load Factors are
applied to the the various calculated loads.
Building Code requirements are conservative
in the methods of distribution and the
weights of loads, which adds to the “factor of
safety.”
However, to maintain simplicity we will not
use any factored loads for the CEA Project.
Loads & Load Paths
 Snow and/or roof load
 Use and occupancy load
 such as DL and LL
 Self weight of structure DL
 Ground reaction
BEAMS AND COLUMNS
LOADS
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The building dead load is the only
known load. All other forces will vary in
magnitude, duration and location.
The building is designed for design
load possibilities that may never
occur.
The structural efficiency of a building is
measured as the ratio of dead to live
load.The building designer strives to
keep the ratio low.
Beam Design
Beams are used in floors and roofs.
Maybe called floor joists, stringers, floor
beams or girders.
Loads on beams are either concentrated
or uniform loads
Beams are designed for Shear, Moment
(bending), and Deflection
Beams
Beams are sized appropriately to safely
support the loads a structure will carry.
Beams are primarily subjected to bending and
shear.
Deflection and deformation can be calculated.
Beams are sized to provide the maximum
result with the minimum materials. A factor
of safety is included in the design.
Beam Deflection
Limit Deflection to
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L/240 of total load (whereas L=length in inches)
L/300 of total load
L/360 of total load (building use throughout life is
unknown) Preferred Limit
WHY??
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Ceiling cracks in plaster
Roof ponding (flat roofs)
Visual or psychological reasons, such as too much
deflection and people think it could be unsafe
Designer’s judgment
Beam Types
Simple
Continuous
Cantilever
Moment
(fixed at one end)
Beam Types
Fixed
Moments at each end
Propped- Fixed at one end supported at other
Overhang
Columns
Columns carry primary Axial Loads and
therefore are designed for compression.
Additional loads from snow, wind or
other horizontal forces can cause
bending in the columns.
Columns then need to be designed for
Axial Load and Bending.
F (External)
Column Forces
Horizontal loads caused by
wind, snow, seismic or
internal building load
WCOL (External)
R1 (Internal)
R2 (Internal)
WFTG (External)
RSoil (External)
LOADS
Building Dead Loads
Weight of the structure
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(steel, concrete, timber)
Partitions/ Walls
Ductwork
Piping
Electrical fixtures
Floor coverings
Roof coverings
Ceiling
Typical Building Dead Loads
Concrete (density 150 lb/ft3)
per 1 inch thickness
12.5 lb/ft2
Steel and Timber based on structural element
weight s
Partitions/ Walls
— Wood stud 2x4 12” to 16” on center
with ½” gypsum board both sides
6 lb/ft2
— Brick (4” thick)
— Concrete Block (8” Wall)
40 lb/ft2
38 lb/ft2
Typical Building Dead Loads
Floor Covering
 Tile
 Hardwood
 Linoleum
 Sub floor ¾” plywood
Ceiling
 Suspended
 Drywall
12 lb/ft2
4 lb/ft2
1 lb/ft2
3 lb/ft2
2 lb/ft2
5 lb/ft2
Typical Building Dead Loads
Roofing
 Sheathing (3/4”)
 Asphalt Shingles
 Insulation Loose
 3 ply ready roofing
 5ply felt and gravel
3 lb/ft2
3 lb/ft2
½ lb/ft2
1 lb/ft2
6 lb/ft2
Mechanical
Electrical, Ductwork and Plumbing
these loads can vary - Estimated 10 lb/ft2
Estimate depends on the type of building
Some may use a percentage of Dead Load
Typical Building Uniform Live Loads
Retail
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First Floor
Upper Floors
100 lb/ft2
80 lb/ft2
Stadiums and Arenas
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Bleachers
Fixed Seats
100 lb/ft2
60 lb/ft2
Library
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Stacks
Reading rooms
Offices
150 lb/ft2
60 lb/ft2
50 lb/ft2
Typical Building Uniform Live Loads
Schools
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Classrooms
First floor corridors
Corridors above first floor
40 lb/ft2
100 lb/ft2
80 lb/ft2
Stadiums and Arenas
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Bleachers
Fixed Seats
Residential (one and two family)
Hotels and Multifamily
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Private rooms and corridors
Private rooms and corridors
100 lb/ft2
60 lb/ft2
40 lb/ft2
40 lb/ft2
100 lb/ft2
Snow Load
Snow Load depends on your location.
Almost all building codes have Snow
Load requirements.
Ground Snow Load ( in New York State)
 Rochester, NY
50 lb/ft2
 Albany, NY
55 lb/ft2
 Watertown, NY
65 lb/ft2
 White Plains, NY
45 lb/ft2
Design for Wind Loads
Dead Loads figure in the evaluation of a building
when designing for Wind Load.
The building Dead Load can help resist the
Overturning and Uplift conditions caused by wind.
Typically, a building framed with steel beams and
columns will have some type of bracing, such as steel
cross bracing or masonry block walls on exterior or in
elevator shaft to handle the wind load conditions.
The floor slab also helps resist wind loads and shear
loads
Building Design
Steel Frame with Concrete Floors and
Flat Roof
RETAIL BUILDING
Design notes:
Revit File is for illustrative purposes only. It is a
preliminary framing plan and therefore not all
steel framing members are accurately noted and
resized for final design.
Visibility of Wall, Roof, and Slab can be changed
to see total framing plan
Not all walls, slabs, or the roof are shown
Building left in “Under Construction” stage
Steel framed building designed for retail space
Girder
Beam
Footing
Column
Partial View of 2nd floor Framing
For Clarity the Ground Floor Slab, 2nd Floor Slab and Roof
Framing and Roof Deck are not shown
3D View of Retail Building
Steel Framing and 1st Floor Slab Shown
Steps in Calculation
1. Analysis of structural members, designing
for Moment and checking for Deflection
2. Evaluate and compare to preliminary design
3. Redesign or Recalculate as necessary, such
as repeat the above steps as this is an
iterative process
4. Calculate Beams loading, transfer loads to
Girder, and carry the load to the column and
then down to the footing
“Load Chasing” for Structural Design
The structural design is done by “chasing
the loads” of the Dead and Live Load
though the slabs, to beams, to girders
then onto the columns or walls. The loads
are then carried down to the footing or
foundation walls and then to the earth
below.
Chasing Loads for this project
Calculate Beam loading and obtain
reactions
Transfer reaction loads to Girder
Carry the girder reactions to the
column and then down to the footing
FOUNDATION PLAN
Design Area
Partial 2nd FLOOR FRAMING PLAN
Column B-3
Beam B.3
Girder 3BC
6’-8”
Width
Partial 2nd FLOOR FRAMING PLAN
Tributary or Contributing Area for Beam B.3 is shown
Column B-3
Partial Roof FLOOR FRAMING PLAN
Steps for Calculating Beam Loading
1.
2.
3.
4.
5.
6.
7.
Find weights of building elements
Compute weight carried per linear foot of beam
and multiple by Tributary Width
Assume weight of beam per lineal foot
Add beam weight to superimposed dead load to
get Total Dead Load (DL)
Select Design Live Load (LL) use applicable
building codes
Combine DL + LL, this will be the Uniform Load
on Beam, w
Calculate any Concentrated Loads on Beam
Steps for Calculating Beam Loading continued
8.
9.
10.
11.
12.
13.
Use MD Solids to set up Beam Loading and
generate the Moment, Shear and End Reactions
for the beam
Select Member Shape using the Standard Steel
Shapes
Define Stress Limits (set Steel Yield Stress
Fy=36ksi or 50 ksi)
Compare Beam Design to Allowable Deflection
Limits ( L/360)
Select most economical beam ( typically the
lightest beam weight)
Deflection may control beam size
Beam and Girder Calculations
Second Floor
2nd Floor Loading for Beam B.3 - Dead Load
Span Length 18’-0”
Dead Load
4” thick concrete slab
Flooring- Ceramic Tile
Partitions (Drywall with metal stud)
Suspended Ceiling
Mechanical/ Electrical Items
Total DL
Assumed Dead Load Weight of Beam
50 lb/ft2
10 lb/ft2
8 lb/ft2
2 lb/ft2
10 lb/ft2
80 lb/ft2
20 lb/ft
2nd Floor Loading for Beam B.3 - Live Load
Live Load
Retail Space
80 lb/ft2
Total Load DL + LL
(per lineal foot of beam)
[80lb/ft2 + 80 lb/ft2 ] x 6.67 ft = 1067.2 lb/ft
Add the Beam Weight of 20 lb/ft
Total DL + LL + Beam Weight = 1087.2 lb/ft
Use 1090 lb/ft
2nd Floor Loading for Beam B.3
Uniform Load w= 1090 lb/ft
Assume: Simple Beam Loading Condition
Span Length is 18 feet.
Uniform Load w = 1090 lb/ft
2nd Floor Beam B.3 - Shear and Moment
Shear
Moment
Max. Moment = 44,145lb-ft
Max. Shear = 9,810 lb
Design Results for Beam B.3
Note: Beams were sized using MD Solids
By Limiting the Deflection to L /360
Where L = 18ft x 12 in/ft = 216 inches
Limit Deflection = L/360
= 216/360
= 0.60 inches
Design Results for Beam B.3
Typically you design for Moment and then check Deflection
Before finishing using MD Solids, use this method that
looks at the Moment and Allowable Bending Stress to find
out the Required Section Modulus.
Where:
SRequired = M/Fb
S is the Section Modulus Required
M is the maximum Moment
Fb is the Allowable Bending Stress
Fb= o.66Fy
For Fy=36,000psi Fb= 24,000 psi
For Fy=50,000psi
Fb=33,000 psi
Design Results for Beam B.3
SRequired = M/Fb
M=44,145 ft-lbs
SRequired = (44,145 ft-lb)(12 in/ft) / 24,000 lb/in2
SRequired = 22.07 in3
This is the Required Section Modulus for Beam B.3
Using this value and a reference for Steel Beams, you
can select a beam section that fits this requirement.
Design Results for Beam B.3
MD Solids calculates the
following :
Standard steel shapes
that will be acceptable
for the specified bending
moment and shear
force.
You must select
Standard Steel Shapes
for the U.S. and use
Fy=36,000 psi for Yield
Strength of Steel
Selecting Beam Sizes
In selecting wide-flanged structural sections , keep
in mind the following:
Section Modulus of beam should be large enough so
that the Allowable Bending Stress is not exceeded
NOTE: MD Solids considered this
Limit Deflection to L/360 where L is in inches
Moment of Inertia of beam should be large enough
so that deflection limits are not exceeded, MD Solids
calculates the deflection based on the selected
structural shape.
Comparisons of the results for Beam B.3
Beam
Sz (in4)
Deflection (inches)
W10x22
23.2
0.7523
W12x22
25.4
0.5691
W14x22
29.0
0.4461
W10x26
27.9
0.6165
W12x26
33.4
0.4352
W14x26
33.5
0.3624
SELECT
Limiting Deflection to L/360
This most likely will control the beam design.
2nd Floor Loading for Girder 3-BC
Uniform Load w= 50 lb/ft (Estimated weight of Girder)
P1 = P2 =19,620 lb. These are the reactions from each
beam similar to Beam B.3 that rest on the Girder
2nd Floor Shear and Moment Girder 3BC
Max. Moment = 133,365 lb-ft
Max. Shear = 20,120.0 lb
Design Results for Girder 3BC
The following standard steel shapes will be
acceptable for the specified bending moment and
shear force.
W16x45
Sz= 72.7 in3
Deflection=0.5773”
W18x46
Sz= 78.8in3
Deflection=0.4761”
Deflection Limit = L/360 = (20 ft x 12 in/ft)/360
Deflection Limit = 0.666”
In MD Solids you should have selected Standard
Steel Shapes for the U.S. and used Fy=36,000 psi
for Yield Strength of Steel
Roof Calculations for
Column Loading
Column B-3
Tributary Roof Area Carried by Column
Column B-3 Loads
We will not size the columns for this
project as that is more involved than what
we need to do for this CEA project.
In addition to the Axial Loads, other loads
from snow, wind, or other horizontal
forces can cause Bending in columns.
Columns are therefore designed for Axial
Load and Bending.
Footing Loads for Column B-3
We will size the footing for Column B-3
Use Allowable Soil Bearing Capacity =
3000 psf
Loads transferred to footing are generated
from:
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Dead and Live Loads from structural elements
above ( 2nd Floor and Roof )
Columns Dead Load ( Self Weight)
Loads from 1st Floor slab
Dead load of Footing itself
Roof Loads
Dead Load
Roof Type:Corrugated Steel Deck with Insulation and 5
ply Membrane Roof and gravel
Ceiling Suspended
Mechanical Equipment
Steel Deck
Insulation
Roof Membrane and Gravel
Roof Framing
Total
2 lb/ft2
10 lb/ft2
5 lb/ft2
2 lb/ft2
6 lb/ft2
10 lb/ft2
35 lb/ft2
Roof Loads continued
Snow Load
Rochester, NY
Total Load on Roof
DL + SL = 35 lb/ft2
55 lb/ft2
+
55 lb/ft2
=
90 lb/ft2
This load may seem high, but consider that no
additional load was added for Mechanical Roof top
equipment
Roof Loads continued
Axial Load On Column B-3 from Roof
Tributary Area of Roof = 18 ft x 20 ft= 360 ft2
DL + SL = 90 lb/ft2
(DL+SL)( Trib. Area)=(90 lb/ft2)(360 ft2)=32,400 lb
Size Footings Under Columns
Loads on Column and Footing
•Loads on Column B-3
have been generated from
the Beam and Girder
reactions at the Roof , the
2nd Floor
•Additionally, the self
weight of the column and
footing will also be added
to the total load used to
Size the Footing
Soil Bearing Reaction
Roof Loads
COLUMN
2nd Floor Loads
1st Floor/ Slab Loads
Column B-3
2nd Floor Partial Plan
Loads on Column and Footing
Loads on the Column
2nd Floor
Girder x 2
=
(20,120 lb) 2 = 40,240lb
Beams x 2
=
(9,810 lb) 2 = 19,620lb
Roof
Concentrated Load
= 32,400 lb
Column Self Weight
21 ft height x 50 lb.ft estimated = 1,050 lb
TOTAL
93,310 lb
USE 94,000 lbs
Loads on Footing
Total Load on Footing = 94,000lb
The Soil is capable of resisting a total bearing
pressure of force of 3000 lb/ft2
Using the following formula:
Pressure = Load /Area
q= P/A
q = 3000 lb/ft2 is the allowable bearing capacity
of the soil
Soil Bearing Capacity Available
Pressure = Load /Area
q= P/A
We will need to deduct the weight of the
footing, which the footing thickness is 12
inches. This is an estimate, typically standard
thickness, but the footing load is high.
(1 ft thick) x 150 lb/ft2 = footing weight in lb/ft2
Weight of Footing
= 150 lb/ft2
Soil Capacity Available = 3000 lb/ft2 - 150lb/ft2
Soil Capacity Available = 2850 lb/ft2 = qnet
Sizing the Footing for Column B-3
Soil Capacity Available = 2850 lb/ft2 = qnet
Total Load of Footing = 94,000 lb
Pressure = Load /Area
q= P/A
Rearranging the formula so that we can get the
required Area of the footing
P/ q net = Area
94,000 lb / 2850 lb/ft2 = 32.98 ft2 = Area Req’d
Footing Size = 5.75 ft X 5.75 ft
USE 6’-0” x 6’-0” Square Footing
Reference Sources
– Jefferis, A., & Madsen, D. A. (2001). Architectural
Drafting and Design. Albany, NY: Delmar, a division of
Thomson Learning.
– Kane, K., & Onouye, B., (2002). Statics and Strength of
Materials for Architecture and Building Construction.(2nd
ed.). Saddle River, NJ: Pearson Education, Inc
– Shaeffer, R. E., (2002). Elementary Structures for
Architects and Builders (4th ed.). Columbus, OH: Prentice
Hall.
– Manual of Steel Construction, (8th ed), American Institute
of Steel Construction
– http://www.emporis.com/en/
– http://www.pbs.org/wgbh/buildingbig/lab/forces.html
– ASCE Minimum Design Loads for buildings and Other
Structures,ASCE 7-98