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Single Angle Flexural Design question...

Single Angle Flexural Design question...

Single Angle Flexural Design question...

(OP)
Hi everyone, First let me say that I have been reading a lot of the posts in here regarding single angle design. I have looked into the steel manual and examples. I just would like if someone can help me clarify one thing. Let me explain, I am adding 2 single angles of equal legs to a girder which spans 46' in a building done in 1912 (Yeah the owners want to keep almost everything), now I don't know if this will work obviously but I want to see what my design results are. Now being that I will be adding bolts to connect both angles on each side of the girder, this is a 2nd floor by the way,and I have the joists connecting to the girders @ 18" O.C. I figure that I can design this angle as "continuous lateral-torsional restraint". Yes? So am I to use F10-2 or F10-3 equation and simply use my 18" as the "laterally unbraced length of a member"?

Thanks in advance.

RE: Single Angle Flexural Design question...

It sounds like from your description that you are adding the angles as flange reinforcement, although they are attached to the web. So they are essentially in tension, or maybe in compression, depending where they are being added relative to the depth of the existing girder. Although part of a flexural member, the angles themselves would be axially loaded, provided my interpretation is correct. Perhaps you can provide a sketch or a better word picture.

RE: Single Angle Flexural Design question...

Where on the girder are the angles? Near the top (compression) flange? Just under the flanges? Bolted to the web?

A sketch might help.
Thanks.

RE: Single Angle Flexural Design question...

I thought this was a steel girder, but see now that you are still working on the grossly undersized wood beam in your earlier post. I don't hold much hope for your approach of adding steel angles, as the beam is still much too shallow...maybe big channels each side would be a starting point...or a central column.

RE: Single Angle Flexural Design question...

(OP)
Well I proposed a central column on each one of those girders and it was a hard NO from the owners and architect. Channels was my next approach, but I wanted to see my results on adding angles. If it were up to me I would start working on possible plate girders, but its not up to me and I just want to put as many options together as are probable to work. This has proved to be a difficult task for me. I appreciate the input.

RE: Single Angle Flexural Design question...

I'm guessing that those are not continuous 46ft 2x10's? So they are spliced at least once? That seems pretty optimistic... even with channels at 46ft. - but to get closer to your question I would think the angles are acting as tension elements more than flexure if you are trying to make them composite with the beam and locating them on the bottom.

Have you thought about making it a truss with the existing girder as top chord?

RE: Single Angle Flexural Design question...

The first thing you need to do is to assert who is in charge of the structure, which must be the structural engineer. Architects often want to do impossible things (refer to the current Skyhooks thread in this forum), but we engineers deal in reality. My starting point would be to accept that the existing beams have failed, as there is no possibility of them being adequate in either strength or serviceabily, notwithstanding their age. And my second decision would be that the wood beams are to be ignored in the flexural capacity of the reinforced floor.

RE: Single Angle Flexural Design question...

If the Arch. wants to keep that beam, tell him you’ll cut it up into 8' length and pile it on his drafting table. That will get it out of this bldg. and maybe prevent him from drawing stupid architect things. Design a beam or truss which will actually carry the load, maybe add a column or two and you might actually be able to save the existing joists. Someone who knows what they are doing and has some Structural Engineering experience and judgement has to take charge of this job, and tell the Arch. and owner what is practically possible, irrespective of their dreams and wishes.

RE: Single Angle Flexural Design question...

(OP)
Bookowski: Yes the girder is spliced at least once. I will look more into the problem to see if I can see it your way regarding the tension vs. bending. I was hoping more for the angles to just take over most of the needed capacity. At first I wanted to set the angle on top, but for construction reasons I figured on bottom. In our meeting we discussed making the girder a truss, adding a beam underneath, and the channel/angle approach. I went with the channel/angle approach first. Im just not optimistic about adding any wood members for the 46' span. I really wanted to just make it steel.

hokie66: I couldn't agree with you more. I just want to have more calculations in hand before I tell my boss I don't see any other possible option but to add a vertical support.

Now, as to my original question regarding angles that are laterally supported. Is equation F10-4a or F10-4b x 1.25 appropriate using 18" O.C. for L? Being that the angle is laterally supported and not just @ midspan.

Thanks for the replies everyone!

RE: Single Angle Flexural Design question...

Have you run any initial numbers? I think you're going to find that the unbraced length is irrelevant - assume 0" unbraced length and see what kind of angle you'd need. You didn't specify what kind of loads you are carrying but even in a wf it would make sense to have a w18 or 21 for this span - an angle isn't going to cut it. Run some numbers.

RE: Single Angle Flexural Design question...

Agree with all said preceding. A second floor girder spanning 46', wood 2x10s? Sounds more like maybe a 1910 roof originally. what is your tributary width? Single angle flex is tricky. I wouldn't even look at it here, not because of trickiness but because it is self evident (to me at least) that it is a non viable solution for deflections for sure and almost for certain for bending due to the shallowness of the section. Good luck!
A quick confirmation for you if you wish, try running a channel .. U shape...flat, oriented as your drawing indicates, thus removing the dual axis asymmetry from the problem. I am sure you'll find it doesn't work. If that doesn't work, adding the single angle torsional item only makes it worse. I think boss will agree.

RE: Single Angle Flexural Design question...

Is your boss trying to punk you? Seems like she/he should already know, or at least have a sneaking suspicion, that a 10" deep wood girder won't span 46'.

Share with us your uniform design load for the girder. Maybe it's not really that bad...

RE: Single Angle Flexural Design question...

Hemi, are you sure that thing isn't the bottom chord of a timber truss? The bolt pattern in your sketch makes me think that it could be. Well, that and the fact that floor didn't collapse 100 years ago.

RE: Single Angle Flexural Design question...

I agree with Archie. The 6 - 2x10 members are probably the bottom chord of a truss. Check it out.

BA

RE: Single Angle Flexural Design question...

(OP)
Tirangled: Tributary width for joist is 18", spacing between girders is 20', girder span is 46'. The angle or channel didn't work. Not even close. Thanks for the input.

PUEngineer: Im pretty sure he wants to train train me. He never said he thought it would work, he simply asked me to put together solutions. UDL is 500 lb/ft.

Archie264: Its not a truss. I went in there and observed it. It was hot as hell and not much space to maneuver, but If this is a truss its the first one I see like this. Please see the attachment and tell me what you all think. Sorry for the bolt pattern, that's just something I put in to throw out an idea as how to bolt the angle to the girder, but the angle and channel are out of the question. The floor has deflection. Several inches in some areas, and when standing at the girder location it can most certainly be felt as to when standing over weak joists area. This I feel is mainly because every joist is notched with only 2" (out of the 10") bearing over the girder.

Going with a W21x55, so far it makes it. Gonna keep at it.

Thanks for tuning in.

RE: Single Angle Flexural Design question...

So those 6-2x10's are actually spaced as per your previous sketch?

thread507-355131: Deflection analysis aproach? Please suggest.

Sounds like you are on the right track now, with a big steel beam. If you only use a single beam at each "girder", then you will need headers to transfer the joist loads to the beam. And that connection of the joist to the "girder" is no good...install joist hangers. What a mess!

RE: Single Angle Flexural Design question...

(OP)
hokie66: Yes those are it. That drawing from my previous post I did to include in my report. Thanks for the advice on the headers and everything. I really appreciate all the help. Being that I solved for the inertia by inputting the allowable deflection my results are marginal, but flexure design is easily covered.

Thanks again everyone.

RE: Single Angle Flexural Design question...

I would remove the wood beams and replace with steel.

Wood-steel composites just don't work with that connection. Too much crushing at the bolt/nail connection to make composite action viable. Look up some of the experiments from the army regarding this (don't have time at the moment).

RE: Single Angle Flexural Design question...

The new beam will be narrower than the existing wood beam, so the joists will not reach far enough. How is that going to work?

BA

RE: Single Angle Flexural Design question...

If the joist line up on each side of the girder you can remove remove the existing beam, splice across to make the joists continuous and then add the new steel beam to the underside of the splice.

Probably easier to just add a new beam on each side and leave the existing in place.

RE: Single Angle Flexural Design question...

If the issue is maintaining the appearance, install a new steel beam, take out the wood beam and use the wood from the existing beam to build a sort of facade box around the new steel beam.

RE: Single Angle Flexural Design question...

I suggest a nice HSS member. You wont have to worry about bracing the top flange, and you could just put it to one side or the other and get the full bending capacity of the element. Then use a long threaded rod thru-bolt with a plate washer to grab all those wood beams.

Ignore the beams altogether, design the new beam for all the loads. Check the wall for the loads and get the wall connx to work.

RE: Single Angle Flexural Design question...

Adding those angles won't do anything. What I suggest is removing the beam, installing an I-section of sufficient capacity, and then cladding the I-section using shaped pieces of the original beam to maintain the appearance.

RE: Single Angle Flexural Design question...

Hemi79:
In your previous thread about deflection calcs., the 2x10 joists spanned 20' btwn. beams and were spaced at 16" on center. Now they are spaced at 18" o/c, and it seems to me there was some mention of the jst. end bearing on the beam being notched. And, you have not really shown us a good section of the girder and the jsts. bearing on it, to scale, so we can understand the relative locations, sizes, etc. Rather than pick a WF off the self, I might be tempted to fab a box beam, where I can very the width and the depth to better fit this situation. Then, I might extend the bot. flg. on this box beam 4" beyond the two webs, as a seat for a 2x4 or 4x4 onto which the old jsts. can now bear. I would support the existing jsts. on either side of the existing girder, shore them up, remove the existing girder, snap a line on the jsts. and cut them to a new, slightly shorter length, getting rid of the notching. I would nail a new rim jst. to the ends of the existing jsts., then move my new box beam up into position. All we’ve seen so far is some photos which show a whole bunch of junk going on around the existing girder, so you have to really study the details and how you will clean this mess up. So, you can finally get a new girder in there, and properly support it. Also, you still have to study the existing jsts., they might have actually checked under older versions of the bldg. codes, for 2nd floor loadings, but may need some exceptions/exemptions under the latest codes.

RE: Single Angle Flexural Design question...

dhengr,
In the sketch in his previous thread, he does give a good idea of how the 'girder' is configured. With spacers/splices, the wooden beam is probably 24" wide. Thus my comment above that he will need headers under the wood, over the steel, to bring the load to the steel beam...if he only uses one beam. A box beam might be a bit much. The joists are poorly supported, but that can probably be accommodated with joist hangers. May be simpler, as CANEIT suggested, just to provide a steel beam on each side. Whichever solution is adopted, there will need to be a lot of jacking and shimming to level everything up.

RE: Single Angle Flexural Design question...

(OP)
dhengr: I apologize for the O.C. mistake. They are actually @ 18" O.C. My calculations reflect the 18".

CANEIT: It seems like a great idea. I will look into it. Its not easy adding vertical supports at the ends of the girder because at the end of the 46' span there are rooms on each side. I say this regarding the idea of adding the beams beside the girder. It would put my connection too far from the new girders.

hokie66: I will be implementing headers. thanks.

I have not looked into an HSS member to be honest. I have thought about adding at least one plate over the steel girder for bearing improvement, but the H21x55 works without the plate (except for bearing). As for the facade, the box (fake beam) that is exposed in the ceiling is framed from the girder down to the ceiling height. There is a few ft. difference between the loaded girder and the exposed molding.

I appreciate all the help. I am going to propose the existing girders be considered for removal. I will recommend a thorough inspection to all parties for deflection confirmation. I don't see how the steel girders will eliminate the existing deflection of the wooden girders. Yes it will not deflect for years to come but what about all the movement that has happened. I agree that all this is pretty messy. I have reiterated that to, well I should say WE, we have voiced our concerns strongly in not having complete access to the whole structure. I have personally mentioned it might even end up costing more for them if we end up over designing due to wrong assumptions. I don't like the idea of keeping the existing joists. All that notching is just a hard NO for me. We will propose appropriate connections for all joist to headers over steel girder.

Thanks again.

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