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Reinforcing existing Beam
2

Reinforcing existing Beam

Reinforcing existing Beam

(OP)
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.  

RE: Reinforcing existing Beam

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

RE: Reinforcing existing Beam

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.  

RE: Reinforcing existing Beam

(OP)
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.


 

RE: Reinforcing existing Beam

2
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
 

RE: Reinforcing existing Beam

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.

 

RE: Reinforcing existing Beam

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
 

RE: Reinforcing existing Beam

(OP)
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
 

RE: Reinforcing existing Beam

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

RE: Reinforcing existing Beam

(OP)
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.  

RE: Reinforcing existing Beam

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.

RE: Reinforcing existing Beam

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

Mike McCann
MMC Engineering

RE: Reinforcing existing Beam

(OP)
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.
 

RE: Reinforcing existing Beam

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.

  

RE: Reinforcing existing Beam

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.

RE: Reinforcing existing Beam

(OP)
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.   

RE: Reinforcing existing Beam

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

RE: Reinforcing existing Beam

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.

RE: Reinforcing existing Beam

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.

RE: Reinforcing existing Beam

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

RE: Reinforcing existing Beam

Simplest solution is to put a new channel in on either side.

Welding adjacent to the timber joists is a potential fire hazard.

RE: Reinforcing existing Beam

The most effective solution that I found was to weld a plate to the bottom flange and a pair of angle to the underside of the top flange. Use a bottom plate wider than the flange so that the weld can be from the top instead of overhead. The angles should be placed so one toe is against the bottom of the top flange and the other toe is against the wide flange's web. You may have to clip the hoizontal toe if the beams flange is too narrow and it may help to clip the vertical leg in order to use a thick angle and get more mass near the top flange.

RE: Reinforcing existing Beam

rcd445:

The weak appeal of adding angle to bottom side of flange is its back corner (right angle) against the round fillet of rolled beam. It is feasible only if the angle is attached to a plate, as thick as the fillet, that is welded prior. Then, it is a lot of field weld.

RE: Reinforcing existing Beam

Place the angles with vertical legs to the outside to avoid welding near the fillet. A section through the composite beam would appear to have two hollow tubes as the top flange.

RE: Reinforcing existing Beam

(OP)
Thanks every body.
BAretired, At this moment Column is a 3.5'' dia post with a thin cap plate. it is welded to bottom flange. what do you think is it a secure connection. To me it sounds secure. what is your opinion.

RE: Reinforcing existing Beam

rfd23,

Hopefully the cap plate is not too thin.  It is almost assuredly adequate but it never hurts to check it.  Consider the column spanning vertically between the floor deck and the grade slab.  Apply a horizontal load, H = 0.02*Cf where Cf is the compressive force in the bottom flange.  The moment at the cap plate is then Hab/h where a is distance from cap plate to floor deck, b is distance from cap plate to grade slab and h = a + b.

Since you are making the steel beam continuous, you will have inflection points each side of the column.  Please note that inflection points are not braced points, so in addition to bracing the top flange, it is a good idea to brace the bottom flange of the beam at or near each  inflection point.

 

BA

RE: Reinforcing existing Beam

rfd23:

How many spans in direction of the continuouse beam? Span lengths?

Is there a beam in parallel to the above beam on each side. How they will be handled? Will they be made continuous as well, with columns alinged? Or they are remain in present condition?

I think you need to provide broader information on layout to get feed backs on the whole, rather than piece by piece.     

RE: Reinforcing existing Beam

(OP)
Ok.
Let me scan/draw the plan and then we can discuss.
actually.
BAretired,
Cap plate appears to be very thin. May be 3 to 5 mm.

RE: Reinforcing existing Beam

rfd23,

If the cap plate is 150 x 100 x 5 and is welded to the exterior edge of the beam flange, it is too thin to develop the required moment.  You would have to add support under the plate.

At one point, you said the simple span moment is 32'k.  You later said the continuous moment is 20'k.  That seems like too much difference for the effects of continuity.  Please check it again.

To arrive at a simple span moment of 32'k with a 10.35' span, the total load on one span would be 24.7k or about 2400 pounds per lineal foot.  How is it possible to get so much load into the beam with 2 x 6 joists?  Is there a bearing wall above?  It doesn't sound right.  

Please include a cross section through the beam when you post the plan.


 

BA

RE: Reinforcing existing Beam

(OP)
BA retired,
attached is plan for the house.
yes this beam is carrying GF, First floor and Roof load. There is a bearing wall on that beam.
In my plans I am proposing 2 new posts, but client did not want any of them. in this way I have to reinforce existing footing, psot and beam. Existing footing is very thin. it is 3ftx3ftx4 in thick. it is only 4 in thick. How I can reinforce this kind of footing.
please advice.

RE: Reinforcing existing Beam

I have never run into a four inch thick footing.  It is difficult to understand how such a footing would have been approved by the authorities in the first place.  I would suggest that the existing footing be replaced with a properly designed foundation.  Meanwhile, the structure must be suitably supported.  

BA

RE: Reinforcing existing Beam

It looks like the latest scope is to remove the 2nd column from the right, thus double the span length from 8'-1" to 16'-2". The column pads are 3'x3'x4", the beam has bearing wall directly on top.

Ok, you are facing least 2 immediate problems:
1. As BA point out - inadequate footings.
2. It would take some effort to simply BEEF-UP the beam (make the 2 8'-1" as a single 16'-2" beam), if possible. Then, how about the columns?

You may include the following in your action plan:
1. Replace the inadequate footings, or do strengthening.
2. Check the capacity of the columns, replace or strengthening.
3. Replace the beams (2 @ 8'-1") with stronger shape to span 16'-2" (simply supported at columns). Or, make the existing beams (4 @ 8'-1", 1 @ 6'-3") continuous, and beef up the beam at the longest span, if feasible.

   

RE: Reinforcing existing Beam

kslee,

The drawing is a little hard to read, but I believe there will be a span of 10'-4" with a 3'-0" cantilever each end.

BA

RE: Reinforcing existing Beam

BA:

He stated the owner does not want additional columns in his latest response that was addressed to you. I could be wrong though.

RE: Reinforcing existing Beam

(OP)
Yes, Ksle
Owner want middle span to be 16 ft 3 in.
Final spans would be 8-1, 8-2, and 16 ft-3 inch.
Attached plan is my proposal which owner does not agree.
just ignore my propsal.
I checked W8x18 even with additional w8x18 at bottom flange does not work for 16 ft span.
Udl on beam is 2.35k/ft.
Max. moment is approximately 60K-ft.
Please advise.

RE: Reinforcing existing Beam

Anything can be done if the owner has the budget and the headroom for it.   

BA

RE: Reinforcing existing Beam

A quick cal indicates you need at least a W8x48. However, I have problem with the load you provided, looks like the original design load was higher than that. I will double check tomorrow and let you know my finding.

But, as pointed out by BA, $$$ is the main issue here (not me, the owner :).

RE: Reinforcing existing Beam

More questions here.

1. Is there a column above to column that is to be removed.
2. What is material of the bearing wall?
3. What the bearing wall supports, roof structure, or another floor?

Be prepared, this project is not simple. Everything has to started from ground (Foundation) up.  

RE: Reinforcing existing Beam

(OP)
kslee,
let me scan my background load calculations and put on this forum.
 

RE: Reinforcing existing Beam

Ok, your load is correct. I figured out the original design was governed by deflection, which was limited to L/800 for total load (ie. 2.2 klf).

Now, let's get to your trouble, assuming there is only wall above/along the proposed 16'-3" span (no column on 2nd floor in the middle). Again, design will be controlled by deflection. Let's find out required Ix.

Del = 5WL^4/384EIx = L/800
Ix = 4000W(L^3)(144 in2/ft2)/384E
   = 4000(2.2)(16.25^3)(144)/384(29000)
   = 488.3 in4

W8 - NG
W10x88, Ix = 534
W12x58, Ix = 533
W14x48, Ix = 485

Note, if there is a column in the middle, the situation is getting much worse.

One other thinking is to replace the top floor wall with lighter one to reduce the dead load, thus the deflection. (The existing wall needs to be removed for new beam construction, if the owner elect to go ahead for the new proposal).

Well, I think now is the time for the owner to make tough decision based on how much he is willing to spend - 2 new foundations, 2 new columns (old might work, but does not look promising), 1 new beam, 1 new wall.

Good luck. Keep us posted.

RE: Reinforcing existing Beam

rfd23,

I can't open your file because I don't have Excel 2007.  Sorry.

BA

RE: Reinforcing existing Beam

Hmm...My 2 cents

Reinforcing the existing beam:
It's never good to mix hot work (welding) around a combustible source (wood).  the beam itself will get very hot.

I have had good success with the attached sketch.  You are essentially eliminating the beam supporting the first floor framing, but still supporting the load bearing wall above, and still sending the forces to your new footings.

RE: Reinforcing existing Beam

Chip:

Good work. But it seems the owner does want adding any column in between the lengthened span (16'-3").

RE: Reinforcing existing Beam

Thanks for the pdf, kslee.  The floor live loads seem a bit high to me, but I am basing my opinion on the Candadian code.  For residential occupancy, we would design the Main Floor for 40 psf and the Second Floor for 30 psf.  This would reduce the total live load by 349.5 plf.

I cannot comment on the snow load as I don't know the location, but 30 psf seems reasonable.

Just to clarify the arrangement of beams.  From left to right will we have 8'-2", 16'-3, 8'-2"?

BA

RE: Reinforcing existing Beam

BA:

I figured the W8x18 has a flexural strength to resist the moment resulted from either 3.6 klf (Fb =0.66 Fy), or 3.27 klf (Fb = 0.6 Fy) uniform load. Both are high when check deflection - using 2.2 klf, for 8.17' span, the deflection turns out to be 0.123". For 8.17' span, it represents L/797, round off to L/800. So, looks like his load is accurate.  

RE: Reinforcing existing Beam

(OP)
Thanks every body for these useful feedback.
I will keep you posted whenever we reached to any decision.

RE: Reinforcing existing Beam

rfd23,

We will await your further comments.   

BA

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