Door Penetration for Concrete Shearwall of Historic Building 4

[cruzinbear]

I am looking at a historic church - I believe the construction was in the the 1920s. In plan, the dimensions of this building are 40' x 90'. In the 90 ft dimension, the concrete walls are 20 ft tall and span the entire length with window penetrations at mid-height that are roughly 3 ft wide spaced at 15 ft apart. The client wants to add a door along this dimensions which will be 6 ft wide and wants to extend one of the end window penetrations to make the door.

I need some guidance since I do not have much experience with concrete shearwalls or historic buildings. But I want to help this client since I am conducting community service and not charging the client. I do have my PE however, it's just couple years of experience.

There are no building plans. I do know however that the 90 ft dimension the walls are concrete and thus these walls are the seismic resisting elements of the structure in this direction. Both the interior and exterior walls have finishes which makes it hard to see joint lines. I believe it's cast-in-place and not tilt up - but then again I am not sure.

Are there any guidelines which I can follow to cut out the additional length of wall in safe manner? I am trying to limit the destruction of the original finishes of the inside and outside. Also, since the extension is only 3 ft, that's technically removing 3% (3'/90') of the shear capacity. Do I have to prove, show any calculations, that the shear capacity is sufficient? If so, what assumptions can I make about the existing shearwall built in 1920s without any available plans or specifications. Any other things I should watch out for?

Any advice or recommendation would be much appreciate. Thanks.

 

[KootK]

@Cruzin:

In renovation, problems are often identified and initially solved on paper. And on paper, everything looks peachy. Part of the reason for this is that renovation designs seldom account for the damage that is done to the existing structure as a result of the renovation. And there almost always is some damage. I'll share an example from my own experience.

I have had to reinforce steel roof joists for new loads on many occasions. On paper, I identify a deficiency in bending capacity. Then, again on paper, I come up with a clever solution. This usually takes the form of welding steel rods into the knuckles of the joist chords which are generally made, in my area, of very thin cold formed hat sections. Yay me! I've addressed that bending capacity issue.

The welds that I specify are overhead welds, usually done from a lift. They connect a rather thick piece of hot rolled steel to a rather thin piece of cold formed metal. Moreover, the fellow doing the welding usually spends his days welding 6' diameter pipes together in the oil fields nearby. The result: there's a decent that the weld heat will burn through the existing joist chords and/or create atrocious heat affected zones. If that happens, not only have I not fixed anything, I've probably made matters worse. I feel this way about many renovation details that I encounter in practice.

One common solution to your lintel issue might be to bolt some channels to the concrete above the new door opening. The channels address an "on paper" need for for more capacity. They likely won't do a damn thing, however, because the channels will be too flexible relative the 13' deep concrete header present over the door. The bolts, however, will do something. They'll probably cause micro cracking in their immediate vicinity and possibly marry two materials (steel and concrete) that will respond slightly differently to temperature changes. Is it a big deal? Probably not. Is it a good reason to rethink spending the owner's money on the repair? I think so.

Whenever I dream up some spiffy renovation scheme, my first check is to visualize the finished product to the best of my ability and then make sure that I can convince myself that I won't actually making things worse.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[cruzinbear]

@KootK, you are brilliant!! Thanks for your perspective.

 

[CELinOttawa]

Experience with CFS and other steel structures do not make good examples for why historic concrete should not be reinforced. Shear failures in concrete are often sudden and catastrophic. Not providing sufficient, known, shear reinforcement for a new opening in an old concrete wall is just asking for trouble.

Incompetent reinforcing schemes aside, adding external reinforcement both ensures that the required capacity is present and breeds an environment where builders aren't of the opinion that new opening can be cut into existing concrete because of the existing reinforcement.

The logic is not the same for flexure, but similar. Under reinforced flexural conditions are deaireable and provide safety through their slow and gradual failures, unless ludicrously insufficient in reinforcement.

Shear is not to be dismissed through arching actions unless the concrete strength is know and the performance conditions favourable and controlled.

 

[KootK]

In this thread, I haven't made anything like a general statement to the effect of historic concrete should not reinforced. Often historic concrete should be reinforced and that reinforcing is done in a manner that inspires confidence. In this particular case, my point is simply that:

1) There are perfectly plausible load paths available that do not involve reinforcing and;
2) Those non-reinforcing load paths should be evaluated before we jump straight to a reinforced solution.

This is just classic Blodgett logic: don't design with your heart (Link). I also don't think that preemptively humbling contractors, by way of spending the owner's money, is a valid reason to pursue a reinforcement scheme.

And it wasn't my intent above to suggest that an anecdote from cold formed steel repair is wholly transferable to a historic concrete repair. The OP asked me to explain my "sometimes reinforcing makes things worse" comment. I felt that the joist reinforcing example was the most salient. Of course, there are no shortage of concrete repair examples if we want to explore that.

I think that we need to retain our grasp on the proportions of our "lintel". It's six feet long and about thirteen feet deep. As a consequence of the need for stiffness compatibility, the only steel reinforcement strategy that would do a lick of good for gravity loads would be something like a full depth truss. Baring that extreme option, the shear resisting mechanism is arching / strut & tie. Either it works based on material properties that are established / conservatively assumed -- and it almost certainly does -- or the OP is in trouble and will need to explore other options. Given the lintel proportions, a diagonal tension crack through the beam would have to occur at an angle of around 75 degrees. That's not shear... it's shear friction. And though a plane that crosses a compression strut. I really just don't see a shear failure here.

Also because of the proportions of the lintel, the flexuraly reinforced scheme is going to behave as a tied concrete arch where the tie is redundant because of the thrust resistance available on either side of the opening. As a result, it will be exceedingly difficult to force tension steel yielding in the reinforcing to be the first mode of flexural failure.



The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[CELinOttawa]

I don't disagree Kootk; I just think that a number of your posts could be easily misinterpreted as representing that existing concrete should often be allowed to arch. It is not sufficiently reliable. In the case of such a large panel above a new opening, I am more concerned about panel stability and secondary stresses.

I think it is clear that you and I are not always going to agree, but I greatly respect your posts (and, by extension, your work). I simply believe that the existing structures should always, or nearly always, be reinforced.

 

[KootK]

I just think that a number of your posts could be easily misinterpreted as representing that existing concrete should often be allowed to arch.

In this, you've got my number. I do think that in many instances existing concrete arching should be exploited. As you say, we'll just have to leave that as a philosophical Gentleman's Disagreement.

As for real concerns in this situation, mine would actually be in regards to out of plane behaviour as well. Whether or not the "lintel" could serve as a girt and whether or not the "piers" on either side of the opening could do their job as wind posts etc.

Reinforced or not, my gut feel is that the OP will end up with a single diagonal crack emanating from the newly created top corner and extending up and away from the opening. Any chance this is a permanently exposed wall CruizinBear?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.