Fb to use when strengthening an existing Wf with a tee

[samdamon]

I am strengthening a variety of wide flange shapes made from A7 steel (Fy=33 ksi) by welding tees on the bottom. When checking the combined superimposed stresses on the top flange of the strengthened section using ASD methods, is there any justification for using a higher allowable Fb than .66Fy? Don't see it in the literature but was wondering what others do in this situation.

 

[racookpe1978]

Not sure of your final configuration:

Is the added "T" welded "upside down" with the bottom of its web to the middle (bottom flange) of the WF? This would give you a combined beam with three separate flanges, each separated by a web.

Or is the T welded right-side up so its web is sticking down into space? This gives an upper WF flange, the WF web, the lower WF flange, the T flange, and the T web to make up the beam.

 

[Ron]

No justification for changing Fb in this situation.

 

[Lion06]

no, use the appropriate Fb, based on your strength and the spec. That being said, if you determine Mp you will likely get a boost over using Fb.

 

[rb1957]

are the Tees the same material as the beam ?

if the Tees have a higher Fcy(Fb) then you've got a rationale (allow the beam to work up to it's allowable Fb (Tees'll strain as well) but instead of the whole thing failing, the Tees'll keep the flange stable and provide a stiffer loadpath for additional load.

are you reinforcing the tension side as well ?

 

[miecz]

Agree that there is no justification for increasing F_b. In fact, depending on the unsupported length of the compression flange, you may have a decrease in F_b.

 

[samdamon]

Thanks for the replies. The WT's are A36 steel, attached web-up to the bottom flange of the existing beams. The top flange of the existing beams are braced by an existing concrete slab which is to remain. Structural EIT, I will try calculating Mp and see if that gets me greater capacity.

 

[jike]

The justification for going from 0.6 Fy to 0.66 Fy in ASD is the average shape factor of 1.1 (that actually comes from plastic design). The criteria for allowing this is:
1) minimum width-thickness ratios (so local buckling does not occur with the higher strain levels)
2) unsupported compression flange not exceeding Lc
3) SYMMETRY

Obviously, you do not meet the 3rd criteria.

 

[Lion06]

jike-

If you use LRFD (I know that isn't the case when using Fb=0.66Fy) you can get much higher shape factors if the PNA doesn't lie at the same location as the ENA. This is the reason that WT designs limit Mp to 1.6My (to ensure elastic behavior under service loads, because shape factors for WT's can exceed 1.6).
I don't see anything wrong with checking it using LRFD and taking advantage of the higher shape factor.

 

[civilperson]

Fb of 0.6 Fy is now listed as the ASD flexure maximum.

 

[Lion06]

civilperson-

Where did that come from?

 

[samdamon]

I am trying an alternative approach now, using the Spec in the AISC 13th edition with LRFD to design these WT's, but can not decide which Section of Chapter F is appropriate for this situation. As Jike pointed out the strengthened section is asymetric on one axis, and its also not a simple I shape, which appears to rule all sections of Section F except F12, Unsymmetrical shapes. Best bending capacity we can do using F12 is the yield moment rather than plastic moment. Is using Section F12 the right approach?

 

[jike]

Another consideration is that symmetrical WF sections have been tested for the extreme strain that they undergo in reaching plastic moment but you have a unsymmetrical section with weld between the WF and the WT that will now be subjected to that strain level.

I do not know of any testing on this type of application. What about the weld? Will it undergo this level of strain? Is it intermittent or continuous? I would not risk it.

 

[samdamon]

Jike-

I normally use an intermittent weld along the length of the WT and a continuous one at the ends of it,as I think most engineers would for economy. You have a point, there is no way to realistically be able to say for sure that you will get plastic behavior at the intermittent welded joint, such that the material at the joint will behave in a similar way to the other material around it. In fact it doesn't seem realistic at all to expect that. I suppose I could ask for continuous welds all along the tee, but that will probably create other problems with the heat, be costly and generally raise some eyebrows.

The bottom line is in this situation it appears its better to use ASD methods and stay in the elastic range of stresses. Haven't got into trouble doing it that way in the past.

 

[kslee1000]

For mixed use of two different materials, there are two methods in my book to check the stress:

1. Conservatively using the lower allowable to simplify the process, and make sleep better.
2. Use equivalent section method, transform the lower grade material into a equivalent section with higher grade (at constant strain). Usually the transformed section shall maintain its depth for design controlled by flexural and deflection. This method validates the use of single fy.

 

[ldp]

I have a crane boom section that is trapzoidal in shape, it has a side to side bow of about 1/2 inch, how Can I strighten this with heat?

 

[ldp]

I have a hydraulic crane boom that is trapzoidal in shape, it has a bow of about 1/2 inch side to side hwo can I strighten this with heat?

 

[racookpe1978]

To klp: Your question will be better discussed in its own thread, since it is not realated to reinforcing WF steel beams.

 

[Lion06]

kslee-

A36 and a 33ksi steel might have different yields, but you can't do a transformed section as E is the same for both.

There is nothing wrong with using the appropriate Fb for each section you're checking (i.e. Fb of the top flange is different than Fb of the bottom of the WT reinforcement)- I think it is overly conservative (and unnecessary) to arbitrarily pick the lower Fb of the two pieces and use it everywhere.
That being said, it is highly unlikely that the WT would ever govern the design. The stress at the top of the (E) WF section would almost always control.

 

[kslee1000]

EIT:

Please review transformed/equivalent section method in older texts (for concrete as well as for steel), as long as the materials and loading remain in linear elastic range, the method is valid.

For mild steel with 36 & 33 yield, the Es should be very close to each other, if not identical. But for argument sake, assume the materials having different elastic modulus, then the two sections will have deiifernt curveatures at theirs interface (remember deflection is a function of E?), then how can you capture/handle the difference (which E to use)?

My thinking is, for quick and safe, use the lower fy and get over with it. For personal curiousity/experience, try the transformed section method, it's not quite straight forward, but worth to know the theoretical background of it.