How to Select a Correctly Size Steel I Beam?

Written by: Suvo • Edited by: Lamar Stonecypher
Updated Nov 4, 2011

Selection of a correctly size steel I beam is very important for your structural design. This article will talk about the selection procedure for a steel I beam for given load specifications.

A steel I beam typically has the following important features or dimensions. (Please refer to the picture below):

I-Beam Terminology

Flange thickness: Top and bottom horizontal plate-like sections of an I-beam are called flangen. The thickness of the flanges is called the flange thickness.
Flange width: The width of the flanges is called flange width.
Beam depth: The height between the top and bottom surface of the steel I beam is called beam depth.
Web thickness: The vertical section of steel I beam is called web, and the thickness of the web is called web thickness.
Fillet radius: The curved portion, where the transition between the web and flange happens is called a fillet. The radius of the fillet is called the fillet radius.

Steps for Selecting a Correctly Sized I Beam

The overall procedure of selecting the correct size of the I beam is based upon basic mechanical design calculations as follows:

The first input you need is the steel I beam load specifications or loading details on the steel I beam.

Draw bending moment diagram for the given loads and you will find the value of maximum bending moments (say M) that the steel I beam is expected to experience.

Choose an approximate size of steel I beam from a standard I beam table.

Find out the area moment of inertia (say I) of the selected steel I beam.

Get the beam depth (say d) of the selected steel I beam.

Now use the following formulae for calculating stress developed (f) in the beam:

f/(d/2)=M/ I

f is the bending stress

M - the moment at the neutral axis

y - the perpendicular distance to the neutral axis

I - the area moment of inertia about the neutral axis x

Compare the calculated value of the bending stress with the yield stress of the steel in order to check the safety factor of your design.

Conclusion

Correctly sized I beam selection is the first step toward correct structural design. The procedure explained above is based upon static I beam load specifications. In the cases where dynamic loads are involved, you need to use FEA tools like ANSYS, Pro Mechanica, etc.

Sinong.....see ASCE Manual of Steel Construction for tables of steel beams. Manufacturers may also provide tables online. However, if you know beam dimensions (depth, flange width, flange thickness & web thickness)you can get very close answer with the following formula;

I = Af*(d/2 - tf/2)^2 + tw*(d-2*tf)^3/12

where;
Af = area of flange = b*tf

b = flange width
tf = flange thickness
tw= web thickness

Of course all dimensions must be in inches (or centimeters). Answer is in^4 or cm^4.

sinong Sep 13, 2011 9:51 PM

moment inertial of builup structural I beam?

how to find momemt inertial i beam?

Curtis Sep 12, 2011 4:29 AM

I-Column selection for long spans

I am working of designing steel structure to span 18 meters. What size of I-Columns do I use? The structure is a single floor; more like a hall. Nothing complex except for the long span: 18m x 45m

Joe W Sep 8, 2011 5:43 PM

Calculate Roof Load

For a City permit we were asked to document the roof load of an already built school auditorium. On the plans it shows the steel beams are L3x3x1/4x0'-8" spaced every 5'4-7/8" apart. With this limited information is it possible to document the roof load?

wesley Aug 16, 2011 8:48 PM

beam size

please help, I need to know what size wide flange beam to us to support 5000 lbs center point loaded. with a span of 50 ft. suported on each end..It can only have 1/2" deflection

Anonymous Jun 8, 2011 11:19 AM

quick question, when do we apply loading factors like 1.25 and 1.5 for dead and live loads? do we use them when testing against ultimate strength only? or do we use them as a general guide for design?

thanks!

Anonymous Jun 3, 2011 10:46 PM

beams for simple structure

hi. i am currently planning to build a decking leveled with the roadway next to it. this decking will be required to support loads up to 3000kg, when completed, it is going to be 24 x 24 feet and the foundation has been set up to hold a three story warehouse. can i have an idea as to what size of beams do i need to use as the columns and girders? it is a static load type design.. (I am gathering a second opinion on this)

Chi Apr 8, 2011 2:24 AM

I beam

HI,
I have a question>
The tiles floor on a shop im making is swollen. The explanations i heard was that there are 2 I beams undeground that have been reinforced and that are put very close to each other (like less than 10cm) this floor existed before but they added one more floor above and this is why they reinforced this area, we took out the tiles and made an expansion joint of 1cm on the damaged area on the screed.
I want to know if making the expansion joint will be the right solution for this and if this swollen was caused by the effects of the I beams?
Thanks!

ANDY Apr 3, 2011 9:50 AM

I WANT TO SPAN 26' WHAT SIZE I-BEAM DO I NEED. THANKS

John F Mann, PE Mar 19, 2011 8:11 AM

Beam design

Bill......this is not a simple design. You have relatively heavy load and a long span. Also, beam stiffness must be much greater than usual to support brick without cracking. A steel beam is likely required.

Of course requirements for supporting the steel beam.......all the way down to foundation must be carefully considered. Then, the foundation must have adequate capacity to support the relatively large concentrated load at each end of the beam.

Another potential issue to be considered is adequate resistance to wind load against the remaining wall.

You should obtain desgin plans and specifications from a qualified professional engineer licensed in your state.

Bill Mancini Mar 14, 2011 8:49 PM

Size for I beam

Need size for beam to support 2nd floor of residence outside wall. Span 23 feet, floor on one side deck on other side with brick siding on span. Weight on beam would include 8 foot high brick wall plus outside flat roof and deck. On inside there is the second floor bed room. Currently there are 3- 2 X 10s together over a span of 11.5 feet with a column in the middle. On one side of the span there is also a 5/8 inch steel plate sandwiched between the 2 X 10s. We want to eliminate the column and make it a clear 23 foot span.

ND2 Mar 6, 2011 11:53 AM

standard I beam table

Hi there,
How do I determine the correct size of an I beam from the standard I beam table if the maximum bending moment is 12.75kN?

krish Mar 1, 2011 12:20 AM

Support tie beams between columns

Hi
please advise me if its necessary to have to install tie beams between columns at a spacing of 5000mm apart at a height of 4500mm the colums are of IPE 180 and the rafters of IPE 160 grade 300WA
the perlines are of 125x75x20x2mm lip channel

janakiraman.m Feb 17, 2011 5:46 AM

WHAT TO KNOW ABOUT STRUCTURAL STEEL BEAM ANALYSIS

hello sir, i need some good information from u or urs memeber.
right now i am working in offshore structural design desinger, almost i had learned little about beam analysis also.
now i have to fully intrest to learn about analysis software like *SACS,*STADD PRO.
can u plz suggest me how to learn these thinks.

please do need ful as soon as possible.
thanks & regard`s
janakiraman

Dave Feb 11, 2011 3:14 PM

Bridge loading enhancement

I need to know what weight a wide flange, 84# per foot I beam, 40' long, 24" high can handle over a 10' centered dynamic load if the beam is supported 24" on either end? I beleive the web is 1/2" and the flanges are 3/4".

farroha Feb 11, 2011 3:10 AM

Question

I am from Sweden and i need som more information about choosing the write beam ,if you like kontakt me at my e-mail

John F Mann, PE Jan 19, 2011 3:47 PM

Beam for driveway bridge

Based on your brief description, you need a bridge design for the driveway. Bridge design is more speciazed than building design since the beams must support dynamic (moving) loads. Of key importance is to determine applicable design loads......which might have to include heavy truck loads.

You need a qualified professional engineer for this design, which will not be free.

A. ALEXANDER Jan 16, 2011 10:53 PM

WHAT IS THE BEST I-BEAM SIZE

COULD U RECOMMEND THE APPROPIATE SIZE OF I- BEAM PLEASE.
AM CONSTRUCTING DRIVEWAY WITH A 30FT SPAN, AT EACH END ARE TAPERED ROMAN PILLARS WITH A BASE OF 18 INCHES AND TOP OF 12 INCHES. NB WHATEVER I- BEAM IS RECOMMENDED IT MUST WITHSTAND A 3 FEET ROW OF CLAY BLOCKS ACROSS ITS HORIZONTAL 30FT LENGTH.
REMEMBER NO CENTRAL POST IS TO BE USED.
THANKS FOR ANY HELP GIVEN.

akideas Oct 19, 2010 1:19 PM

Building using pilings & steel???

I am looking at building a home. My vision is to have the living space elevated with half on top of the garage built with commercial split face concrete block and the other half on pilings. I want to use steel beams for abiiity to have greater spans with less vertical support as well as less pilings...my entire thought process is to utilize discounted materials that I can get from left stock of commerical projects, to have a stable solid structure that will out last traditional stick frame construction, to minimize impact on the nature forrest, and most important to avoid the water issues of my neighbors. The challenge is that the builders and engineers I have talked to are trying to convince me that pilings are too expensive and excavating all the organics and hauling in 200,000 tons of fill is a better way to go...knowing how high the water table is and knowing what a good deal I can get on the materials, I do not buy it. What is your opinion of building using pilings and steel floor system?

John F Mann, PE Oct 12, 2010 1:20 PM

shine

Basically............you would have to take a course in school to learn how to perform proper design of steel beam.

Much more information is required before the design can be performed.

See my previous posts for list of information required.....as well as some idea of calculations. However, you might also contact me directly.

For the amount of load you must support, design should be performed by a qualified professional engineer.

John F Mann, PE Oct 12, 2010 1:11 PM

Variations of beam dimensions

dhinesh...........both horizontal flanges (top & bottom) and the vertical web are essential for an I-beam.

The entire beam resists bending stress. However, the flanges (one in tension, the other in compression) resists almost all of the bending stress.

For design purposes...........we consider that only the web (with height taken as full depth of beam) resists shear stress.

Flange and web dimensions are varied to provide different beam properties (strength and stiffness).

shine Oct 9, 2010 5:05 AM

lifting beam

i want to make a lifting beam(square type frame) to lift a valve of weight 17tonne . What type of Beam size do i need to use for that . could you explain me , how to do the calculation.Please advise

dhinesh Oct 7, 2010 12:44 AM

structural design

In some of the applications the web plate size of an I beam is larger (thick and the web length) and in some applications the flange plate size is greater... Why is it varying? Which one has more strength... which is the load acting member (whether flange or web).....

Waiting for response...

Thanks in advance

John F Mann, PE Sep 28, 2010 10:16 AM

Beam requirements; point load

Manuel.......for a load of 20,000 pounds and a span of 30 feet, you should obtain complete design from a qualified professional engineer. Discussion below does not represent a complete design.

As for any steel beam design, the beam must have adequate lateral bracing at each end. Also, for a 30-foot span, it is highly likely that the compression flange of an I-beam must also have lateral bracing between supports. Without adequate lateral bracing, the beam will fail (in sideways buckling) at a load much less than for the beam with bracing.

For any design, both strength and deflection must be considered.

The following discussion is based on the condition that the load is static (not dynamic). For a dynamic load, the beam would have to be stronger.

For strength, with A36 steel and adequate lateral bracing, the section modulus must be equal to or greater than 1800 inch-kips (midspan moment) divided by an allowable bending stress of 0.6Fy (21.6 ksi), or 83.3 in^3.

Deflection must be limited to a value that may depend on various factors, including the effect of movement on supported elements or even on elements under the beam. For general building construction, the limit of Span / 360 is often used, which is 1.00 inch for this case. Moment of inertia must then be equal to or greater than 670 in^4.

There are other issues to be checked, which may depend on how the 20,000 pound load is to be applied to the beam (on top or hung from below).

Capacity of supports must also be adequate.

Again.......consult a qualified professional engineer for a final design.

Ian Smith Sep 27, 2010 10:28 PM

When is this dangerous?

And requires a multiplication of 2, when home work is not done and estimates are off to the wrong side. Both of which are possable. Be safe, not stupid.

Manuel Gonzalez Sep 26, 2010 1:52 PM

Simple Beam

What steel Beam can I use for a load of 20,000 Pounds at the center on a 30 ft span ? (suport at both sides

Ian Smith Sep 2, 2010 10:48 AM

I bought some beams

I bought some beams cheap I believe W6 *15 by thier weight and the critical dementions. I really exspected to find a simple chart with like modulus numbers. I don't think they make such a chart but the american institute of steel construction has a nice pub. 518 pgs. and if one can under stand the greek symbols and all the funky equations in there then you don't need an engineer. I'm going to use these beams I just need to know how many I have to buy, and I will use an engineer to tell me.

John F Mann PE Jul 16, 2010 5:08 PM

Curtain wall design

Response to "Anonymous" 7-11-10;

Much more information is required.........including one or more drawings to show conditions (existing or proposed).

Loads from curtain walls are often "uniformly" distributed.........however these loads can also be concentrated at discrete support points.

John F Mann PE Jul 16, 2010 5:05 PM

Design of beam to support roof

Response to question from Jim (6-6-10);

For design of any such beam .......to allow for removal of bearing wall.........the following information is required (at a minimum & in general);

1- Length of span (for new beam)

2- Length of framing members to be supported by new beam.........on each side of beam.

3- Type of space that framing members must support.......such as roof or interior space.......as well as type of building (residential, office, industrial). Essentially, this determines uniform design loads (such as 40 psf for residential, 50 psf for office).

3A - Identification of any bearing walls supported on floor framing members to be supported by new beam. Loads supported by any such bearing wall must also be determined.

3B- Any cross beams to be supported (other than floor or roof joists).........and loads.

4- For beam supporting roof........geometry of roof must be evaluated to determine roof snow load per building code. Snow drifting and sliding (from higher roof) must also be carefully considered.

5- Allowable deflection. If beam supports floor space with tile flooring...........allowable deflection is much less than "normal".

6- Details of elements that will support columns (at each end of beam). Reinforcement of such elements may be necessary..........especially for a long beam. This evaluation must include foundation elements. New (larger) foundation elements may be required.

7- For beam to support roof...........requirements for wind uplift resistance must be considered..........all the way down to foundation. Just as for downward loads, uplift load will now be concentrated at each end of the beam...........instead of uniformly distributed along entire length of wall to be removed.

Anonymous Jul 11, 2010 9:01 AM

direction of the dead load in a vertical beam

I would like to design a curtain wall fascade, so im trying to calculate the acting loads on it, they will be mainly dead load (own weight of glass and aluminum) and wind load, i just wanted to check if the effect of the dead load will be distributed along the span of the fascade or it will affect as pointed load at the top of the fascade.
Thanks alot

ali Jun 12, 2010 5:43 AM

house chart

i need house chart and dizine consist of 3 floors
steels and cement

Jim Jun 6, 2010 7:35 AM

Removing a load bearing wall

Looking for what size I beam I would need to support a low picth roof which I believe is about 20 lbs. per sq. ft., the span is 18 ft..

Daniel Sandoval Jun 2, 2010 11:43 PM

trailer

i like to make a 70 ton trailer what thignes of i beam do i need 29 deck

Ananth.K Jun 2, 2010 12:19 AM

Plate material

sir,
can u give feed back for the use of SA 516 Gr.70 & SA 302 Gr. B plate material in the following aspects:

1. Raw material cost
2. Manufacturing cost.
3. Lead time for procurement.
4. Fabrication feasibility in India & any special fabrication process requirement.
5. Lead time for fabrication.

The thickness requirement is approx. 120mm, 15000mm shell length, ID 1450mm.
its very very urgent... please reply positively...

George May 27, 2010 6:34 PM

draw one beam,estimate the weight that it will support and a distributed load per unit length,calculate the reaction,draw the shear and bending moment diagram,and know what are the maximum shear and maximum bending moment.
calculate the maximum shear stress,and the maximum bending stress,and compare it to the yield stress in each case.
finally,calculate the maximum deflection caused by the weight,and compare it with the maximum deflection the beam can support,and do not forget to multiply by a factor of safety which is equal to 2,because this is dangerous.
choose the beam with less weight.

Carlos Leiva Apr 14, 2010 10:24 PM

A little "knowledge" can be dangerous

As a yet-to-be licensed engineer with 5 years of experience in Missouri, I'll agree with John and Phil.

This article is a gross oversimplification designed to "illustrate" the non-engineer public, and little else.

Also, on a bit more technical note, you can't simply take a load out of thin air and size a beam. What about dead loads (other than self-weight), live loads, wind, seismic, etc???

Aaron Petersen Jan 26, 2010 3:18 PM

i beam size

I am building a support to lift a valve that wieghs approx. 3000lbs a single i beam overhead with a roller and chain fall on it. With two i beams supporting it up. one on each end. 14 ft across and 14 ft tall with i beam on laying flat on the ground with the upright beam gusseted on 45 degree angle off of the beam on the ground on each side. i hope this is enough info i just need to know what size of i beam ito use. hanks Aaron

Phil, B., PE Jan 15, 2010 3:23 PM

As a licensed engineer in Virginia I completely agree with John.

But thankfully some of the most difficult portions of determining the loading are not discussed. For example, what are the load increase factors? What are the allowable stress reduction factors (they differ depending on the type of support, primary or secondary, that is being sized). Not too mention bending stress is not the only stress you need to check. What about the shear stress in the beam and what about buckling, is this a slender beam or column which would be determined by the unbraced length, etc., etc., etc.

I guess this would be a decent "background" or "refresher" for someone who wants to know how something is done, but an untrained person should never attempt to size a structural member.

John F Mann, PE Jan 10, 2010 1:56 PM

This "article" should be REMOVED.

I am a structural engineer with 32 years experience, licensed as a professional engineer in three states.

The suggested calculation procedure may be well meaning. However, it is grossly inadequate and DANGEROUS.

Much more information is required to properly design a steel beam.

First.........there is no practical way for anyone without engineering training to calculate the bending moment (M). So the advice should not useful for any practical purpose (thankfully). Yet, you never know.

More important is the need to consider essential requirements, including........lateral bracing of top flange to prevent buckling failure.....support capacity....... and deflection.

The last instruction........to compare calculated stress to "yield stress" to allow for checking "safety factor"....... is too confusing for any untrained reader to make much sense. However, to the extent that anyone might try to interpret ........it is grossly incorrect and UNSAFE.

Calculated bending stress must be compared to allowable bending stress per standard code provisions.........which require substantial training to understand and apply correctly. There is no leeway for flyby "designers" to determine their own safety factor.

For any suggestion like this........the person providing the information should describe their qualifications.

Offering structural design services without a professional engineer license is also a violation of state regulations governing practice of engineering. The point of such regulations is to protect the public against just the type of deficient "advice" demonstrated by this "article".

lionel Jul 6, 2009 4:06 PM

depth for a basement

I will like to know how fur can I build a basement, with I beams with out self destuction. I dont live in a earth quake area?

How to Select a Correctly Size Steel I Beam?

I-Beam Terminology

Steps for Selecting a Correctly Sized I Beam

Conclusion

See also

Comments