steel sections and load combinations

8/13/2019 Steel sections and load combinations

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LOAD FACTORS AND LOADCOMBINATION

It is impossible that all loads like liveload, wind load and earthquake alloccur together with their maximum

intensity. A load combination combinesdifferent types of loads depending onthe probability of occurrence of these

loads, considering their expectedintensity in the combination comparedwith the maximum load intensity.

1



The factors of safety are also included

in the LRFD load combinations and

hence the output of the expressions is

a design load.

The alphabets used in the

combinations mean different types of

nominal service loads and thenumerical values with them are all the

load factors.2



When intermediate floors have

full live loads, any type of roof

load may be considered equal to

half of its normal service loadintensity.

Similarly, in case of maximum

intensity wind storm, live load

may be half.3



The last combination, given afterwards,

is very important for uplift of structureor reversal of forces.

The wind load on roof is upwards inmajority of the cases and if the

downward gravity load is less, thestructure may be blown up or saggingbending may change into hoggingbending.

A list of most commonly usedcombinations are as under:

4



LRFD Load Combination

1. 1.4 (D+F)2. 1.2 (D+F+T) + 1.6(L+H) +0.5(Lr or S

or R)

3. 1.2D + 1.6(Lr or S or R) + (L or 0.8W)4. 1.2D + 1.6W + 1.0L +0.5(Lr or S or R)

5. 1.2D + 1.0E + 1.0L + 0.2S

6. 0.9D + 1.6W + 1.6H

7. 0.9D + 1.0E + 1.6H5



LRFD Load Combination D

dead load L live load Lr roof live load W wind load S snow load

E earthquake load R rainwater or ice load H load due to lateral earth pressure,

ground water pressure or pressureof bulk materials

F load due to fluids with well definedpressures and max heights.

T self-retaining force6



LRFD Load Combination

Study the remaining discussion by yourself.

7



ASD Load Combination

1. D + F

2. D + H + F + L + T

3. D + H +F + (Lr or S or R)

4. D + H +F + 0.75(L + T) + 0.75 (Lr or S or R)

5. D + H +F + (W or 0.7E)

6. D + H +F + 0.75(W or 0.7E) + 0.75L +

0.75(Lr or S or R)7. 0.6D +W +H

8. 0.6D + 0.7E + H8



LRFD

1. 1.4D

2. 1.2D + 1.6L + 0.5Lr

3. 1.2D + 1.6Lr + (L or 0.8W)

4. 1.2D + 1.3W + 1.0L + 0.5Lr

5. 0.9D + 1.3W

10



ASD

1. D

2. D + L

3. D + Lr

4. D + 0.75L + 0.75Lr

5. D + 0.8W

6. D + 0.6W + 0.75L + 0.75Lr

7. 0.6D + 0.8W11



Live Load Reduction

The intensity of live load may be

reduced if the contributory area for

the live load exceeds certain limit.

It is due to the fact that, under these

circumstances, all the area may notbe subjected to the full load.

12



All uniform live loads, except the roof

loads (for which separate provisionsare given in ASCE-07), may be

reduced as follows:

Where

Lo = the unreduced live load

AT = tributary area in m2

K LL = live load element factor13

T LL A K

L L 57.425.0



a Interior columns and exterior

columns without cantilever

slabs.

K LL

= 4

b Edge columns with cantilever

slabs.

K LL

= 3

c Corner columns withcantilever slabs, edge beams

without cantilever slabs and

interior beams.

K LL = 2

d All other members including

slabs.

K LL

= 1

14



TYPES OF STRUCTURAL STEEL

Steels are divided into four

categories depending on the

carbon percentages (C) asfollowing:

1. Low carbon steel C < 0.15%

2. Mild carbon steel C = 0.15 - 0.29%3. Medium carbon steel C = 0.30 – 0.59%

4. High carbon steel C = 0.60 – 1.70%15



E-Value of steel = 185 GPa to 230 GPa

(Average 200 GPa)

Unit weight = 7850 kg/m3

= 77 kN/m3

= 7.85 g/cc

For comparison, the unit weight of

concrete is 23.6 kN/m3.

16



Most of the structural steel fallsinto the mild carbon steel orsimply mild steel (MS) category.

Hot rolled structural shapes may

be made to conform to A36M, A529M, A572M, A588M, A709M,

A913M and A992M.

17



Sheets are manufactured according

to the standards ASTM A606, A1011MSS, HSLAS and HSLAS-F.

Bolts are made according to ASTM

standards A307, A325M, A449,

A40M and F1852.

18



Most commonly used structural

steel is A36M having the

following properties:Fy = 250 MPa

Fu = 400 MPa

E = 200 GPa19



Weld Electrode And Filler Material

Weld electrodes are classified as E60,

E70, E80, E100 and E110.

The letter E denotes electrodes.

The two digits indicate the ultimate

tensile strength in ksi. The

corresponding SI equivalents are

E425, E495, E550, E690 and E760.20



HOT ROLLED STRUCTURAL SHAPES

These are the steel cross-sectionalshapes that are hot rolled in themills. Some of these shapes areshown in next slides, whereas, the

steel bars, plates and hollowsections are reproduced in slidesnext to the above slides.

An HP h x w is a bearing pile section,which is approximately h mm deepweighing w kgs/m.

21



HOT ROLLED STRUCTURAL SHAPES

Bearing piles are made with theregular W rolls but with thicker web

to provide better resistance to the

impact of pile driving.

HSS are hollow structural sections

that are prismatic square,

rectangular or round products of a

pipe or tubing.22



23



HOT ROLLED STRUCTURALSHAPES

Every hot rolled shape has its

unique standard designation, which

not only tells about the type of

cross-sectional shape but also

about its size.

The details about some of thecommon hot rolled shapes are

given in the next slides.24



25

Slope ≈ 0°

W-Section



26

16.7% Slope

S-Section



27Angle-Section



28Channel-Section

16.7% Slope



29Tee-Section



30HP-Section

Thicker than flange



31Pipe Section



32Structural Tubing



33Bars



34Plates



1. W-Shapes

The letter „W‟ stands for an I-shapewith wide flange. The cross-sectionis doubly symmetric in the form ofthe letter “I”. The width / depth ratio

varies from about 0.3 to 1.0.The US Customary designation W16 x 40 means that the nominaldepth of the section is 16 in andthe weight per unit length of thesection is 40 lbs/ft.

35



1. W-Shapes

Nominal height is the rounded

off height to be used for common

use.

Actual depth of the section may

be in decimals and somewhatdifferent from this depth.

36



The equivalent SI designation

W410 x 60 means that the W-

section has a nominal depth of 410

mm and a weight of 60 kgf/m.37

Less or no slope Flange

Web



This kilogram-force weight per

unit length may be converted in

kN/m by multiplying it with the

factor 9.81/1000.

38



2. S-Shapes

oDoubly symmetrical I-shapes.

oPreviously called standard I-beams

or American Standard Beam. 39

16.7% Slope



2. S-Shapes

oThe inner edge of the flange has aslope of approximately 16.7%.

o An S510 x 112 section means thatthe section is S-shape havingnominal depth of 510 mm andweight of 112 kgf/m.

40

16.7 % Slope



2. S-Shapes

oThe width / depth ratio varies

from about 0.25 to 0.85.

41

16.7 % Slope



3. M-Shapes

oMiscellaneous I-shapes.

oDoubly symmetrical I-shapes notclassified as W or S shapes.

oRelatively lightweight used forsmaller spans and lesser loads.

o An M310 x 17.6 means that it is M-shape section having nominal depthof 310 mm and weight of 17.6 kgf/m.

42



4. C-Shapes

The C-shapes have the followingdistinguishing features:

oChannel shapes with standardproportions.

o Inner flange slope is 16.7%.43

16.7 % Slope



4. C-Shapes


O Previously called Standard or

American Standard Channels. 44

16.7 % Slope



4. C-Shapes


o A C150 x 19.3 is a standard channelshape with a nominal depth of150mm and a weight of 19.3 kgf/m.

45

16.7 % Slope



5. MC-Shapes

These sections have thefollowing properties:

oChannels not classified as C-shapes.

oPreviously called Shipbuilding or

Miscellaneous Channels.

46



6. L-Shapes or Angle Sections

The various types of anglesections are shown in Figure andtheir salient features are givenbelow:

O The single angle sections are inthe form of letter „L‟. 47

a

b




The various types of anglesections are shown in Figure andtheir salient features are givenbelow:

O If a = b, these are called equalangle sections.

48

a

b




The various types of angle sectionsare shown in Figure and their salientfeatures are given below:

o If a ≠ b, these are called unequalangle sections.

49

a

b




oSides of the angle are called „legs‟ or „arms‟.

oL89 x 76 x 12.7 is an unequal legangle with longer leg dimension of

89mm and shorter leg dimension of76mm with a leg thickness of12.7mm.

50

a

b




oDouble angle sections arecombination of two angles withlonger or shorter sides close to eachother.

oDouble angle sections are denotedby 2Ls.

51

a

b




o2L89 x 76 x 12.7 means two

angles L89 x 76 x 12.7 placed

side by side in one of the waysshown in the figure.

52

a

b



7. T-Shapes

oThese are called structural tees.

oThese are obtained by splitting W, S

or M shapes and are called WT, ST,or MT shapes, respectively.

53



7. T-Shapes

o A WT205 x 30 is a structural tee with

a nominal depth of 205mm and a

weight of 30kgf/m and is obtained by

splitting the W410 x 60 section.54



COLD – FORMED SHAPES

These sections are formed fromthin high strength steel alloy

plates under normal

temperature.

Some of the common shapes of

these sections are drawn inFigure:

55



56Channels



57Zees



58

I-Shaped Double Channels



59 Angle



60Hat Sections



BUILT-UP SECTIONS

Sections made by combining two ormore standard hot rolled sections,

joined together at intervals with the

help of direct welding, stay plates orlacing, are called built-up sections.

Examples are four angles section,

double angle section and double

channel section shown in Figure.61



BUILT-UP SECTIONS

However, double angle sectionis sometimes excluded from

built-up section category and isconsidered as a regular hot

rolled member because of

difference of its behavior from

other built-up sections. 62



63

4-Angle Box Section



64

Double Angle



65

Two Channels connected back-to-back



66

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steel sections and load combinations

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