Steel to wood
Steel to wood
(OP)
I've been asked to compare the strengths of a couple different sections, without the use of loads. (consider it to be flexural failure)
I seem to recall a procedure that was used way back when I took mechanics of materials.
I have the sizes of the steel and the wood member. To ballpark it I used the prismatic method, using the section moduluses, but I have a very strong memory of a more specific example in my mind. I have the 4th ed hibbler if anyone else recalls what I am talking about.
Thanks
Will
I seem to recall a procedure that was used way back when I took mechanics of materials.
I have the sizes of the steel and the wood member. To ballpark it I used the prismatic method, using the section moduluses, but I have a very strong memory of a more specific example in my mind. I have the 4th ed hibbler if anyone else recalls what I am talking about.
Thanks
Will






RE: Steel to wood
You want to look at transforming sections, which uses the elastic moduli to equate one material to another.
Used in analyzing composite beams.
RE: Steel to wood
Mike McCann
McCann Engineering
RE: Steel to wood
RE: Steel to wood
When comparing the STRENGTHS of two different members made of two different materials, I would compare the allowable moments (that is the strength after all, right?). I guess you should compare allowable shears to be correct, also.
I would compare S*(fb) for wood - and S*fb for steel.
That will give you a comparison of strengths.
Please let me know if I missed something in the OP.
RE: Steel to wood
That is assuming that it has continuous lateral restraint, if not buckling will need to be considered.
csd
RE: Steel to wood
If I understand the problem correctly, I believe you've both missed the point of the problem here...
I understand that the same shape is being used, just different materials. Consequently, any properties relating to the section used would fall out of the equation since they are equal - A, S, and I in particular, so they are irrelevent in the solution. The only factors that do not fall out, are E, G and the ultimate / allowable bending and shear stresses, which vary with the materials used.
Did I miss something?
Mike McCann
McCann Engineering
RE: Steel to wood
The OP says, "I've been asked to compare the strengths of a COUPLE different sections". It doesn't say the strength of A section using a couple different materials.
The way csd and myself have interpreted this problem is the only one that seems to make sense. If you are comparing a single section, one made of wood and one made of steel, there really is no comparison. Steel is MUCH stiffer and MUCH stronger. You don't need to go beyond comparing E and fb. Nothing more is needed unless you are comparing a steel plate in strong axis bending, but that doesn't seem to be what the OP is implying as no information regarding that is given.
He also says, "I have the SIZES of the steel and the wood member." To me, sizes means that there is a different size wood member and a different size steel member.
WBUW-
Can you please clarify your question.
RE: Steel to wood
The root of the problem is that they want to replace a 4x12 we'll call it a DF-L#1 with a 2x10x1/2 HSS (assume A36 for now) steel section. I have not been given any loads to size the member as one normally would.
My first through was to use the transformation method, but I could not remember where it was. Knowing the properties of the HSS I was going to try to calc out its wood equivalent, or visa versa.
As I could not remember how to do this, I thought I would ballpark it using the prismatic method. Section Mod = Mmax / Allowable Stress. => Mmax = Section Mod x Allowable Stress.
Steel => Mmax = 36ksi x 11.1 in^3 = 399.6 k-in
Timber, as timber is not a homogenous material comparison using this method is not really that accurate...looking at the NDS the flexural strength (lets just assume flexural failure) is about 1 ksi.
Wood => Mmax = 1ksi x 78.83in^3 = 78.83 k-in
Comparison of max moments = 399.6 / 78.83 = 5.07
Hense the steel section is roughly 5x flexurally stronger than the wood.
I have not yet ran through the transformation calcs...will post when I do. Got busy yesterday, will be out of the office for a while today too...
RE: Steel to wood
RE: Steel to wood
Also, I don't know why you are saying that this isn't a good (accurate) comparison. I think this comparison is fine and see no problems with it at all.
RE: Steel to wood
How are you coming up with 23.76 for the allowable. I throught about taking some reductions off the yield strengths, but I wasn't sure what the appropriate reduxs were.
I am just assuming 36ksi, for HSS's isn't it more common to see 42 ksi? What is the common strength for them, anyone? I seem to remember pipe steel being stronger...
RE: Steel to wood
A transformed section of a single material is meaningless as far as strength comparison is concerned.
Transformed section is for composite or built up sections as the stress induced in the materials is relative to the stiffnes of the material. n is meaningless when you are talking about 2 separate sections .
Deflection and shear will not be an issue on this section, but you may want to compare the I and Vmax values just for completeness.
You are definately thinking too complex for this simple problem.
csd
RE: Steel to wood
Yes, 42ksi is more common for tubes, but you did say assume 36ksi.
I came up with 23.76 from fb=0.66fy. Check out the green book for tubes. For fy=42ksi, your fb would be 27.72ksi and produce an allowable moment of 308 k-in - still way higher than the wood, but still lower than 36ksi using the full fy.
RE: Steel to wood
Running the transformation (I'm was to see)
n = Ew/Es = 1.7/29 = 0.059
Bst = nBw = 0.059 * 3.5 = 0.207 inches
This equates to a 0.207 x 11.25 steel plate. Calculating I = bh^3/12 = 24.56 in^4
I'm unsure if a linear comparison is very accurate here, but the Ixx of the steel is 78.83 / 24.56 = 3.2
So, 3.9 to 3.2, unless someone knows quick and dirty better relationship to use than the linear comparison of the Ixx values.
StructuralEIT
Yeah, honestly I haven't looked up the spec yet, so I wasn't sure what the steel strength was.
fb = 0.66fy sounds familiar, I do mostly wood and concrete so not as familiar with steel as I could be. I have the 9th edition of the AISC ASD, do you have a page for the formula and where you are finding the allowable moment so I can toss in a couple markers?
Think that pretty much takes care of it, thanks guys.
RE: Steel to wood
RE: Steel to wood
RE: Steel to wood
RE: Steel to wood
RE: Steel to wood