Reinforcing existing Beam 2

[rfd23]

I have existing W8x18 beam. I need to reinforce this beam due to change in span. I did some research on welding of plates to wxisting beam. Most of the time we weld horizontal plate on bottom of bottom flange or weld a vertical plate along web, welded to undrside of top flange and top of botom flange.
Can I use two vertical plates on both sides of web and welded to web. depth of plates would be to suit welding and k of beam.
All thoughts are greatly appreciated.

 

[msquared48]

1. You can, but the "I" gained vs material used is better with the flange locations. The vertical orientation would contribute more to the shear capacity.

2. Also, the horizontal bottom flange location is less impacted by existing transverse connections to the beam than the vertical connections at the periphery of the flange.

3. If the beam is composite, seems like the connection to the top flange could be tricky unless the vertical member was placed flush with the bottom of the top flange, and the top of the bottom flange, and inset slightly.

Mike McCann
MMC Engineering

 

[kslee1000]

The solution you proposed is not quite effective, unless you can bear on both the top & bottom flanges similar to an enclosed box. Or else, you will have a hard time to make the plates as fully composite with the web, thus share the loads.

Another method you didn't mention is to add transverse braces to reduce the unbraced length. From which you may pick up adequate Fb that is required for the new set-ups. Sure it may not easy, or even feasible. Also, the Fb could be already at code allowed maximum. But, worth the investigation, and can be a part of your retrofit plan.

 

[rfd23]

Thanks both of you guys.
Welding a plate to the bottom of flange is not a problem.
It is a two storey single family house(average deatached house). 2x6 joist @ 12'' are supported on top flange of beam.In my opinion wood joist are just resting on steel beam and i can not take steel beam as fully braced.Hence I am using 10.35 as unbraced length.
Any suggestion.
I am welding 1/4''x 6.75'' plate at bottom. Ixc=84.39
y1(top)=5.089, y2(bottom)=3.301
Span=10.35ft, Mx=32Kip-ft, SX top=16.58 in^3,
Sx bottom (extreme bottom fiber)=25.57
Fb allowble Top=17.5ksi (Lb>Lu) (compression flange)
Fb bottom=21.6
fb top=M/Sxtop=32*12/16.58=22.85>17.5
fb bottom=32*12/25.57=15.01<21.6
It appears to me that welding a bottom plate only is not a solution. May be I am doing something wrong. Any suggestions.

 

[Lutfi]

The following summarizes my findings and recommendations that I found in various references when I reinforced beams under load (Last Project that I did, I reinforced a 120 foot long 8 foot deep trusses under load:

1. It is normal practice to reinforce steel members while carrying load.
2. It is desirable to reduce or relief the load on the member being reinforced if practical.
3. If proper procedure is established and followed, the heat effects should be minimized.
4. The anticipated allowable stress can be based on the properties of the new built up section.
5. The stress to be used in calculating the area of the new steel will be based on the new allowable stress of the new section less the actual stress at time of reinforcement.
6. Member connections should be evaluated (at splices and ends).
7. The use of intermittent welds is professionally recognized practice and is incorporated by AISC’s specifications.
8. Based on AISC specifications, for properly designed compression member reinforcement, it is not required to consider residual stresses for overall member.
9. It is preferable that welding is done under static loading conditions.
10. It is important to evaluate the condition of the existing steel (corrosion, loss of section, damage etc.)
11. Evaluate local buckling.
12. It is not recommended to weld transversely on loaded tension members.
13. It is recommended that all welds be done parallel to stress lines.
14. It is recommended that welding start from the middle then towards the ends and weld on both sides simultaneously.

One comment regarding finding number 13 is that it is acceptable if welding across the member is less that 50 percent of the member section. This is due to the small area of superheated steel (size less than a dime).

Reinforcing structural members under load is not a text book example. However, I found several references that have a wealth of information. Here is a short list of these references:

a. Reinforcing Steel members and the effects of welding, R.H. R. Tide, AISC Journal, fourth quarter, 1990.
b. Field welding to existing steel structures, David Ricker, AISC Journal, first quarter, 1988.
c. Reinforcing loaded steel compression members, J. H. Brown, AISC Journal, fourth quarter, 1988.
d. Welding on existing structures, Omer Blodgett, The Lincoln Electric Company, dated May 10, 1962.
e. How to retrofit and rehabilitate steel structures, Christopher Hewitt and William Liddy, Structural Engineer, September 2003.
f. Positive reinforcement, Anthony J. Panotta, Structural Engineer, April, 2001.
g. Welding repairs of historic bridges, Omer Blodgett, The Lincoln Electric Company.
h. Reinforced structures under load, The Engineering Foundation welding research council, supplement to the Journal of the American Welding Society, February 1944.
i. Modification of roof trusses and columns to support air pollution control equipment, Timothy E. Donovan,
j. Steel Interchange, American Institute of Steel Construction.
k. Verbal discussions with several well established steel fabricators and steel detailer.


I hope this helps!



Regards,
Lutfi

 

[lkjh345]

Any reason you must limit yourself to just a plate on the bottom? Can you weld a tube or another section to the bottom flange of the beam?

A TS6x4 (6" dimension horizontal) could work nicely here if you have the head room.

The increase in Ix and Sx is significant when reinforcing with another member that adds depth.

JMHO.

 

[Lutfi]

rfd, first let me suggest that you take a peek and see how the joists are supported. As engineer who will be responsible for revising an existing structure, this is a must even if you have to remove part of the ceiling. Take pictures for documentation. You state that 2x6s are supported on the top flange. This means that there must be a wood nailer that is attached to the top flange. Additionally, the joists will be nailed to the wood nailer. This is the construction that I am familiar with.
For lateral stability, it does not take much to provide lateral bracing for the compression flange. I think you are being too conservative.

I have a question, Can you add a post and reduce the span so that W8 will work?

Let me suggest that you consider adding a WT to the bottom of the W8 assuming headroom will allow this. You may not need the WT to run the full length of the beam based on the new bending moment.


Regards,
Lutfi

 

[rfd23]

Thanks.lutfi for so detail in put.
I am not sure, i can weld a member at bottom flange. Definately I will explore this option.

Uptill now, although beam is contineous on some posts, I was calculating moment as wl^2/8 (simply supported)just to be on safe side.
Actually ,
this beam is contineous over couple of 3.5" dia pipe post.
Just ordinary post as you guys have seen in a basement of single/two storey houses. Post has very thin cap plate and welded to bottom flange of beam.
I think I must take this as continuity. It will greatly reduce my moment.

Lutfi, any comment.

I am sure I can safely use this as a contineous beam.
if I go by this, max. moment is 20 Kip-ft, and I do not need any reinforcement.
please advise and discuss. Thanks

 

[BAretired]

My suggestion is to brace the top flange of the beam at 4' centers by tying into the joists with steel straps welded to the beam and nailed or bolted to the joists. If that brings the beam up to capacity, forget about welding a plate on the bottom.

BA

 

[rfd23]

Thanks Lutfi,
This is a unfinished basement and I can investigate more on that. on my first visit I did not notice any nails going through the top flange.
2x6 joists rest on wooden plank and this plank is resting on top flange. I could not see any nails. But definately i will look at it once more.
Regarding posts, client want to remove one middle post and makes span double. But I am insisting him to add two more posts on edges so i can add new footing without overstressing existing footing.
As you know housing market builders do not have much safety margin.

 

[hokie66]

Sounds to me that the top flange is already braced by the joists. If the beam is continuous over columns, the bottom flange probably requires bracing. That way, reinforcing the beam may not be necessary.

 

[msquared48]

What about welding a channel to either side, not quite the height of the web so they fit flush?

Mike McCann
MMC Engineering

 

[rfd23]

hokie66,
If there is no mechanical connection(welding/nailing) between joist and top flange, how we can say top flange is braced. It is only a bearing of wooden plank on top of flange.
Regarding bottom flange,
I think we can safely assumed that bottom flange is braced at pipe support location.
Max. span is 10.35 ft. Can I safely assume that bottom flange unbraced length is 10.35.
ln this situation Mx=20Kip-ft
and my stress ratio is 0.78 works fine. However
Lets suppose loads are high.How Can I reduce unbraced length of bottom flange in this situation.
I think if I add stiffener plates on both side of web can reduce the unbraced length. Any suggestions.

 

[kslee1000]

To your senond post.

Add bottom cover plate alone has never been an effective solution, as the NA would shift lower towards the bottom flange, thus increase the upper lever arm. The result is an increase in compression on top flange that negate the benefit gained. Always try to reinforce both flanges if in doubt. Also, watch out adequacy of end supports.

 

[apsix]

This is the best suggestion, as posted by BA(ret);
"My suggestion is to brace the top flange of the beam at 4' centers by tying into the joists with steel straps welded to the beam and nailed or bolted to the joists. If that brings the beam up to capacity, forget about welding a plate on the bottom. "

Providing lateral restraint so that it works as a simply supported beam has got to be easier than other strengthening methods.
As hokie says; make sure the bottom flange is restrained if required, especially at the columns.

 

[rfd23]

I would like to treat this beam as a contineous beam over cols (this is actual situation).
Thanks BAretired for top flange, it is a wonderfull idea and can easily be achieved.
Now,
How to add lateral restraint to bottom flange in this case.
Thanks everybody.

 

[abusementpark]

If it is a continuous beam, then you can use plastic analysis to squeeze more capacity out of it.

 

[kslee1000]

Assume you have resolved all issues up to this point, especially adequate transverse bracing, I don't think you need to worry about bottom flange at column, at which it is stabilized by bearing pressure. Unless you have a very loose column base, or lateral load, kicking out at beam level is an unlikely event.

For high lateral load concern, you may provide transverse members with the same depth as the beam, or bring a kicker from bottom of the transverse members to the bottom flange of the beam. However, I kind feel it is an over kill for most situations.

 

[hokie66]

My view is the opposite of kslee's. The beam to column connection is the most likely point of instability. The column cannot brace the beam flange, neither can the beam brace the top of the column, unless there is sufficient continuity between the two. Welded plate stiffeners both sides would probably do the job adequately.

On the other hand, I would not be concerned about the top flange. In a continuous beam, the top flange at midspan will be lightly stressed in comparison to the bottom flange at the column. Bracing, whether strictly in accordance with a code or not, is present in the form of friction. The more load, the more friction. I can't imagine the top flange moving laterally in this situation.

 

[BAretired]

Weld a 1/4" plate stiffener each side of the web directly over the column for 3/4 of the beam height. Make sure that you have a decent connection to the column. The stiffeners will be, in effect a continuation of the column through the beam and that will brace the bottom flange.

BA