What is the section modulus of 3/8" A36 steel plate

[WHPE]

I'm running an analysis of a 11.25" tall (deep) Flitch Plate - iLevel Truss Joist beam utilizing a "BLT sandwich" of a 1.75" 1.9E LVL Microllam, a 3/8" ASTM A36 steel plate, a 3.5" 1.55E LSL Timberstrand, another 3/8" plate, and finally, another 1.75" 1.9E LVL (all being 11.25" and bolted together with 3/4" A325 bolts staggered at 16" o.c. but with 4 stacked bolts at each end). Total uniform load is about 15k including 15% for seismic; i.e., max end reaction is about 7.5k and I'm using L/480 (shooting for less than .3" maximum deflection).

Will my BLT Flitch beam work or what is the section modulus of a 3/8" X 11-1/4" ASTM A36 steel plate?

 

[HgTX]

I must be misunderstanding your question. It sounds like you're saying you can't calculate the section modulus of a rectangle, and surely that is not the case.

Hg

Eng-Tips policies: faq731-376

 

[Lion06]

The section modulus (elastic) is 7.91 in^3.

 

[ctcray]

Most steel plate comes in 1-inch increments, ie 11" wide x 20' long pieces, so i would recommend working with what is readily available to save the contractor. Might be better off using a 10" steel beam with packed out webs. Have you figured the self weight of this monster?

 

[Lion06]

I should clarify that is per plate.
Also, because everything is the exact same depth, the only reason you are bolting them together is for your own peace of mind.
Because the load is bearing on all parts of the "beam" and the load is going into the supports through all parts of the "beam" there is no need to transfer load into or out of the wood via the bolts, It will all deflect together naturally and therefore naturally behave the way you would try to get a typical flitch plate beam to behave (by deflecting together, thereby sharing load based on relative stiffnesses). You have it pretty easy, though, as mentioned above. Just bolt at some nominal spacing and maybe two bolts at the ends, I wouldn't use four stacked. that is giving a small spacing and cutting into the shear capacity of the wood (which may or may not be a concern, but there is no reason to use four bolts at the end).

 

[WHPE]

Thanks for all your input. Let me clarify. I did spec a W10X26 w/nailers both sides thru-bolted to pick up joist hangers but the contractor did not want to handle the 416# of the W10X25, so I broke it down as follows:

15'- 3" span, total load is -15k (uniform max ld = 980 plf)
Max end vert reaction = -7,500 using L/480 & 40LL + 20DL

Max beam depth permitted by existing 12" floor joists is 11.25" +/-

Flitch Bm Solution:

(3)1.75" 1.9 EOM Microllam LVLs + (2) 5/16" A36 Plates* bolted at 16" oc w/.625" A307 bolts/nuts/washers; glueing adds 15 psf. Total Flitch Bm wt less bolts is 656# versus W10X26 w/DF wood nailers = 575 or diff of 81#

End bearing @ 4.5" using 3.5" X 5.5" X 8' 1.8 EOM Parallam PSL

*may get away with 1/4" plate???

 

[WHPE]

Thanks for all your input. Let me clarify. I did spec a W10X26 w/nailers both sides thru-bolted to pick up joist hangers but the contractor did not want to handle the 416# of the W10X26, so I broke it down for easier handling, as follows:

15'- 3" span, total load is -15k (uniform max ld = 980 plf)
Max end vert reaction = -7,500 using L/480 & 40LL + 20DL

Max beam depth permitted by existing 12" floor joists is 11.25" +/-

Flitch Bm Solution:

(3) 1.75" 1.9 EOM Microllam LVLs + (2) 5/16" A36 Plates* bolted at 16" oc w/.625" A307 bolts/nuts/washers; glueing adds 15 psf. Total Flitch Bm wt less bolts is 656# versus W10X26 w/DF wood nailers = 575 or diff of 81#

End bearing @ 4.5" using 3.5" X 5.5" X 8' 1.8 EOM Parallam PSL

*may get away with 1/4" plate???

 

[UcfSE]

You didn't respond to Hg's question, and it isn't clear what you want to know. No one here can design it for you and say if you can use something smaller or not.

Try reading this article:

Structure Jun07

 

[JStephen]

The strength of a plate on edge like that will be based on buckling, not just bending stress. Refer to the AISC-05.

"Flat bar" is available in widths of 10", 12", etc., but not normally 11". "Plate" implies a wider width cut to that dimension.

 

[csd72]

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Next time please search for the topic using the google at the top of the page before you post a new topic.

 

[Lion06]

the whole point of sandwiching the steel plate between the wood beams and attaching them is to take the buckling out of the equation.

 

[MiketheEngineer]

How about 4 - 1 3/4 x 14'' Microlam beams??

 

[ronster]

Feel the need to clear up of few things stated here:

I'm in agreement with HgTX, what is the question. Someone posting as a structural engineer better know how to calculate the section modulus of a rectangular plate.

3/4" diameter A325 bolts are not used in a flitch beam. A307 carriage bolts are used. Typically 1/2" bolts unless larger are needed.

Plates come in 1" increments (as ctcray stated). 11" flitch plates are used all of the time. 11 1/4" plate would need to be cut from 12" plate and should not be specified. Not only would it be expensive to cut the steel, the steel plate and wood beam would not have the same camber so the steel plate would stick above or below the wood beam along the member.

There "IS" a need need to transfer load through the bolts. All of the load is transfered from the wood through the bolts and into the steel and back through the bolts into the wood for bearing. The top of steel is set down below the top wood and does not get loaded in bearing. Also, the elastic modulus of steel and wood are not the same so they will not deflect the same unless bolts transfer load between them. The steel will support most of the load over the span as it is stiffer. You need the bearing value of the wood at the supports as the steel plate will typically not have enough bearing area.

The whole point of attaching a steel plate to wood beams is not to prevent buckling. That is just part of the point. The wood is used also for compatability with the floor system, for connecting and transfering loads to the steel plate, and also for end bearing.

 

[Lion06]

Ronster-
While I agree with your post in general and for a generic situation, I disagree a little for this particular post and situation.

1.)While it might be cheaper to use an 11" plate, that wasn't what was asked.

2.) "Also, the elastic modulus of steel and wood are not the same so they will not deflect the same unless bolts transfer load between them."
This is only true if the wood and plates are different depths - which was not the case in this post. But in the case where they are different depths, the fact that they will not deflect together has nothing to do with different E values, it has to do with the geometry of the condition (i.e. that the load must be transferred through the bolts). The point of the bolts (generally speaking) is to ensure that the members DO deflect together.

3.) "There "IS" a need need to transfer load through the bolts. All of the load is transfered from the wood through the bolts and into the steel and back through the bolts into the wood for bearing"
Again, this is only true if the wood and plate are different depths and/or are cambered differently. Again, not the case for this original post.

4.) "The steel will support most of the load over the span as it is stiffer. You need the bearing value of the wood at the supports as the steel plate will typically not have enough bearing area."
If the steel is bearing on the support (again, as was the case in this post), all of the bolts in the world won't get the load out of the plate and into the wood for bearing on the support.

 

[youngstructural]

There is a need to bolt the sections together, both because of the fact that these plates and timbers are almost definately not going to be cut perfectly to the same depth, as well as to address any possible warpage in the timber. Trust me, warping is one of your biggest enemies with a home-made flitch beam...

Also, without the bolting, your beam's "plies" will just buckle away from each other. You need to provide bolting to ensure the built-up section acts as a composite.

Due to the stiffness of the two sections, the timber will not support the design-intended load (although it would support a load proportionate to the deflection) without composite bolting.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...