moment connecting old beams to new columns

[ralphi]

Hi, the office has received this retrofit project study where an old house with wooden joists and floors but concrete columns and beams and the neighbor has proven that there is a trespassing into his property line that is about 2 feet. This would put the perimeter columns directly at the neighbor property.

Has anyone actually tried replacing columns with new ones that are adjusted 2 feet shorter (instead of 18 feet beam span.. it becomes 16 feet)? This is a general question so even without knowing the layout you can imagine it

Some team members suggested putting new steel columns and putting the steel I-beams underneath the original concrete beam edge and welding the rebars of the original concrete beam to the steel i-beam then removing the original concrete columns that got trespassed. Would this work? But we are worried about lateral movement that can affect the interface. How would you design the interface (is there a industry standard for this).

Or has anyone tried putting a new concrete column by chipping away the old column-beam joint.. but a cold joint could form right at the face of the new joint.. would this be recommended?

If these are all not recommended or done anywhere. We'd jus advise client to demolish the entire house and build a new one (but he won't agree so we may just back out from the project). The neighbor won't accept any payments over the trespassed area because he need the critical space too. Thank you!

 

[KootK]

Is this column and beam assembly a moment frame for lateral loads or just a gravity load system? If it's just gravity, I'd explore:

1) Install a steel or concrete column in the new location.

2) Assume a pinned beam connection at the new column.

3) Run some numbers to see if the beam can work without a fixed connection to the column.

As long as it's not a moment frame, I'd think that your odds are pretty good. Reconstituting a fixed connection between the beam and a new column would be pretty tough.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[ralphi]

Column and beam assembly is a moment frame for lateral loads.

Moment resisting system would be mainly the columns & beams. The old house has no shear wall due to open spaces in the ground floor for parking space. Some practical tips how to moment connect the end of the beam and the new steel column or even steel beam?

Or perhaps just pinned beam connection at the new steel column fixed strongly at its foundation?

 

[dhengr]

Ralphi:
Reread your OP, and consider the fact that we can’t see it from here, we’re not looking at what you are. Then imagine that you get 10 responses, and I’ll wager you’ll get at least 10 guesses at what you are talking about and the arrangement and conditions you’re talking about; each requiring a different solution, layout and details. We know you are “replacing columns with new ones that are adjusted 2 feet shorter,” and ‘the beam span becomes 16', instead of 18'.’ And, we know “ This is a general question so even without knowing the layout you can imagine it.” Well... KootK gave you your general answer, since we can’t imagine what you are talking about. Get on the stick, if you want meaningful answers; some sketches, plans, details, dimensions, sizes, loads, etc. some info. for us to start to really understand your problem. Then, you might start getting some meaningful help. There are plenty of smart people here on E-Tips willing to help, but they don’t like wasting their time guessing at the unknown.

 

[KootK]

dhengr's right, we would really benefit from a sketch of your situation. How many columns are involved in the moment frame along each framing line? Perhaps you can do without a moment connection to the last column and still be viable laterally. One way to recreate the moment connection would be to use a column wider than your beam and continue the column a few feet above the beam. Of course, not many floor plans can accommodate that. What size is your beam? What does the rebar look like in the area where the new column would be installed?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[ralphi]

Above shows the layout. 5 horizontal beams with span about 18 feet with columns at each end (no middle columns). The blue line indicates the actual property lines. So 6 columns will be removed all the way to second floor roof. All special moment frame seismic R=8. See actual picture of property.

Would like some tips for the following.

Rebars can be exposed after columns are demolished.. but there is a cold joint where new column would be moment connected to old beam (as can be seen above). What's the chipping pattern to recreate aggregate interlock? Would it be 100% aggregate interlock?

In the event of shoring failure or slip for instance in the 1/6 inch, would the right joint suffer crushing failure at bottom part of the joint or yielding failure at rebars above the joint? Or bending and failure of the column? Any video of actual tests?

How would you do the retrofit?

 

[spieng89]

ralphi, how are you going to address the foundations? They are infringing on the property line as well. You could consider some type of haunch between the beam and column and model the effects. I truly can't envision any joint you could make though that would be considered a moment joint without some laboratory testing to support the claims. Could you get away with a chevron bracing scheme to allow for a car to fit through?

 

[canwesteng]

Jackhammer enough of the beam to expose the rebar so you can use mechanical splices to make the top and bottom bar continuous, dowel in shear interface reinforcement with epoxy, and cast a new column. I don't see why a cold joint can't transfer moment as long as the proper checks are done.

 

[ralphi]

I appreciate all the assistances. Thank you. Raspivey. New foundation system (strip footing) would surely be put when the new columns are placed... no problem about that.

Canwesteng. The rule says splices can only be put at top of midspan due to the large amount of shear at support and if the mechanical splices were put at top of support.. there is a pulling action that could jeopardize the splice connection. Could mechanical splices be exempted from placement locations?

Have you guys seen any details of connecting old concrete beams to wide flange steel beam perpendicular to it below (this would be another idea where steel column and steel beam would be made first close to existing columns where they can carry the old beams prior to old columns removal. Think beam to beam connection)). How do you detail the moment connection between concrete beam and steel beam (below it carrying the concrete beam)? Someone suggested in cases like this. They put dowel to the old beams and weld this to the new steel beam (perpendicular... emphasis) below . But big question, has this been done anywhere before? How would it perform under seismic cyclic loading. Im afraid the bars connected to steel beam and old beam would break the old beam apart.

 

[ralphi]

Client wanted the column replacements to be done with their rented tenant still using the place due to their just renewing the contract and it is a call center with lots of employees. They can't leave even one month and the neighbor would build a 10 storey building besides it with basement parking. Their architect warned that soul erosions can cause any new columns to sink endangering the building. So they are forced to sell the property to the neighbor and agreed to relocate the tenants. Therefore we backed out of the project and part of my report says:

"Unable to establish robust special moment reconnection in multiple columns.
Initiate destruct (demolition) protocol"

Thanks a whole lot to Kootk for the very useful tips mentioned earlier.

For knowledge in case similar situation is encountered in the future. Do ACI, Eurocodes or any codes have guidelines for repair of special moment frame (such as replacing entire columns, beams and joints ) in situations such as this where the neighbor has encrouched on the property lines?

Has any of your actually done any such replacement? Please share how you do it. Thank you!

 

[canwesteng]

Can you point me in the direction of where you found a rule saying splices can't be used at a column face?

 

[ralphi]

Can you point me in the direction of where you found a rule saying splices can't be used at a column face?

See page 14 of

http://nehrp.gov/pdf/nistgcr8-917-1.pdf

"If beam longitudinal bars are lap-spliced, hoops are required
along the length of the lap, and longitudinal bars around the
perimeter of the cross section are required to have lateral support
conforming to ACI 318 - 7.10.5.3. Beam longitudinal bar lap
splices shall not be used (a) within the joints; (b) within a
distance of twice the member depth from the face of the joint;
and (c) where analysis indicates flexural yielding is likely due to
inelastic lateral displacements of the frame. Generally, if lap
splices are used, they are placed near the mid-span of the beam.
See Figure 5-11 for hoop spacing requirements."

Remember the beam-joint can have plastic rotations as it uses seismic reduction factor R=8 and the concrete can break apart with only the longitudinal bars and stirrups remaining. If they are spliced (even mechanical), they can be pulled apart. Also when you use sledge hammer or jackhammer on the concrete.. won't the bars suffer any micro brittle failure (supposed you would retain the original bars).

But the main problem is we cant put any shoring inside because of the 50 employees using the building. We need to do it with them inside so we backed out.

 

[BAretired]

Good for you. I would have backed out too.

BA

 

[canwesteng]

Agreed that lap splicing is inappropriate, but the document you provided explicitly allows mechanical splices in these locations, with the caveat they must be type 2. I don't disagree with turning down the job, since I don't think it's reasonable to do the demo and shoring while the building is occupied, but I would consider this as an option next time you're in this situation. I've done it before and the contractor didn't seem to have too much difficulty exposing the bar without breaking them, but maybe someone else can chime in.