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Removing existing columns
4

Removing existing columns

Removing existing columns

(OP)
Hi

Please find the attached file.
The object is to remove the two columns to have a larger open space. The thicker walls are stone masonry built about thirty years ago; the thinner walls (extension) are hollow concrete blocks built about ten years ago. I have the thought of introducing permanent W12 beam supported by two W8 columns spaced 9.8m clear distance located in the the thicker and thinner wall interaction (before cutting off the columns). The columns are to be supported on single footings. I have a limited depth for the beam of 30cm due to the clear height required for the ceiling. My concerns are
1- The lateral- torsional buckling of the beam
2- Constructability due to the enclosed area. I suppose the beam to be cut into two pieces and spliced.
3- Immediate deflection of the slab
4- Should I use nonlinear analysis?

RE: Removing existing columns

Im not a buildings guy but:

1. I think youll struggle to get a 300mm deep beam to work (depends on the loading regime and lateral restraint) but thats why you do a quick back of fag packet calculation.
2. Constructability I think you'll struggle as well although Im not sure about the context of this open space (is it in isolation or inside a larger building?"). Even if you splice it into two pieces youll have difficulty getting the splice to work due to the maximum moment occuring in the middle of the beam.
3. You will likely need to use props or similar, surely theres some design guidance about that illustrates how this is done in a pdf etc

gl

RE: Removing existing columns

2
Hell, do as the famous architect did it. Frank LLoyd Wright just took them out. So the ceiling sagged, so what.

RE: Removing existing columns

Are the foundations adequate to support the new loads on the wall?

What loads is the new beam supporting?

What is the construction of the roof/ceiling?

For the span you could be looking at a beam in excess of 18" deep. You have to minimise deflection with masonry.

With any change in loading regime, you could be looking at cracking and the client should be made aware of this.

Dik

RE: Removing existing columns

OG: Great post...

RE: Removing existing columns

dik: In his Spring Green, WI house you could see the result, but with his short stature, what the heck, he didn'b hit his head. Small height doors also his idea.

RE: Removing existing columns

Oldestguy:
Frank LLoyd Wright was actually just thinking ahead. Once he knew where the low spot in the roof was, he knew approx. where he would need to place a bucket to catch the leaking rain water which he was also well known for.

RE: Removing existing columns

(OP)
Thanks

Quote (ukbridge)

2. Constructability I think you'll struggle as well although Im not sure about the context of this open space (is it in isolation or inside a larger building?"). Even if you splice it into two pieces youll have difficulty getting the splice to work due to the maximum moment occuring in the middle of the beam.
It is inside a larger building

Quote (dik)

Are the foundations adequate to support the new loads on the wall?

What loads is the new beam supporting?

What is the construction of the roof/ceiling?
The beam will replace the two columns.
The new beam is supporting 18cm thick roof slab, self-weight and service load of 3kN/m2 as dead load and 1kN/m2 as live load
Do you mean the material? It's RC concrete

RE: Removing existing columns

OG: I understand that he was somewhat abusive to his staff...

Dik

RE: Removing existing columns

Putting two and two together, it doesn't seem particularly viable to me.

How are you going to get the beam in, through the window? Let alone all the equipment needed to break out the columns, retrofit the beam into the wall etc etc. Maybe a more experienced engineer has a good idea, draw some sketches showing the construction sequence and show it to the contractor if it cant be done it cant be done.

Nevertheless I don't think 300mm will be enough, do a simple 5minute hand calculation to size the beam wl^2/8 etc assume its unrestrained as theres no composite action between the slab and the beam.

loaded width B = 0.5*11 = 5.5m
1.35gk+1.5qk = 1.35*(3+0.25*0.18)+1.5*1 = 5.6kN/m2
UDL = 5.6*5.5 =31kN/m
M = wl^2/8 = 31*10^2/8 = 387kNm

For a LTB Lcr = 10m, a UB 356x171x67 gets you about 120kNm need a new plan i think

RE: Removing existing columns

UKbridge... I just used a WAG for spans and trib width due to the 2 span condition... might have an increase in 20% for this. Due to the flange being supported, I wouldn't consider LTB. Also based on CSA Standards... not Eurocode.

Dik

RE: Removing existing columns

@dik yes apologies, agree with you there reasonable to assume fully restrained.

RE: Removing existing columns

No apologies necessary... fully understood what you had done. Purpose was to get a magnitude of the size. The spreadsheet is written using variables, and, I can cut and paste the beam design part so I may have 20 beams in s single sheet.

Dik

RE: Removing existing columns

(OP)
Hi

Quote (dik)

UKbridge... I just used a WAG for spans and trib width due to the 2 span condition... might have an increase in 20% for this. Due to the flange being supported, I wouldn't consider LTB. Also based on CSA Standards... not Eurocode.
WAG for spans?
The spans are clear from the drawing
You wouldn't consider LTB?
Can you assume the flange being supported?
Do you assume composite section? There isn't shear studs.
I'm apologizing if I missed something.

RE: Removing existing columns

hoshang:

I was in a hurry and just 'roughly' did the sums... I would secure the flange to the slab and not worry about LTB... yes, the flange would be supported...I would not consider composite action... my spreadsheet is for simple span... I didn't even consider composite action, else I'd have used a different spreadsheet. I think you pretty much touched on everything.

Dik

RE: Removing existing columns

(OP)

Quote (dik)

I would secure the flange to the slab and not worry about LTB... yes, the flange would be supported
Thanks.
Isn't there dry pack between the new beam and the existing RC roof?
How the flange is secured to the slab and supported?
A sketch of this situation would be helpful.
LTB of the beam is my first concern.

RE: Removing existing columns

Drypack if needed, and bearing pads at the ends...and, the natural camber of the beam upwards.

Dik

RE: Removing existing columns

JAE: clicked the forum policies, by accident and nearly read them <G> Just realised that I've never read the forum policies. Good link and the summation of the work was good. Missed it, somehow.

Dik

RE: Removing existing columns

Just noticed the loading on my sheet was wrong... corrected. W18x55 no longer OK, need a W21x55. I've highlighted the loading, trib width and span. Your live load seems a little light. Ignore B02 and B03... that was from another project and has used your design loads. I'll update my spreadsheet so that the loading is carried forward.

Dik

RE: Removing existing columns

Why add a channel when you can secure the flange to the concrete? Just an added expense and a more difficult shape to move/transport.

Why the hangup on LTB. Best if you can avoid it by securing the flange. Using a channel is a backward approach, and you may not find a channel large enough. With a channel comes greater instability.

An HP16x183 is 3 times the weight. It is far more costly, being a less common section and may not be available in higher strengths and is far less common. It is also far more difficult to install in a confined location. Consideration of same is really 'silly' to quote Monte Python. I've updated my spreadsheet to include for Class of section, and if you use Zx and not Sx, the W18x55 works (I don't know what other codes allow).

Dik

RE: Removing existing columns

(OP)
Hi

I tried W12x210. It seemed OK. What do you think?

RE: Removing existing columns

hoshang, you seem to be trying to use shallower and heavier sections than others are proposing. Is there a depth limitation for the beam? Otherwise, a deeper and lighter section is far less expensive, stiffer, and doesn't add as much load to the foundation.

RE: Removing existing columns

(OP)

Quote (HotRod10)

Is there a depth limitation for the beam?
Hi
Please read the original post (the first post). Besides, LTB is the main concern. So I try finding section with wider flanges to have less lateral torsional buckling concerns.

RE: Removing existing columns

Quote (OP)

LTB is the main concern.

From an earlier post, "Why the hangup on LTB. Best if you can avoid it by securing the flange."

Dik

RE: Removing existing columns

I thought maybe there was something later about the depth limitation that I missed, since Dik was proposing W21x and W18x sections. Anyway, it doesn't take anything very strong to provide lateral support against LTB, but it probably would mean attaching an angle or something to the top flange to lock it in. Heavier and simpler vs. lighter and more complex - I guess whether it's worth it, depends on how much weight you can save by providing lateral support. It wouldn't have to be continuous; maybe discrete bracing could be an option?

RE: Removing existing columns

(OP)

Quote (dik)

again, your live load seems light.
Hi
I changed Live load to 2kN/m2. I tried W12x190. It seems OK.
Other concerns:
1- My thought is to use 3 pieces of the beam (2.5m on each sides, about 4m in the middle) and splice them. What do you think?
2- I'm familiar with American steel sections (not familiar with Europe steel sections); here only Europe steel sections are available (Turkish make). Does this section has equivalent in Europe?

RE: Removing existing columns

If it has to be 3 pieces, then so be it, but if you can do it with 2 pieces, even if the splice ends up near the middle, one larger splice is more economical than 2 splices. Presuming your design code is similar to the AASHTO bridge code, a minimums for the splice (size, number of bolts, etc.) may dictate the 2 splices end up the same size as if you just used one. Do you know the maximum piece length you can use?

RE: Removing existing columns

(OP)
Hi dik
Thank you.
I'm sorry. I tried a little bigger span (I used 10x10m)
W12x170 doesn't verify.
Mry/(Fib*Mny) = 1.06 > 1.00 LRFD (H1-1b) Not verified
uzt = 4.4 cm > uzt max = L/240.00 = 4.2 cm Not verified
What about my concerns in my post on 9 Jul 18 at 17:20?

RE: Removing existing columns

W12x170 doesn't verify... are you using the Plastic section modulus? I get a Mr=1030+ just a quick calc... and a Mf=695 based on the trib width and span. Def'l is slightly below L/180. Maybe different codes? Fy = 350 MPa, and phi = 0.9 and Zx = 275 in^3. For Canadian codes, if you have a Class 1 or Class 2 section, you can use the plastic section modulus and not the elastic one.

I'm not sure what the difference between 4.4 and 4.2 cm is... looks like a couple of millimeters.


Dik

RE: Removing existing columns

(OP)
Hi dik
Thank you
Not very different. I think W12x170 is OK.
What about my concerns in my post on 9 Jul 18 at 17:20?

RE: Removing existing columns

(OP)
Hi
Other concerns:
1- My thought is to use 3 pieces of the beam (2.5m on each sides, about 4m in the middle) and splice them. What do you think?
2- I'm familiar with American steel sections (not familiar with Europe steel sections); here only Europe steel sections are available (Turkish make). Does this section has equivalent in Europe?

RE: Removing existing columns

(OP)
Hi

Quote (dik)


Are there any thoughts on my concerns?

RE: Removing existing columns

(OP)
Hi

If I couldn't find equivalent Europe steel shapes for wide W12x170, I'm thinking of using built-up section.

RE: Removing existing columns

Well, if you have only 12" of depth to work with, you'll have to go that way anyway - a W12x170 is 14" deep, as is the European HE 320M, which would be the rough equivalent.

Is the roof deck something you could make composite with the beam? If you're going to have something fabricated, it may become advantageous to add some type of shear connectors, if the roof deck can be used as a composite top flange. Otherwise, providing lateral bracing at a few location should become easier.

RE: Removing existing columns

(OP)

Quote (HotRod10)

a W12x170 is 14" deep, as is the European HE 320M, which would be the rough equivalent.
Thanks. maybe HEM320 the best choice.

Quote (HotRod10)

If you're going to have something fabricated, it may become advantageous to add some type of shear connectors, if the roof deck can be used as a composite top flange
Please read the original post carefully. The slab is already existing. What do you suggest as shear connectors for this case? Provide a sketch, please.

Quote (HotRod10)

Otherwise, providing lateral bracing at a few location should become easier.
How providing lateral bracing? In early posts I suggested using Cap Channel on the W12 beam. What do you suggest for lateral bracing? Provide a sketch, please.
https://www.screencast.com/t/FimMajImEN
What about using 3 pieces of the beam (2.5m on each side and 4m on the center) and splicing them (instead of using one heavy and long piece) as a construction method due to the enclosed (limit) space.

RE: Removing existing columns

You don't need lateral bracing if the beam isn't susceptible to lateral torsional buckling. LTB is only a concern where the top (compression) flange is susceptible to lateral displacement and rotation, the mechanism which causes a beam to fail due to LTB. I think so long as the slab fits snugly against the slab soffit, and the fact that the beam is roughly equally loaded both sides its acceptable not to consider lateral-torsional buckling even if theres no composite action between the beam and slab.

I would absolutely not use a channel, the last thing you want to do is introduce any more potential lateral stability here.

Ultimately its a bit of a judgement call, but unfortunately makes a massive difference in the capacity of your beam. I think @HotRod12 makes a good point, if theres any doubt in your mind use an angle or something to fix the beam to the slab or at the least spend a bit of time reading about it if nothing else for a bit of robustness. If you can get a beam that works for Lcr = 10 within the construction depth, then happy days just use that and move on, its a single beam at the end of the day and its peace of mind for you.

Regarding splicing its basically a calculation isn't it to see whether or not its viable. It comes down to what you can actually fit into the building, if 2 pieces can fit, and you can get away with a single splice at midspan then great.

gl



RE: Removing existing columns

"The slab is already existing."

I assume that means it's concrete? Making it composite may not be an option worth considering then, as it would likely require chipping out a significant amount of concrete while leaving the reinforcement and then grouting the "pockets" around the shear connectors. Currently, the maximum spacing for groups of shear connectors in the AASHTO bridge design spec is 2 ft. Based on new research, there is a proposed change to increase it to 4 ft, but you would have to comply with your governing design code in that regard, anyway.

"How providing lateral bracing?"

I don't know what the configuration of the roof system is, so I wouldn't know what to propose. What I can tell you is that it doesn't take anything particularly strong to prevent LTB, so long as it doesn't allow any significant lateral movement before it engages. In other words, as long as the beam remains straight, there is very little lateral force exerted. Generally, even small angles placed tight against the edges of the flange, and attached solidly to the roof diaphragm, should provide adequate lateral support. Your governing code should be consulted as to the capacity requirements for the beam to be considered braced. I would not recommend attaching the beam directly to the roof diaphragm, as the resulting composite action could induce large shear forces in the connection, unless provision is made to allow the beam and roof to move independently in the longitudinal direction while preventing transverse movement (i.e. slotted connections).

RE: Removing existing columns

During the houseparty that floor will be jumpin'.

RE: Removing existing columns

"I think so long as the slab fits snugly against the slab soffit, and the fact that the beam is roughly equally loaded both sides its acceptable not to consider lateral-torsional buckling..."

You can't count on friction to prevent LTB, nor does the loading being equal help. If the beam is susceptible to LTB, then it must be rigidly braced or the design capacity will be reduced. However, given the large flange sizes that are required here, and the short span, the capacity reduction due to LTB may be small or nonexistent. at most, one braced point at midspan would likely eliminate any LTB capacity reductions.

RE: Removing existing columns

(OP)

Quote (HotRod10)

In other words, as long as the beam remains straight, there is very little lateral force exerted
Thanks
What's the purpose of the new beam? It takes loads from the existing slab (by tributary area or whatever method). This load induces moment in the new beam (compression stress on top flange and negative stress on bottom flange). The compression stress in the top flange will tend to buckle the top flange while the tension stress in the bottom flange tends to straighten the bottom flange. This will induce LTB in the new beam. Every beam strength formula is based on the restrained length along the beam (whether it's at third point, quarter point, center point,etc.). So, LTB is the main concern for the new beam. This is my understanding of LTB.

RE: Removing existing columns

You could put beam above the roof, then put rods down through roof to pick up slab. Done that a couple of times. It will remove head-knocker at ceiling. Just need to make weather tight and brace for LT buckling.

RE: Removing existing columns

(OP)

Quote (hoshang)

What about using 3 pieces of the beam (2.5m on each side and 4m on the center) and splicing them (instead of using one heavy and long piece) as a construction method due to the enclosed (limit) space.
What about this splice arrangement?
https://www.screencast.com/t/gegFBiOCp

RE: Removing existing columns

Best to talk to a fabricator to see what problems there are.

Dik

RE: Removing existing columns

hoshang, the only thing missing from your description is the mechanism by which LTB occurs, that being the eccentricity. The lateral moment in the compression flange which causes the buckling is a P-delta effect - axial load multiplied by the eccentricity. No eccentricity = no moment = no buckling. It's similar to the ponding effect, where the farther the flange displaces laterally, the more moment there is, and the more moment there is the farther it displaces. Limit the unbraced length, and you limit the lateral displacement.

"So, LTB is the main concern for the new beam."

Depends on the lateral moment of inertia of the compression flange. The wider the flange and the shallower the beam (the displacement is restrained by the bottom flange which wants to stay straight), the less susceptible it will be to LTB. Your beam may be shallow and wide enough that the capacity reduction may be very small or zero, even if unbraced. You'll have to do the calculations for the beam configuration you choose, see what the reduction in capacity is, and decide if it's worth the effort to brace it.

RE: Removing existing columns

(OP)

Quote (dik)

Best to talk to a fabricator to see what problems there are.
Thanks. What problems do you expect with this detail? Mine I would suppose leveling two heavy pieces of the beam.

RE: Removing existing columns

(OP)
Hi

I have other concerns. Now, it's about interfering of the new foundation for the 2 steel columns supporting the new W12x170 beam with the existing 2 single column foundation. These 2 existing foundations can't be removed before the construction of the new steel frame is completed. So my thought is that we don't remove the existing footings and drill holes and insert reinforcing bars into the existing footings so that it spliced with the reinforcing bars of the new footings.

RE: Removing existing columns

Drilling into existing concrete to install dowels is fairly common. A couple of words of caution based on our experience, though:

1) Proper installation of the adhesive anchorages for these doesn't happen without strict oversight. If there isn't someone watching the installation carefully, the holes don't get cleaned thoroughly, so the epoxy doesn't adhere to concrete. Also, the two-part epoxy often doesn't get mixed properly and doesn't cure.

2) Because of the questionable installation practices, we don't count on splices with dowels to transfer moment. If we need the connection to transfer moment, we require chipping away the concrete to expose enough of the existing reinforcement to install mechanical rebar splices.

RE: Removing existing columns

(OP)

Quote (HotRod10)

based on our experience
Thanks
based on your experience, Placing the new steel column foundation on top of the existing column foundation (using adhesive anchors for connection between the two foundation) is better? Or placing the new steel column foundation at the same level as the existing column foundation (using adhesive anchors for connection between the two foundation) is better?

RE: Removing existing columns

Not sure what you're asking, but essentially my caution is to not rely on tension capacity from a bar set with an adhesive anchor. They should be ok for shear.

RE: Removing existing columns

(OP)

Quote (HotRod10)

my caution is to not rely on tension capacity from a bar set with an adhesive anchor. They should be ok for shear.
Thanks.
If the new and existing footings are at the same level, adhesive anchors (on sides) can be beneficial for resisting differential settlement between the new and existing footings.
I'm sorry, I think I should switch to Foundation Engineering thread if you (Structural engineering) agree, since the discussion is now about foundation.
The discussion about the steel frame was very helpful.
The discussion ended with using W12x170 for the beam (as dik suggested).
Thank you all, especially dik, HotRod10, etc.

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