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Types of Structures & Loads- of 21

ENGR. ATIQ UR REHMAN AWAN

STRUCTURAL

ANALYSIS

Department of Engineering & Technology, The University of Lahore


A structure refers to a system of connected parts used to support a load. Important examples

related to civil engineering include buildings, bridges, and towers; and in other branches of

engineering, ship and aircraft frames, tanks, pressure vessels, mechanical systems, and electrical

supporting structures are important.

When designing a structure to serve a specified function for public use, the engineer must

account for its safety, esthetics, and serviceability, while taking into consideration economic

and environmental constraints. Once a preliminary design of a structure is proposed, the

structure must then be analyzed to ensure that it has its required stiffness and strength. To

analyze a structure properly, certain idealizations must be made as to how the members are

supported and connected together. The loadings are determined from codes and local

specifications, and the forces in the members and their displacements are found using the

theory of structural analysis, which is the subject matter of this text. The results of this analysis

then can be used to redesign the structure, accounting for a more accurate determination of

the weight of the members and their size. Structural design, therefore, follows a series of

successive approximations in which every cycle requires a structural analysis.

It is important for a structural engineer to recognize the various types of elements composing

a structure and to be able to classify structures as to their form and function.

Types of Structures

The combination of structural elements and the materials from which they are composed is

referred to as a structural system. Each system is constructed of one or more of four basic types

of structures.

Trusses: When the span of a structure is required to be large and its depth is not an important

criterion for design, a truss may be selected. Trusses consist of slender elements, usually

arranged in triangular fashion. Planar trusses are composed of members that lie in the same

plane and are frequently used for bridge and roof support, whereas space trusses have

members extending in three dimensions and are suitable for towers.

Types of Structures & Loads 1

Introduction

Classification of structures


Due to the geometric arrangement of its

members, loads that cause the entire truss to

bend are converted into tensile or

compressive forces in the members. Because

of this, one of the primary advantages of a

truss, compared to a beam, is that it uses less

material to support a given load. Also, a truss

is constructed from long and slender

elements, which can be arranged in various ways to support a load. Most often it is

economically feasible to use a truss to cover spans ranging from 30 ft (9 m) to 400 ft (122 m),

although trusses have been used on occasion for spans of greater lengths.

Cables and Arches: Two other forms of structures used to span long distances are the cable

and the arch. Cables are usually flexible and carry their loads in tension. They are commonly

used to support bridges, and building roofs. When used for these purposes, the cable has an

advantage over the beam and the truss, especially

for spans that are greater than 150 ft (46 m).

Because they are always in tension, cables will not

become unstable and suddenly collapse, as may

happen with beams or trusses. Furthermore, the

truss will require added costs for construction and

increased depth as the span increases. Use of

cables, on the other hand, is limited only by their

sag, weight, and methods of anchorage.

The arch achieves its strength in compression, since it has a reverse curvature to that of the

cable. The arch must be rigid, however, in order to maintain its shape, and this results in

secondary loadings involving shear and moment, which must be considered in its design.

Arches are frequently used in bridge structures, dome roofs, and for openings in masonry walls.


Frames: Frames are often used in buildings and are composed of beams and columns that are

either pin or fixed connected. Like trusses, frames extend in two or three dimensions. The

loading on a frame causes bending of its members, and if it has rigid joint connections, this

structure is generally “indeterminate” from a standpoint of analysis. The strength of such a

frame is derived from the moment interactions between the beams and the columns at the

rigid joints.

Structural Elements

Some of the more common elements from which structures are composed are as follows.

Tie Rods: Structural members subjected to a tensile force are referred to as tie rods or bracing

struts. Due to the nature of this load, these members are rather slender, and are often chosen

from rods, bars, angles, or channels.

Slab: A slab is a broad, flat plate, with top and bottom surfaces parallel and supported by

beams, by masonry or reinforced concrete walls, by structural steel members, directly by

columns. According to the way loads are transferred to supporting beams and columns,

slabs are classified into two types; one-way and two-way. One way slab is define as the slab

who’s longer to shorter span ratio Ly/Lx ≥ 2. A one-way slab needs moment resisting

reinforcement only in its short-direction because the moment along long axes is so small that

it can be neglected. A slab who’s longer to shorter span ratio Ly/Lx< 2 is called two-way slab.

Two-way slabs have tension reinforcing spanning in both directions.


Beams: Beams are usually straight horizontal members used primarily to carry vertical loads.

Based on the types of support beam is classified as simply supported, Cantilever, overhang,

continuous and fixed beam.

Based on the types of material used beams may be

made from concrete, steel or from wood. Concrete

beams generally have rectangular cross sections, since

it is easy to construct this form directly in the field.

Because concrete is rather weak in resisting tension,

steel “reinforcing rods” are cast into the beam within

regions of the cross section subjected to tension.

Based on cross-sectional shape beams are rectangular,

T, I, L in shape. Precast concrete beams or girders are

fabricated at a shop or yard in the same manner and

then transported to the job site. Based on geometry

beam may be straight, curved and tapered in cross-

section.

Columns: Members that are generally vertical and resist axial compressive loads are referred

to as columns. Tubes and wide-flange

cross sections are often used for metal

columns, and circular and square cross

sections with reinforcing rods are used for

those made of concrete. Occasionally,

columns are subjected to both an axial

load and a bending moment as shown in

the figure. These members are referred to

as beam columns.

Foundation: A structural element who takes the load from super structure and then transfer to

soil is known as footing. Foundation is that part of the structure which is in direct contact with

soil. Here, the part of the structure above ground level is called as the superstructure. E.g. Slab,

beam and column etc. where the part of the structure below the ground level is called as the

substructure. E.g. footing. Foundations are generally divided into two categories:

1. Shallow foundation

A shallow foundation is a type of foundation which transfers building loads to the earth very

near the surface, Shallow Foundations are provided when adequate SBC is available at

relatively short depth below ground level. In shallow foundation the ratio of Df /B < 1, where

Df is the depth of footing and B is the width of footing.


Types of shallow foundation: Isolated Footing

These are independent footings which are provided when

SBC (Soil Bearing Capacity) is generally

high

Columns are far apart

Loads on footings are less

The isolated footings can have different cross

section. Some of the popular shapes of footings

are;

Square

Rectangular

Combined footing

These are common footings which support the loads from are provided when:

SBC is generally less

Columns are closely spaced

Footings are heavily loaded

When exterior column came closer to property line in such cases footings cannot be

extended on one side. Here, the footings of exterior and interior columns are connected by

the combined footing. Combined footings essentially consist of a common slab for the

columns it is supporting. These slabs are generally rectangular in plan. Sometimes they can

also be trapezoidal in plan.


Strap Footing

An alternate way of providing combined footing located close to property line is the strap

footing strap footing, independent slabs below columns are provided which are then

connected by a strap beam. The strap beam does not remain in contact with the soil and

does not transfer any pressure to the soil.

Strip Footing

Strip footing is a continuous footing provided under columns or walls. Strip footing is

provided when more than 2 column comes in same line and their centre to centre spacing

is small.

Mat/Raft Foundation

Mat foundation covers the whole plan area of structure. The detailing is similar to two way

reinforced solid floor slabs or flat slabs. It is a combined footing that covers the entire area

beneath a structure and supports all the walls and columns. It is normally provided when

1. Soil pressure is low

2. Loads are very heavy

3. Spread footings cover > 50% area


2. Deep foundation

Deep Foundations are provided when adequate SBC is available at large depth below ground

level. In shallow foundation the ratio of Df/B ≥ 1, where Df is the depth of footing and B is the

width of footing. Some of the common types are listed below.

1. Pile Foundation

2. Pier Foundation

3. Well Foundation


Once the dimensional requirements for a structure have been defined, it becomes necessary to

determine the loads the structure must support. Once the structural form has been determined,

the actual design begins with those elements that are subjected to the primary loads the

structure is intended to carry, and proceeds in sequence

to the various supporting members until the foundation

is reached. Thus, a building floor slab would be designed

first, followed by the supporting beams, columns, and

last, the foundation footings. In order to design a

structure, it is therefore necessary to first specify the

loads that act on it. The design loading for a structure is

often specified in codes. In general, the structural

engineer works with two types of codes: general building

codes and design codes. General building codes specify

the requirements of governmental bodies for minimum

design loads on structures and minimum standards for

construction. Design codes provide detailed technical

standards and are used to establish the requirements for

the actual structural design. Since a structure is generally

subjected to several types of loads, a brief discussion of these loadings will now be presented

to illustrate how one must consider their effects in practice.

Dead Loads: Dead load is defined as the load which does not change its magnitude and

position. Dead loads consist of the weights of the various structural members and the weights

of any objects that are permanently attached to the structure. Hence, for a building, the dead

loads include the weights of the columns, beams, and girders, the floor slab, roofing, walls,

windows, plumbing, electrical fixtures, and other miscellaneous attachments. Once the materials

and sizes of the various components of the structure are determined, their weights can be found

from tables that list their densities.

Loads

Load

Roof Surface

Roof slab

Beams

Column

Foundation

Soil


Minimum Densities for Design Loads from Materials

Material 𝐥𝐛/𝐟𝐭𝟑 𝐊𝐍/𝐦𝟑

Aluminum

Copper

Iron

Lead

Steel, Rolled

Limestone, Marble

Sand stone

Shale or slate

Oil

Water

Ice

Brick

Cement, Portland, set

Cement, Portland, loose

Earth, dry, packed

Concrete, plain cinder

Concrete, plain stone

Concrete, reinforced cinder

Concrete, reinforced stone

Clay, dry

Clay, damp

Masonry, lightweight solid concrete

Masonry, normal weight

Plywood

Steel, cold-drawn

Wood, Douglas Fir

Wood, Southern Pine

Wood, spruce

170

556

450

710

490

165

147

175

58

62.4

56

120

183

90

95

108

144

111

150

63

110

105

135

36

492

34

37

29

26.7

87.4

70.8

111.6

77

25.9

23.1

27.5

9.12

9.81

8.8

18.9

28.7

14

14.9

17

22.6

17.4

23.6

9.9

17.3

16.5

21.2

5.7

77.3

5.3

5.8

4.5

Frame Partitions & Walls Psf KN/m2

Exterior stud walls with brick veneer 48 2.30

Windows, glass, frame and sash 8 0.38

Wood studs 2×4 in.(51×102mm) unplastered 4 0.19

Wood studs 2×4 in.(51×102mm) plastered one side 12 0.57

Wood studs 2×4 in.(51×102mm) plastered two sides 20 0.96

Floor Fills

Cinder concrete, per inch (mm) 9 0.017

Lightweight concrete, plain, per inch (mm) 8 0.015

Stone concrete, per inch (mm) 12 0.023


Ceilings

Acoustical fiberboard 1 0.05

Plaster on tile or concrete 5 0.24

Suspended metal lath and gypsum plaster 10 0.48

Asphalt shingles 2 0.10

Fiberboard 1/2in, (13 mm) 0.75 0.04

Live Loads: Live Loads can vary both in their magnitude and location. They may be caused by

the weights of objects temporarily placed on a structure, moving vehicles, or natural forces. The

minimum live loads specified in codes are determined from studying the history of their effects

on existing structures. Usually, these loads include additional protection against excessive

deflection or sudden overload. Various types of live loads can be found from table listed below.

Sr.

No Occupancy/Use

Live Load

Kgs/m2

N/m2

Lb/ft2

1 Private apartment, schools class room 200 1900 40

2 Office 250 to 425 2400 to 4000 50 to 85

3 Fixed-seats, assembly halls, library reading

rooms 300 2900 60

4 Corridors in public buildings 400 3800 80

5 Moveable seats assembly halls 500 4800 100

6 Wholesales, light storage, warehouses 610 6000 125

7 Library Stack room 730 7200 150

8

Heavy manufacturing. Heavy storage

warehouses, sidewalks and drive ways

subjected to trucking

1200 12000 250

9 Stairs, generals 500 4800 100

10 Stairs up to two-family residences, 50%

more than specifications 300 2900 60

Snow Loads: In some parts of the country, roof loading due to snow can be quite severe, and

therefore protection against possible failure is of primary concern. Design loadings typically

depend on the building’s general shape and roof geometry, wind exposure, location, its

importance, and whether or not it is heated. Like wind, snow loads in the ASCE 7-10 Standard

are generally determined from a zone map reporting 50-year recurrence intervals of an extreme

snow depth.

Specifications for snow loads are covered in the ASCE 7-10 Standard. If a roof is flat, defined as

having a slope of less than 5%, then the pressure loading on the roof can be obtained the

following empirical formula:

𝑃𝑓 = 0.7𝐶𝑒𝐶𝑡𝐼𝑠𝑝𝑔


Where:

𝐶𝑒= an exposure factor which depends upon the terrain. For example, for a fully

exposed roof in an unobstructed area, 𝐶𝑒=0.8

Whereas if the roof is sheltered and located in the center of a large city, then 𝐶𝑒=1.2

𝐶𝑡= a thermal factor which refers to the average temperature within the building.

For unheated structures kept below freezing 𝐶𝑡 = 1.2

Whereas if the roof is supporting a normally heated structure, then 𝐶𝑡 = 1

𝐼𝑠= the importance factor as it relates to occupancy. For example, for agriculture

and storage facilities 𝐼𝑠= 0.8, and for schools and hospitals 𝐼𝑠=1.2

If 𝑃𝑔 ≤ 20𝑙𝑏/𝑓𝑡2 (0.96 KN/𝑚2) then use the largest value for 𝑃𝑓 either computed from the above

equation or from 𝑃𝑓 = 𝐼𝑠𝑝𝑔.If 𝑃𝑔 > 20𝑙𝑏/𝑓𝑡2((0.96 KN/𝑚2) then use pf =𝐼𝑠 20𝑙𝑏/𝑓𝑡2

Wind Loads: When structures block the flow of wind, the wind’s kinetic energy is converted

into potential energy of pressure, which causes a wind loading. The effect of wind on a structure

depends upon the density and velocity of the air, the angle of incidence of the wind, the shape

and stiffness of the structure, and the roughness of its surface.

Highway Bridge Loads: The primary live loads on bridge spans are those due to traffic, and

the heaviest vehicle loading encountered is that caused

by a series of trucks. Specifications for truck loadings on

highway bridges are reported in the LRFD Bridge Design

Specifications of the American Association of State and

Highway Transportation Officials (AASHTO). For two-

axle trucks, these loads are designated with an H,

followed by the weight of the truck in tons and another number which gives the year of the

specifications in which the load was reported. H-series truck weights vary from 10 to 20 tons.

However, bridges located on major highways, which carry a great deal of traffic, are often

designed for two-axle trucks plus a one-axle semitrailer

as in Fig. These are designated as HS loadings. In

general, a truck loading selected for design depends

upon the type of bridge, its location, and the type of

traffic anticipated. The size of the “standard truck” and

the distribution of its weight is also reported in the

specifications. Although trucks are assumed to be on the road, all lanes on the bridge need not

be fully loaded with a row of trucks to obtain the critical load, since such a loading would be

highly improbable.


Impact Loads: Moving vehicles may bounce or side-sway as they move over a bridge, and

therefore they impart an impact to the deck. The percentage increase of the live loads due to

impact is called the impact factor, I. This factor is generally obtained from formulas developed

from experimental evidence. For example, for highway bridges the AASHTO specifications

require that:

I= 50

𝐿+125 𝑏𝑢𝑡 𝑛𝑜𝑡 𝑙𝑎𝑟𝑔𝑒𝑟 𝑡ℎ𝑎𝑛 0.3

Where L is the length of the span in feet that is subjected to the live load.

In some cases provisions for impact loading on the structure of a building must also be taken

into account. For example, the ASCE 7-10 Standard requires the weight of elevator machinery

to be increased by 100%, and the loads on any hangers used to support floors and balconies to

be increased by 33%.

Design Codes

Building Code Requirements for Reinforced Concrete, American. Conc. Inst. (ACI)

Manual of Steel Construction, American Institute of Steel Construction (AISC)

Standard Specifications for Highway Bridges, American Association of State Highway

and Transportation Officials (AASHTO)

National Design Specification for Wood Construction, American Forest and Paper

Association (AFPA)

Manual for Railway Engineering, American Railway Engineering Association (AREA)


Example 1.1: The floor of a heavy storage warehouse building is made of

6in.-thick concrete. The floor is a slab having a length of 10 ft and width of 8

ft. Determine the total factored concentrated load.

Solution:

Dead Load = 150 𝑙𝑏/𝑓𝑡3 × {(6"

12) × (8𝑓𝑡) × (10𝑓𝑡)}

=6000lb =6 kips

Live Load=250 𝑙𝑏/𝑓𝑡2 × (8𝑓𝑡) × (10𝑓𝑡)

=20000 lb=20 ki

Factored Load =1.2(6) + 1.6(20)

= 39.2 kips

Example 1.2: The “New Jersey” barrier is commonly used during highway

construction. Determine its weight per foot of length if it is made from plain

stone concrete.

Solution:

sin 55

12=

sin 35

𝑥⇒ 𝑥 = 8.4𝑖𝑛

sin 75

𝑥=

1.6

sin 15⇒ 𝑥 = 5.97𝑖𝑛

Cross-sectional Area

= (6×24)+(7.2×12) +2(1

2×8.4×12)

+(4×5.97)+ 2(1

2×1.6×5.97)=364.63 in2=2.53ft2

Load per unit length= 2.53×144=364.32 lb/ft

Exercise


Example 1.3: The floor beam is used to support the 6-ft width of a

lightweight plain concrete slab having a thickness of 4 in. The slab serves as

a portion of the ceiling for the floor below, and

therefore its bottom is coated with plaster.

Furthermore, an 8-ft-high, 12-in.-thick

lightweight solid concrete block wall is directly

over the top flange of the beam. Determine the

dead load on the beam measured per foot of

length of the beam.

Solution:

Dead load of lightweight plain Concrete Slab

= 8𝑙𝑏

𝑓𝑡2.𝑖𝑛× 4𝑖𝑛 × 6𝑓𝑡 =192lb/ft

Dead load of Plaster Ceiling = 5𝑙𝑏

𝑓𝑡2× 6𝑓𝑡 =30lb/ft

Lightweight Concrete Block Wall = 105𝑙𝑏

𝑓𝑡3 × 8𝑓𝑡 × 1𝑓𝑡=840lb/ft

Total dead Load on Beam= 192+30+840=1062 lb/ft

Example 1.4:

The unheated storage facility as shown is

located on flat open terrain in Murree,

where the specified ground snow load is

15lb/ft2. Determine the design snow load on

the roof which has a slope of 40.

Solution:

Since the roof slope is < 50, So Ce=0.8 due to

the open area, Ct = 1.2 and Is= 0.8.Thus,

𝑃𝑓 = 0.7𝐶𝑒𝐶𝑡𝐼𝑠𝑝𝑔

=0.7(0.8)(1.2)(0.8)(15lb/ft2)= 8.06 lb/ft2

Since 𝑝𝑔=15lb/ft2<20lb/ft2, then also

𝑃𝑓 = Is𝑝𝑔=1.2(15lb/ft2) =18lb/ft2

By comparison choose 𝑃𝑓 =18lb/ft2


Example 1.5:

An unheated horse stall has a flat roof with a slop of 80mm/m. it is located

in open field where the ground snow load is 0.72 KN/m2. Determine the snow

load that is requires to design the roof of the stall.

Solution:

𝜃 = tan−1(80

1000) = 4.57° < 5°

Ce=0.8

Ct = 1.2

Is= 1.2

𝑃𝑓 = 𝐶𝑒𝐶𝑡𝐼𝑠𝑝𝑔

=0.7(0.8)(1.2)(1.2)(0.72 KN/m2)

=0.81KN/m2

Since 𝑝𝑔=0.72 KN/m2<0.96 KN/m2,

Then also 𝑃𝑓 = Is𝑝𝑔=1.2(0.72 KN/m2) = 0.576 KN/m2

By comparison choose 𝑃𝑓 =0.81 KN/m2


Example 1.6:

The slab of a library room is supported by beams AB, CD and DE as shown.

Each beam is 15ft long and are spaced 7ft on centers. The slab is made from

normal weight reinforced concrete that is 6in thick. Calculate the factored

dead load per ft of length on edge and center beams.

Solution:

The dead load on the floor is due to the weight of the concrete slab. Since 𝐿𝑦

𝐿𝑥 =

15

7 = 2.14 it is a one way slab and it delivers its load to the supporting

members by one-way action).

Factored dead load on edge beam= 1.2(150×3.5ft×0.5ft) =262.5 lb/ft

Factored dead load on center beam= 1.2(150×7ft×0.5ft) =525 lb/ft

Center Beam Edge Beam

15ft

7 ft

7 ft

A

C

E

B

D

F

15ft

7 ft

3.5

ft 3

.5 ft

15ft

7 ft

3.5

ft

525 lb/ft 262.5 lb/ft

A B C D


Example 1.7:

The floor of a classroom is to be supported by the bar joists shown in Fig.

Each joist is 15ft long and they are spaced 2.5ft on centers. The floor itself is

to be made from lightweight concrete that is

4 in. thick. Neglect the weight of the joists and

the corrugated metal deck, and determine the

load that acts along each joist.

Solution:

The dead load on the floor is due to the

weight of the concrete slab. Since Ly/Lx >2 the concrete slab is treated as a

one-way slab.

Load due to light weight concrete slab=8𝑙𝑏

𝑓𝑡2.𝑖𝑛× 4𝑖𝑛 × 2.5𝑓𝑡 =80lb/ft

The live load for a classroom is 40 lb/ft2

Live load acting on the joist=40 lb/ft2× 2.5𝑓𝑡=100 lb/ft

Therefore the uniform load along on joist=w=80+100=180 lb/ft


Example 1.8:

Consider the framing system in which a reinforced concrete slab having a

thickness of 6in is placed on the beams AB, CD, and EF and these beams are

rest on the girders AE and BF as shown. The length of the beam is 10ft and

these are placed 5ft center to center. Determine the load per ft of length on

beams and concentrated load on each column A, B, E and F.

Solution:

Since Ly/Lx ≥2 the concrete slab is treated as a one-way slab.

Density of the concrete=150 lb/ft3

Load on the beam AB and EF= 150× 2.5 ×6

12 =187.5 lb/ft

Load on the beam CD= 150× 5 ×6

12 =375 lb/ft

Load on the girder AE and BF from beam AB= (187.5×10)/2=937.5lb

Load on the girder AE and BF from beam EF= (187.5×10)/2=937.5lb

Load on the girder AE and BF from beam CD= (375×10)/2=1875lb

375 lb/ft 187.5 lb/ft

A B C D

10ft 10ft

1875

lb 1875 937.5 937.5


Load on the column=937.5+(1875/2)=1875lb

Example 1.9:

A concrete slab is supported by four 10-ft-long edge beams, AB, BD, DC, and

CA as shown in fig. if the thickness of the slab is 8in then calculate the load

per ft length on the beam AB.

Solution:

Since Ly/Lx <2 the concrete slab is treated as a two-way slab. Due to two-

way slab action, the assumed tributary area for beam AB is determined by

constructing diagonal 45° lines.

Density of the concrete=150 lb/ft3.

Load on the beam AB= 150× 5 ×8

12 =500 lb/ft

A E

937.5lb 1875lb 937.5lb

5ft 5ft 1875lb 1875lb

1875lb


Example 1.10:

A slab of length 15ft and width 10ft is supported by four edge beams, AB, BD,

DC, and CA as shown in fig. If the slab is made from reinforced concrete having

a thickness of 8in then calculate the load per ft length on the beam AB and AC.

Solution:

As Ly/Lx <2 the concrete slab is treated as a two-way slab. Density of the

concrete=150 lb/ft3

Load on the beam AB= 150× 5 ×8

12 =500 lb/ft

Load on the beam AC= 150× 5 ×8

12 =500 lb/ft

Example 1.11:

The flat roof of the steel-frame building shown in the photo is intended to

support a total load of 2KN/m2 over its surface. Determine the roof load that is

transmitted to beam BC.

Solution:

In this case Lx=4m and Ly=5m. Since

Ly/Lx=1.25<2 we have two-way slab

action. Load on surface of the slab=

2KN/m2

Load on the beam BC

= (2KN/m2× 2)+(2KN/m2× 2) =8KN/m

8KN/m

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