NS21-1steel

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ARCH 631 Note Set 21.1 F2010abn

2

m = edge distance for a column baseplate

M = internal bending moment

M max = maximum internal bending momentM max-adj = maximum bending moment

adjusted to include self weightM n = nominal flexure strength with thefull section at the yield stress for

LRFD beam design

M p = internal bending moment when all

fibers in a cross section reach theyield stress

M u = maximum moment from factored

loads for LRFD beam designM y = internal bending moment when the

extreme fibers in a cross section

reach the yield stressn = edge distance for a column base

plate

n’ = column base plate design value

n.a. = shorthand for neutral axis N = bearing length on a wide flange

steel section

= depth of a column base plate P = name for load or axial force vector

P n = nominal column load capacity inLRFD steel design

P p

= nominal bearing capacity of

concrete under base plate P u = factored column load calculated

from load factors in LRFD steel

designr = radius of gyration

R = generic load quantity (force, shear,

moment, etc.) for LRFD design

= shorthand for rain or ice load Rn = nominal value (capacity) to be

multiplied by φ S = shorthand for snow load

= section modulus

S req’d = section modulus required atallowable stress

S req’d-adj = section modulus required at

allowable stress when moment isadjusted to include self weight

t f = thickness of flange of wide flanget p = thickness of column base plate

t w = thickness of web of wide flange

T = torque (axial moment)= shorthand for thermal load

V = internal shear forceV max = maximum internal shear forceV max-adj = maximum internal shear force

adjusted to include self weight

V n = nominal shear strength capacity for

LRFD beam designV u = maximum shear from factored loads

for LRFD beam design

wequivalent = the equivalent distributed loadderived from the maximum bending

moment

w self wt = name for distributed load from self weight of member

W = shorthand for wind load

X = column base plate design value

Z = plastic section modulus of a steelbeam

Δ actual = actual beam deflectionΔ allowable = allowable beam deflectionΔ limit = allowable beam deflection limit

Δ max = maximum beam deflection

yε = yield strain (no units)

φ = resistance factor

bφ = resistance factor for bending for

LRFD

cφ = resistance factor for compression

for LRFD

vφ = resistance factor for shear for

LRFD

λ ’ = column base plate design value

c λ = design constant for slenderness

evaluation for steel columns in

LRFD designγ = load factor in LRFD design

π = pi (3.1415 radians or 180°)

ρ = radial distance

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Steel Design

Structural design standards for steel are established by the Manual of Steel Construction

published by the American Institute of Steel Construction, and uses ASD and LRFD design.

Materials

American Society for Testing Materials (ASTM) is the organization responsible for material and

other standards related to manufacturing. Materials meeting their standards are guaranteed to

have the published strength and material properties for a designation.

A36 – carbon steel used for plates, angles Fy = 36 ksi, Fu = 58 ksi

A572 – high strength low-alloy used for some beams Fy = 60 ksi, Fu = 75 ksi

A992 – for building framing used for most beams Fy = 50 ksi, Fu = 65 ksi

ASD

Factors of Safety are applied to the limit stresses for allowable stress values:

bending (braced) Fb = 0.66Fy

bending (unbraced, Lc < L < Lu) Fb = 0.60Fy

shear Fv = 0.40Fy shear (bolts) tabulated

shear (welds) Fv = 0.30Fweld

- Lc is the maximum unbraced length of the compression flange at which the allowablebending stress may be taken at 0.66Fy or “other” equations if applicable.

- Lu is the maximum unbraced length of the compression flange for which the allowable

bending stress may be taken at 0.60 Fy when Cb = 1, which means that the bending moment

at any point within an unbraced length is larger than that at both ends of the length.

LRFD

where γ = load factor for the type of load

R = load (dead or live; force, moment or stress)φ = resistance factorRn = nominal load (ultimate capacity; force, moment or stress)

Nominal strength is defined as the

capacity of a structure or component to resist the effects of loads, as determined by

computations using specified material strengths (such as yield strength, Fy, or ultimate

strength, Fu) and dimensions and formulas derived from accepted principles of structural

mechanics or by field tests or laboratory tests of scaled models, allowing for modelingeffects and differences between laboratory and field conditions

nii R R φ γ ≤Σ

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Factored Load Combinations

The design strength, n Rφ , of each structural element or structural assembly must equal or exceed

the design strength based on the ASCE-7 combinations of factored nominal loads:

1.4( D + F) 1.2( D + F ) + 1.6( L + H) + 0.5( Lr or S or R)1.2 D + 1.6( Lr or S or R) + ( L or 0.8W )

1.2 D + 1.6W + L + 0.5( Lr or S or R)

1.2 D + 1.0 E + L + 0.2S

0.9 D + 1.6W + 1.6 H0.9 D + 1.0 E + 1.6 H

Criteria for Design of Beams

Allowable normal stress or normal stress from LRFD should not be exceeded:

Knowing M and Fb, the minimum section modulus fitting the limit is:

Besides strength, we also need to be concerned about serviceability. This involves things like

limiting deflections & cracking, controlling noise and vibrations, preventing excessive

settlements of foundations and durability. When we know about a beam section and its material,we can determine beam deformations.

Determining Maximum Bending Moment

Drawing V and M diagrams will show us the maximum values for design. Computer

applications are very helpful.

Determining Maximum Bending Stress

For a prismatic member (constant cross section), the maximum normal stress will occur at the

maximum moment.

For a non-prismatic member, the stress varies with the cross section AND the moment.

Deflections

Elastic curve equations can be found in handbooks, textbooks, design manuals, etc...Computer

programs can be used as well.

Elastic curve equations can be superpositioned ONLY if the stresses are in the elastic range.

The deflected shape is roughly the same shape flipped as the bending moment diagram but isconstrained by supports and geometry.

b

d req F

M S ≥'

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Allowable Deflection Limits

All building codes and design codes limit deflection for beam types and damage that could

happen based on service condition and severity.

Use LL only DL+LLRoof beams:

Industrial L/180 L/120

Commercial

plaster ceiling L/240 L/180

no plaster L/360 L/240

Floor beams:

Ordinary Usage L/360 L/240

Roof or floor (damageable elements) L/480

Lateral Buckling

With compression stresses in the top of a beam, a sudden “popping” or buckling can happen

even at low stresses. In order to prevent it, we need to brace it along the top, or laterally brace it,or provide a bigger Iy.

Local Buckling in Steel I Beams– Web Crippling or

Flange Buckling

Concentrated forces on a steel beam can cause the web tobuckle (called web crippling). Web stiffeners under the

beam loads and bearing plates at the supports reduce that

tendency. Web stiffeners also prevent the web from

shearing in plate girders.

The maximum support load and interior load can be determined from:

where t w = thickness of the web

N = bearing length

k = dimension to fillet found in beam section tables

[ ]k N t F P w yend max 5.2)66.0( +=−

[ ]k N t F P w yinterior max 5)66.0( +=−

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6

E

1

fy = 50ksi

εy = 0.001724

f

ε

Beam Loads & Load Tracing

In order to determine the loads on a beam (or girder, joist, column, frame, foundation...) we can

start at the top of a structure and determine the tributary area that a load acts over and the beamneeds to support. Loads come from material weights, people, and the environment. This area is

assumed to be from half the distance to the next beam over to halfway to the next beam.

The reactions must be supported by the next lower structural element ad infinitum, to the ground.

LRFD Bending or Flexure

For determining the flexural design strength, nbM φ , for resistance to pure bending (no axial

load) in most flexural members where the following conditions exist, a single calculation willsuffice:

where Mu = maximum moment from factored loads

φb = resistance factor for bending = 0.9

Mn = nominal moment (ultimate capacity)

Fy = yield strength of the steelZ = plastic section modulus

Plastic Section Modulus

Plastic behavior is characterized by a yield point and anincrease in strain with no increase in stress.

Internal Moments and Plastic Hinges

Plastic hinges can develop when all of the material in a cross section sees the yield stress.Because all the material at that section can strain without any additional load, the member

segments on either side of the hinge can rotate, possibly causing instability.

For a rectangular section:

Elastic to f y:

Fully Plastic:

Z F M M R ynbuii 9.0=≤=Σ φ γ

( ) y y y y y f

bc f

cb f

bh f

c

I M

3

2

6

2

6

222

====

y y pult M f bcM or M 2

32 ==

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8

LRFD - Shear

The formulas for the determination of the shear strength on a section are too complex for routine

use with the variety of shapes available or possible for steel members. For members that possessan axis of symmetry in the plane of loading, and where web stiffeners are not required, two

simplifying assumptions that result in a negligible loss of (theoretical) accuracy are permitted:

1. The contribution of the flanges to shear capacity may be neglected.

2. y

w F

t h 418≤ where h equals the clear distance between flanges less the fillet or

corner radius for rolled shapes.

With these assumptions, the calculated strength becomes simple. Neglecting the flanges, all

symmetrical rolled shapes, box shapes, and built-up sections reduce to an equivalent rectangular

section with dimensions d t wΣ and shear strength becomes nvV φ :

where Vu = maximum shear from factored loads

φv = resistance factor for shear = 0.9Vn = nominal shear (ultimate capacity)

Fyw = yield strength of the steel in the web

Aw = twd = area of the web

LRFD - Design for Flexure

The nominal flexural strength Mn is the lowest value obtained according to the limit states of

1. yielding

2. lateral-torsional buckling

3. flange local buckling

4. web local buckling

Beam design charts show nb M φ for unbraced length, Lb , of the compression flange in one-foot

increments from 1 to 50 ft. for values of the bending coefficient Cb = 1. For values of 1<Cb≤2.3,

the required flexural strength Mu can be reduced by dividing it by Cb. (Cb = 1 when the bendingmoment at any point within an unbraced length is larger than that at both ends of the length. C b

of 1 is conservative and permitted to be used in any case. When the free end is unbraced in a

cantilever or overhang, Cb = 1.)

NOTE: the self weight is not included in determination of nbM φ

)6.0(9.0 w ywnvuii A F V V R =≤=Σ φ γ

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For a laterally braced compact section (one for which the plastic moment can be reached beforelocal buckling) only the limit stat of yielding is applicable. For unbraced compact beams and

non-compact tees and double angles, only the limit states of yielding and lateral-torsional

buckling are applicable.

Compact sections meet the following criteria: y f

f

F t b 652

≤ and yw

c

F t h 640≤

where:

b f = flange width in inches

t f = flange thickness in inches

F y = minimum yield stress in ksi

hc = height of the web in inches

t w = web thickness in inches

With lateral-torsional buckling the nominal flexural strength is

where Cb is a modification factor for non-uniform moment diagramswhere, when both ends of the beam segment are braced:

Mmax = absolute value of the maximum moment in the unbraced beam segment

MA = absolute value of the moment at the quarter point of the unbraced beam segmentMB = absolute value of the moment at the center point of the unbraced beam segment

MC = absolute value of the moment at the three quarter point of the unbraced beam

segment length.

Beam design charts show nb M φ for unbraced length of the compression flange in one-foot

increments from 1 to 50 ft. for values of the bending coefficient Cb = 1. For values of 1<Cb≤2.3,

the required flexural strength Mu can be reduced by dividing it by Cb.

ASD – Bending or Flexure

Allowable stresses are reduced when the unbraced length of the compression flange can bucklecalled Lc. The limiting unbraced length at the lower stresses is called Lu. The maximum

moment that can be applied (taking self weight into account) can be plotted against the unbraced

length. The limit Lc is indicated by a solid dot (•), while Lu is indicated by an open dot ().

Solid lines indicate the most economical, while dashed lines indicate there is a lighter sectionthat could be used. Cb, which is a modification factor for non-zero moments at the ends, is 1 for

simply supported beams (0 moments at the ends). (see figure)

[ ] pbn M s Land sM onbased t tanconsC M ≤= '' )( nbu M M φ ≤

C B A

bM M M M

M C

3425.2

5.12

max

max

+++=

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d t

V

A

V

A

V f

wweb

v =≈=−2

3max

Ib

VQ f maxv =−

Design Procedure

The intent is to find the most light weight member satisfying the section modulus size.

1. Know Fall for the material or FU for LRFD.2. Draw V & M, finding Mmax.

3. Calculate Sreq’d or Z. This step is equivalent to determiningb

maxb F

S

M f ≤= , and

yb

u

F

M Z

φ ≥

to meet the LRFD design criteria that nbu M M φ ≤ .

4. For steel: use the section charts to find S or Z that will work and remember that the beam self weight will increase S req’d. The design charts show the lightest section within a grouping of

similar S’s.

****Determine the “updated” V max

and M max

including the beam self weight, and verify that theupdated S req’d has been met.******

5. Consider lateral stability.

6. Evaluate horizontal shear stresses using Vmax to determine if vv F f ≤

For I beams:

Others:

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7. Provide adequate bearing area at supports:

8. Evaluate shear due to torsion vv F abc

T or

J

T f ≤=

2

1

ρ

(circular section or rectangular)

9. Evaluate the deflection to determine if allowed LL LLmax −≤ ΔΔ and/or lallowed TotaTotal −Δ≤Δ max

**** note: when Δcalculated > Δlimit , I required can be found with:

and S req’d will be satisfied for similar self weight *****

FOR ANY EVALUATION:

Redesign (with a new section) at any point that a stress or serviceability criteria is

NOT satisfied and re-evaluate each condition until it is satisfactory.

Open-web Joists and Girders

The Steel Joist Institute provides charts listing allowable distributed loads for members based on

strength AND deflection limits. Live load deflections can dominate, and the distributed loads arelisted typically in RED below the total distributed loads in regular text.

If the load is not uniform, an equivalent uniform load can be calculated

from the maximum moment equation:

If the deflection limit is less, the design live load to

check against allowable must be increased, ex.

Criteria for Design of Columns

If we know the loads, we can select a section that is adequate for strength & buckling.

If we know the length, we can find the limiting load satisfying strength & buckling.

ASD

American Institute of Steel Construction (AISC)Manual of ASD, 9th ed:

Long and slender: [ Le /r ≥ Cc, preferably < 200]

( )2

2

23

12

..r

Kl

E

S F

F F cr

allowable

π ==

The yield limit is idealized into a parabolic curve that blends into the Euler’s Formula at C c.

With Fy = 36 ksi, Cc = 126.1With Fy = 50 ksi, Cc = 107.0

y

c F

E C

22π =

p p F A

P f ≤=

trial

it lim

big too

d ' req I I Δ

Δ≥

⎟ ⎠

⎞⎜⎝

⎛ = −400 /

360 /

L

Lww havell adjusted

table limit

wanted

8

2

max

LwM

equivalent =

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Short and stubby: [Le /r < Cc]

( )..2

12

2

S F

F

C

r Kl

F y

c

a

⎥⎥⎥

⎦

⎤

⎢⎢⎢

⎣

⎡−=

with:

( ) ( )3

3

88

3

3

5..

cc C

r Kl

C

r Kl

S F −+=

Procedure

1. Calculate Cc

2. Determine limiting criteria:

strength: Le /r < Cc buckling: Le /r ≥ Cc

(at Le /r = Cc, the allowable stress values are the same for both equations)

3. Select a section

If the load and area of the column are known, set the stress equal to the allowablestress, and solve for Le & L

If the load and length of the column are known, set the stress equal to the allowable stress,

and solve for r or A or I and select a section that satisfies the values found.

4. Continue from 3 until Fa is satisfied

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The critical load with respect to the slenderness ratio is presented in chart format in ASD, 8th

ed,as well as the allowable stress charts for compression members.

LRFD

For compression, φc = 0.85 and Pn = AgFcr nu P P φ ≤

where :

Ag is the cross section area andFcr is the critical stress shown below (in Compact Sections).

Compact Sections

Compact sections are defined as sections with flanges continuously connected to the web or

webs and the width-thickness rations are less than limiting values given in the manual. This is toavoid local buckling of the flange or the web.

Formula parts depend on E

F

r

Kl y

cπ

λ = wherer

L

r

Kl e= ,

5.1≤cλ : 5.1>cλ :

( ) y

λ

cr F . F c2

6580= y

c

cr F F ⎥⎦

⎤⎢⎣

⎡=

2

877.0

λ

where Fcr is the critical stress

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Columns with Bending (Beam-Columns)

In order to design an adequate section for allowable stress, we have to start somewhere:

1. Make assumptions about the limiting stress from:- buckling

- axial stress- combined stress

2. See if we can find values for r or A or S (=I/cmax)

3. Pick a trial section based on if we think r or A is going to govern the section size.

4. Analyze the stresses and compare to allowable using the allowable stress method or

interaction formula for eccentric columns.

5. Did the section pass the stress test?

- If not, do you increase r or A or S?- If so, is the difference really big so that you could decrease r or A or S to make it

more efficient (economical)?

6. Change the section choice and go back to step 4. Repeat until the section meets the

stress criteria.

ASD:

The modification factors are included in the form: 0.1

11

≤

⎟⎟

⎠

⎞

⎜⎜⎝

⎛

′−

+

⎟⎟

⎠

⎞

⎜⎜⎝

⎛

′−

+

byey

a

bymy

bxex

a

bxmx

a

a

F F

f

f C

F F

f

f C

F

f

where:

e

a

F

f

′−1 = magnification factor accounting for P-Δ

( )2

2

23

12

r Kl

E F e

π =′ = allowable buckling stress

Cm = modification factor accounting for end conditions,

= 0.6 – 0.4 (M1 /M2) where M1 and M2 are the end moments and M1<M2.

M1 /M2 is positive when the member is bent in reverse curvature, negativewhen bent in single curvature

= 0.85, members in frames subject to joint translation (sidesway)

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LRFD:

The modification factors are included in the form for two conditions.

For 2.0≥nc

u

P

P

φ : 0.19

8

≤⎟⎟

⎠

⎞

⎜⎜⎝

⎛

++ nyb

uy

nxb

ux

nc

u

M

M

M

M

P

P

φ φ φ

For 2.0<nc

u

P

P

φ : 0.1

2≤⎟

⎟ ⎠

⎞⎜⎜⎝

⎛ ++

nyb

uy

nxb

ux

nc

u

M

M

M

M

P

P

φ φ φ

where:

cφ = 0.85 for compression

bφ = 0.90 for flexure

Criteria for Design of Connections and Tension Members

Refer to the specific note set.

Criteria for Design of Column Base Plates

Column base plates are designed for bearing on the concrete(concrete capacity) and flexure because the column “punches”

down the plate and it could bend upward near the edges of thecolumn (shown as 0.8b f and 0.95d ). The plate dimensions are Band N and are preferably in full inches with thicknesses in

multiples of 1/8 inches.

LRFD required thickness:BNF.

Pt

y

up

90

2l=

where l is the larger of m, n and n′λ

m =2

95.0 d N − n =

2

8.0 f b B −

4

f dbn =′ 1

)11(

2≤

−+=

X

X λ

where X depends on the concrete bearing capacity of φ c P p , with

φ c = 0.65 and Pp = 0.85 f ’cA

BN f

P

bd

db

P

P

bd

db X

cc

u

f

f

pc

u

f

f

)85.0()(

4

)(

422 ′

⋅+

=⋅+

=φ φ

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Beam Design Flow Chart

isΔmax ≤ Δlimits?This may beboth the limit for live loaddeflection and total load deflection.)

Collect data: Fb & Fv

Allowable Stress or LRFD Design?

ASD LRFD

Collect data: L, ω, γ, Δlimits; find beam charts

for load cases and Δactual equations

Collect data: load factors, Fy, Fu, and equations for shear

ca acit with v

Find Vmax& Mmaxfromconstructing diagrams or

using beam chart formulas Find Vu & Mu fromconstructing diagrams or using beam chart formulas

with the factored loads

Find Sreq’d and pick a sectionfrom a table with Sx greater or

equal to Sreq’d

Determineωself wt (last number in

name) or calculate ωself wt. using Afound and 490 lb/ft3. Find Mmax-adj

Calculate Sreq’d-adj using Mmax-adj.

Is Sx(picked) ≥ Sreq’d-adj?

(OR calculate f b. Is f b≤ Fb?)

Yes

Pick a steel section from a chart

having φbMn ≥ Mu for the knownunbraced len th

Is Vu ≤ φv(0.6FywebAweb)

Yes

Calculate Areq’d-adj using Vmax-adj.

Is A(picked) ≥ Areq’d-adj?

(OR calculate f v. Is f v≤ Fv?)

Nopick a new section with alarger area

Nopick a sectionwith a larger web area

CalculateΔmax (no load factors!)using superpositioning and beamchart equations with the Ix for the

section

Nopick a section with a larger Ix

Yes (DONE)

No

Determineωself wt (last number in

name) or calculate ωself wt. using Afound and 490 lb/ft3. Factor.

Find Mmax-adj & Vmax-adj.

trial

it lim

big too

d ' req I I Δ

Δ≥

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