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Steel Beam Design: |
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This design guide is intended to provide guidance for the safe and economical design
of steel beams. This design guide and the corresponding calculations are based on
the 14th edition of the AISC Steel Construction Manual. All calculations
can be performed for Load Resistance Factor Design, LRFD or Allowable Strength Design,
ASD.
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Steel Beam Design Using The 14th Edition AISC Steel Construction Manual:
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W-Section Beam Selection By Zx:
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1. The fastest and easiest way to design a steel beam is to use the beam selection
tables in the AISC manual. From the combination loading on the beam, either
factored loading for LRFD or the unfactored loading for ASD the required design
moment, Mu and the design shear, Vu are calculated. Note:
the design loading on the beam should also include the beam's own weight as
part of the dead load.
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2. Calculate the required plastic section modulus of the beam. For LRFD set
the design moment equal to the nominal moment, Mn multiplied by the safety factor
phi where phiMn = Mu and solve for the required plastic section modulus, Zx.
Where Mu = phiMn = FyZx, thus Zx = Mu/Fy. For ASD set the
design moment equal to the Nominal Moment, Mn divided by the safety factor Omega
where Mn/Omega = Mu and solve for the required plastic section modulus, Zx.
Where Mu = Mn/Omega = FyZx, thus Zx = Mu/Fy.
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3. Enter the "W-section Beam Selection by Zx" chart and find the beam
with the same or larger Zx. Check the flexural strength either phiMn for LRFD
or Mn/Omega to insure it is equal to or greater than Mu.
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4. Check the unbraced length factors, Lp and Lr. If a new design, the beam
selected would require bracing less than or equal to Lp to achieve the required
strength Mn, however if the design will have the beam braced at intervals greater
than Lp then the proper beam will need to be selected using the "Available
Moments vs. Unbraced Length" chart by the method below.
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5. Check the shear strength of the selected beam. The shear strength of the
beam given in the chart, phiVn for LRFD or Vn/Omega for ASD, must equal or exceed
the design shear strength, Vu calculated in step 1 above.
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6. Check the deflection of the beam. Calculations for maximum deflection are
given in tables 3-22 and 3-23
or the alternate method for simple-span beams and I-shaped members and channels
using a loading constant
to calculate beam deflection can be used.
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IBC DEFLECTION LIMITS
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Construction
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Live Load
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Seismic or Wind
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Dead Load + Live Load*
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Roof members:
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Supporting plaster ceiling
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l/360
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l/360
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l/240
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Supporting non-plaster ceiling
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l/240
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l/240
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l/180
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Not supporting ceiling
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l/180
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l/180
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l/120
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Floor members
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1/360
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l/240
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Exterior walls and interior partitions:
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With brittle finishes
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l/240
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With flexible finishes
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l/120
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Farm buildings
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l/180
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Greenhouses
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l/120
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*For steel structural members, the dead load shall be taken as zero.
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W-Section Beam Selection by Uniform Load:
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If the loading on the beam is a uniform load, the W-section can be selected from
the "Uniform Load" selection tables. Uniform loads are tabulated
for each W- section for different span lengths in one foot intervals. Again
the uniform loading on the beam is for a beam with an unbraced length, Lb less than
or equal to the unbraced length limit factor Lp.
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W-Section Beam Selection by Available Moment
vs Unbraced Length:
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1. From the combination loading on the beam, either factored
loading for LRFD or the unfactored loading for ASD the required design moment, Mu
and the design shear, Vu are calculated. Note: the design loading on the beam
should also include the beam's own weight as part of the dead load.
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2. Enter the tables for "W-shapes - Plots of Available Moment vs. Unbraced
Length", and find the required design moment on the vertical axis, Mn/Omega
for ASD or phiMn for LRFD. Next find the unbraced length on the horizontal
axis. Plot the intersection point of the required design moment with the unbraced
length. Choose a beam directly above and to the right of the point of intersection.
The closest solid line will be the most economical beam. If the line is dashed
it means there is a more economical beam.
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3. Find the selected beam in the "Maximum Uniform Load" tables or the
"Selection by Zx" tables and check the required shear strength, Vu versus
the beams shear strength, phiVn for LRFD or Vn/Omega for ASD.
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4. Check the deflection of the beam and if it meets the serviceability and deflection requirements.
Refer to the IBC deflection limits table above for common deflection limits.
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Analysis of an Existing Beam:
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The steps below are for the analysis of an existing beam. An existing beam
can be analyzed quite easily with the "Section Properties" and "Maximum
Uniform Load" tables in the AISC Steel Construction Manual. Also the
properties of the beam can be analyzed by calculation using the "Specifications
For Structural Steel Buildings" which is part of the AISC Steel Construction
Manual, and which contains the calculations upon which all of the various beam tables
are based and the properties and strengths were calculated. Or, a combination
of calculation as well as reference to the tables in the manual can be used.
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1. Determine the beam that is being analyzed. Thru measurement etc.
determine the beam dimensions, the loading on the beam, beam span and spacing of
lateral bracing.
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2. Reference the CECALC.com section
properties tables here, in the AISC Steel Construction Manual, or manually
calculate the section properties of the beam. The following are beam section
property calculations for common shapes used as beams:
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3. If analyzing by calculation check compactness of the beam (if using the AISC Steel Construction Manual tables, or the CECALC.com Lp and Lr value tables for rolled I shapes and channels,
go to step 4):
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4. If the beam being analyzed is an I-shape or channel, determine the unbraced
length limit states for yielding, Lp and for inelastic lateral-torsional
buckling, Lr, (else go to step 5). If using the AISC Steel Construction Manual
tables, Lp and Lr are listed at the bottom of the uniform load tables for each
beam under beam properties or you may use the CECALC.com Lp and Lr value tables, US - 50 ksi, Metric - 345 MPa, US - 36 ksi, Metric - 248 MPa. If calculating
them, the following are calculations for the common I shapes and channels:
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5. If the beam is an I-shape or channel and the unbraced length, Lb of the beam being analyzed is less
than Lp then the flexural strength of the beam is the full moment strength of the
beam listed in the beam properties section of the AISC Steel Construction Manual uniform load tables, and the allowable
design moment, Mu is equal to Mn/Omega for ASD or (phi)Mn for LRFD. The ASD and LRFD Moment values are also listed in the CECALC.com Lp and Lr value tables. If the Lp
< Lb <= Lr then the strength of the beam can be interpolated using the formulas
given in the AISC Steel Construction Manual, Chapter 3 or simple
interpolation between the values given in the CECALC.com Lp and Lr value tables. If Lb > Lr then
the strength of the beam can be found by finding the shape in the "Plots of
Available Moment vs Unbraced Length". If calculating
the strength of the beam, the following are calculations for the flexural strength
of common shapes used as beams.
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6. Check the shear strength of the beam. If the beam properties are listed
in the table, the shear strength of the beam can be directly read from the table
and compared to the required shear strength. If the properties of the beam
are being calculated, the following are the calculations for the shear strength
of common shapes:
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7. Check the deflection of the beam. Calculations for maximum
deflection are given in tables
3-22 and 3-23 or the alternate method for simple-span beams and I-shaped members
and channels using a loading
constant to calculate beam deflection can be used.
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