Increasing Beam Capacity 5

[DMWWEngr]

At work we are looking at changing the loading of a room. The dead load on the floor will be dramatically increased. I have checked and the primary steel beams supporting the load will fail in flexure. However, the beams are sufficent in block fracture, shear, etc. An initial quick check of the secondary steel beams WILL support the added load.

I'm looking for a good method to increase the flexural capacity of the primary steel beams.

What do you guys recommend??

There is a bit of room below the slab to increase the depth of the beams. I'm thinking about welding on a plate (perpindicular) to increase the depth (web). Then weld a plate at the bottom to create another flange. Is this a viable option?? Would the flange plate be necessary, use a tee instead?? What possible problems would this option bring on??

Using the above example, what if the additional plates had to be added in sections due to space limitations (say four 5' sections to make up the 20' span)?? What additional concerns would this bring up??

P.S. Removing the existing beams and installing new ones are not an option in this situation.

Any advice appreciated!! ---
Andrew

 

[JAE]

My suggestion was to add a tube to the bottom of the beam...not to box it in. The tube at the bottom will significantly add to your Zx as well as increasing Ix to limit deflections.

Adding plates to create a box-type beam can work as well...although probably not as efficiently. It will not affect your shear connections at the end of the beam as they are the "stiff" to which the shear will flow at the ends of the beam. The plates can be simply fillet welded to the flange tips - but this again is adding cost as now you have two large plates and four welds which all work less efficiently than the tube at the bottom with two welds (one each bottom flange tip).

 

[DMWWEngr]

JAE,

How do I calculate the Zx if I place a box beam at the bottom?? I would like to check this but I'm not sure how to calculate Zx in unsymmetrical sections!!

Can someone post the method??

TIA!! ---
Andrew

 

[LPPE]

One thing JAE - Welding plates to the flange tips would be almost impossible at the top flange due to the presense of the steel decka and slab. The plates would have to go between the flanges, but then how do you get the plates to stay there while you weld without the plates falling in towards the web?

As for finding your Z-

Go to the Properties of Geometric Sections in your LRFD manual, and break your new shape up into parts, and figure out Z for each appropriate part and add them together.

 

[DMWWEngr]

Re: Z

That doesn't work....at least when I try it. You have to account for the shift in the neutral axis. I can do that with a symmetric shape but the same method doesn't seem to work with an unsymmetrical shape.

I keep trying to find the Z of angles and I can't seem to get them to come out right!! ---
Andrew

 

[Taro]

Check any mechanics of materials text for calculation of the plastic modulus, Z. Basically, you find the plastic neutral axis, then multiply the areas above and below the n.a. times the distance of their centroid from the n.a.. That is, Z=sum(A*y).

 

[JAE]

Taro is right....you must find plastic NA....not add parts together.

pylko...I only suggested adding the tube to the bottom flange.

 

[haynewp]

If you are going to go with plates on the sides, I think you would need one person to hold the plate at each end, so it wouldn't fall towards the web, while another does the welding. Adding a tube to the bottom should increase your section with less steel than plates on the sides.

You also probably do not need to add this new section through the full length of the beam, however, you do need a certain length past that from which your moment capacity without the addition would be adequate. This extra is for development of the welds for your new built up section. I think ASD 9th (maybe 7th)edition has that procedure.

What angles are you trying to find the Z of anyway?

 

[evelrod]

pylko---Welding the plates in the 'boxing the beam' solution is a piece of cake compared to just getting the darn things up there and making them fit a cambered beam!
I still like the WT solution, I have used it before for ease of construction, but it has stability problems while being welded, sooo , on second thought, I like JAE's tube steel solution as long as it is sufficiently wide enough to allow simple stitch welds to the flange from above. Narrow tube sections can be a real bear to weld overhead.

To redhead's solution with the barstock welded at the top flange web juncture (or for that matter the bottom also) this is the solution the GM engineers dearly loved, for the life of me I don't know why. Implimentation of this solution in the field is a MONSTROUS headache. Getting it
welded sufficiently to pass inspection is outlandishly time consuming. ie. round section bar to flat plate.(square bar?)

Briansch--- It's hard to teach an old dog new tricks. With all your research it's hard to disagree, but none the less I do. Heavy welding on loaded beams or trusses has always been a no no. You may get by now and again, but it only takes one time--- This is a fairly small beam and doesn't appear to be overloaded, but it still would be a good idea to ck shear values at both ends and perhaps to use the same jacks you would use to install the bottom plate or tube to perhaps take a bit of the load off the beam until you get everything tacked up. Then perhaps a bit of reinforcement with a couple of seat lugs or such?

DMWWEngr---Whatever solution you choose (all will probably work), keep in mind ---'It's all about time and money, my time and your money!'.


Rod

 

[LPPE]

Sorry about the Z thing. I thought finding the neutral axis was a given.
And finding Z of angles is quite tricky. Why not try to find Z of a shape thats symmetrical at least in one axis and then compare to the value in the book to see if you got it right? Once you get the hang for that, try finding Z of a W shape with a C. Find your answer, then compare with LRFD. If you get the hang of that, finding Z of a W section with a HSS on the bottom should be a piece of cake.

 

[DMWWEngr]

Thanks for everyone's help. I figured out the Zx on the "new" section. It looks like it will be much cheaper to go with a 6x6 tube beneath the existing beam (rather than the plates). I have a few more minor questions on this.....

The tube will stick out a ~1/4" on each side. Since I'm not very familiar with welding practices...Is the 6" tube going to be wide enough to allow for relatively simple welding?? Would it be better to use a slightly larger tube...6-1/2", 7" etc???

What is a good resource for designing welds?? LRFD has very limited information.

Thanks again for all the great information recieved, ---
Andrew

 

[rkchalla]

Good resources for designing with welds are the books by Blodgett from Lincoln Electric. Design of weldments and Design of Welded structures.
If the beam is an old shape, make sure that the weld type and procedure are compatible.
I have used the WT method before with very good results.
-RKC

 

[JAE]

I believe that tubes with 1/2" increments (i.e. 6 1/2&quot are more difficult to obtain. You might want to check with a local steel fabricator on this.

The tube extending only 1/4" on either side sounds tight to me. For sweep variations in the field, you may have difficulty getting the weld installed properly. An 8" tube would probably work the best.

 

[hakimi]

you can also check the pretension method applying thermal initial constrain o n the lower flange of the beam.

 

[measure]

Good discussion. In similar situations I have used wide flange sections to reinforce the bottom of existing beams because they can be positioned and clamped in place before welding. A flat plate would be first choice if the required increase is small.

I have never had to reinforce the entire span. Determine the required extents of reinforcing and perhaps you don't have to splice. I avoid splices in these critical elements if at all possible.

In extreme cases where we may need to reinforce the top flange we have used angles on each side, sort of forming tubes with the flange and web. The welds are fillets along the toes.

Historically, we have always specified unloading the beam (shoring) while welding.

I like the Blogett reference also. The minimum connections are spelled out in the AISC ASD Section B and J among others. The end welds are based on F=MQ/I and the intermediate connections are based on f=VQ/I. There are also rules about proportions. The existing connections should also be verified with the new reactions.

 

[DMWWEngr]

Thanks for all the comments. I'm looking at using a larger tube to make the welding easier. I'll also look into getting my hands on one of the welding books that was recommended.

Thanks everyone for their responses. It is much appreciated. ---
Andrew

 

[LPPE]

One other - If your looking at an HSS6x6, why not try an HSS8x4 or similar. You then get your 8" for easier welding, plus gain 2" clearance.

 

[DMWWEngr]

That's exactly what I just switched my design too!! Looks like I'm starting to think correctly ---
Andrew