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Door Penetration for Concrete Shearwall of Historic Building
4

Door Penetration for Concrete Shearwall of Historic Building

Door Penetration for Concrete Shearwall of Historic Building

(OP)
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.

RE: Door Penetration for Concrete Shearwall of Historic Building

No good deed goes unpunished! Be careful working for churches, for a fee or for free! Do you have separate professional liability insurance?

RE: Door Penetration for Concrete Shearwall of Historic Building

I have provided engineering services as part of community service projects. In the US, the federal government has laws that protect and indemnify volunteers. Even with no fee, you should provide a contract that references those clauses of your jurisdiction.

As far as the engineering part: I doubt the shear capacity of the wall is an issue even with 1,000 psi concrete (capacity is like 400 kips for the 90-ft wall). The check I would be concerned about is the cold joint between the shear wall and the foundation. Not know the reinforcing, you can't really rely on shear friction per se. I would just use friction between the two concrete surfaces.

That's my 2 cents.

RE: Door Penetration for Concrete Shearwall of Historic Building

Be careful about how you model this if you are in a seismic zone. This is not hard work, but it does take some knowledge of either detailed modelling, or some of the strength and stiffness (sum of squares | sum of cubes) tricks in order to show you are okay.

Don't forget your gravity case; Often in cutting a new opening for an existing concrete wall you will need to add steel reo or another form of lintel.

In detailing the cuts you can easily cause stress risers and a long term maintenance (or even safety) issue.

Good luck, and listen to Ron's warning. CYA.

RE: Door Penetration for Concrete Shearwall of Historic Building

(OP)
Thanks for all the replies.

Ron, I do not have a separate professional liability insurance. You are making me think about something I did not pay attention to really. I will look into this. What do you suggest?

MacGruber, thanks for your insight. I actually visited the jobsite this afternoon. The tenants have removed some of the interior finish to expose the concrete shearwalls. There are no cold joints vertically where we want to extend the opening. I did not examine for any cold joints of the foundation since this is not part of the scope of work. My plan is not to seismically retrofit the building. However, I will check the foundation and wall joint the next time I am there.

CELinOttawa, thanks for the heads up. Also, I appreciate you mentioning lintel. I will provide a frame detail within the opening to avoid excess stresses and distribute the future vertical and out-of-plane loads. I will have them install the lintel first, so any vertical forces induced by extending the existing penetration can be transferred to the the stiff element and down to the foundation via the adjacent concrete wall segments. Any advice on the type of cut to to minimize the chance and affect of the stress risers?



RE: Door Penetration for Concrete Shearwall of Historic Building

A few points:

- [] since this is not part of the scope of work. That argument is never going to fly. You need to investigate any and all issues/areas/problems which could be caused by your work or cause problems for your work. You stamp it, you're responsible for the lot.

- Core the corners of the new opening, rather than have cuts run into one another.

- Do a thorough locate of all bars around the opening. This can be done for a nominal fee by very capable techs who will draw the bars onto the wall and provide a report as to approximate sizes. Last time I saw this done it was approx $2k for the area around a half-dozen columns.

- Sawcuts should be done by a contractor with a good deal of experience cutting existing concrete. If they aren't isolating the area with hung sheets, undertaking dust suppression techniques (wet saw, vacuum air removal, air supply PPE, or similar) they likely aren't the right team. You don't need to be cleaning dust out of a church's rafters!

Effectively you want to make the opening with as much knowledge as possible, and then make the opening as low-impact (to the rest of the structure) as possible. The corners of concrete cuts into panels often sprout cracks as the building shifts about the load path changes... This can be entirely innocuous, or indicative of a problem, but are always disconcerting to the client and pubic. SLS fail.

RE: Door Penetration for Concrete Shearwall of Historic Building

If this is a "Historic Building" and is on the "National Historic Register", can you even add the opening, legally speaking?

Mike McCann, PE, SE (WA)


RE: Door Penetration for Concrete Shearwall of Historic Building

Legality aside, I like your 3% argument. I'd be tempted to stop there. I expect that the 13' concrete lintel above the new door can be justified using just the tensile strength of really weak concrete. I'd make sure it's not tilt up though. That could throw a wrench into things.

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

RE: Door Penetration for Concrete Shearwall of Historic Building

I would never allow just the tensile strength, or even minor/nominal reinforcement for a lintel design... Just not wise with heritage concrete.

If you test the core strength of old concrete, you quickly find that it can be all over the map. Too weak/soft in the wrong spot and your lintel fails.

RE: Door Penetration for Concrete Shearwall of Historic Building

Assuming that the roof load is delivered near the top of the wall and there's some meat on either side of the opening, this isn't really a "lintel" at all. Rather, it's a concrete arch with a wedge of concrete over the door that requires some nominal tension capacity to keep it from falling away from the arch. Similar things are done with masonry all the time.

If there's significant thrust capacity either side of the opening, then there's no point in installing installing "lintel" rebar that will never see any tension. And there would be even less point in installing a common steel angle/channel lintel that will never see any flexure.

When renovating, I use every creative trick that I can think of to steer things towards a "do nothing" solution. Firstly, I believe that it is my duty to the owner not to waste his/her $$$. More importantly, I've seen a lot of "reinforcement" that probably does more harm than good from a strength perspective.

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

RE: Door Penetration for Concrete Shearwall of Historic Building

And I've been the forensic investigating engineer on chucks of concrete that have fallen out of such an "arch".

Arches are a pure compression solution. Where you have concrete connected to, but beneath, the "arch", you have an insufficiently reinforced beam/lintel. Do you know where the micro-cracking is in that panel or wall? Depending on a great deal of factors, there could already be an inverted V of cracking and a section ready to mobilize once the wall is cut out from beneath.

I always introduce at least nominal steel reinforcement. I've also gone back and seen this steel taking load.

RE: Door Penetration for Concrete Shearwall of Historic Building

If we're going for a trip down anecdotal lane, I've seen 16' openings cut into unreinforced, unshored, load bearing masonry walls where not a damn thing happened. Not even a loose block. I think that some faith in our construction materials is justified.

In this context, the arch is not a pure compression solution. It's the compression dominated component of what is simply a deep, unreinforced beam. You can, and do, have both compression and tension stress trajectories in such a beam. If the tensile stresses are manageable, I'm happy to take advantage of them.

It's an often overlooked fact that we use concrete in tension all the time. Were it not possible to rely on it, we'd have to kiss the following goodbye:

1) Vc in concrete.
2) All of the much maligned Appendx D voodoo.
3) Unreinforced punching shear.
4) Unreinforced concrete design (don't like it much anyhow).
5) Rebar, since most of it depends on cover not splitting off.
6) Pretty much any code provision containing the term SQRT(f'c).

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

RE: Door Penetration for Concrete Shearwall of Historic Building

Easy Kootk; Not an attack on your work... Every job is a judgement call. I just won't rely on tension in existing concrete. It is not reliable.

RE: Door Penetration for Concrete Shearwall of Historic Building

KootK,

Just curious: is that the origin of your signature line?

RE: Door Penetration for Concrete Shearwall of Historic Building

Oh no... don't think that I'm feeling attacked CEL. This is just what you and I do from time to time. I cherish it. I count on you to call me out on my "unwise" engineering when that's how you see things. But you have to know that I'll return the volley if I've got anything left up my sleeve.

Continuing along those lines, I feel compelled to point out that, per my list of six above, I suspect that you rely on concrete in tension all the time. All of your past concrete projects are existing concrete after all.

I think that APEGGA should give me CEU credits for my participation in our debates.

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

RE: Door Penetration for Concrete Shearwall of Historic Building

Haha... Love it. Yes, yes, tension is unavoidable in most every fastening... I simply don't believe in having tension as a primary load path in modifications to an existing structure. Too many times I've see core test results with break strengths all over the map between 12MPa (and, though rare, far less) to the 40MPa range in the same structure.

RE: Door Penetration for Concrete Shearwall of Historic Building

2
It's not Archie. Although I think that I get your drift if you're referring to my point #5 above.

I used to work with an older, brilliant engineer who would often correct rebar detailing based on his understanding of bond stress. He stopped paying attention to code updates around 1970. I'd never dealt with bond stress so I procured some older codes and checked it out. I found it fascinating that our understanding seems to have proceeded along this trajectory:

Phase 1) We designed for bond stress, made all the right decisions, and detailed our rebar correctly. This is the majority of the last century

Phase 2) We dropped bond stress and switched to anchorage and development length concepts because we realized that, at the level of individual cracks, bond stress just doesn't reflect the mechanical reality of things (sometimes the stress actually reverses direction). In my opinion, the quality of rebar detailing suffered terribly during this phase. This is the later portion of the last century.

Phase 3) Now we have strut and tie models which have allowed us to "discover" a bunch of new important rebar detailing concepts. We're doing a better job again but we've really just relearned the great bond stress lessons that we somehow forgot during phase 2.

I find all of this forgetting and rediscovering to be morbidly humorous. Additionally, there is a parallel between how I feel about the devil and how I feel about bond stress. I believe in neither but, from time to time, thinking in those terms helps me to make good decisions. That is the genesis of my signature. Sorry you asked?

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

RE: Door Penetration for Concrete Shearwall of Historic Building

>>>Sorry you asked?<<<
Quite the contrary! That's why I asked and thanks for the education.

>>>He stopped paying attention to code updates around 1970.<<<
He sounds like my kind of guy!2thumbsup

I love perusing old codes, drawings and construction references because those old-timers really knew how to build. I guess since they didn't have computers at their disposal they had to have a good understanding of how structures behave. Also, they used good practical approximations and rules of thumb that have since been proven rather accurate.

And while bond stress might be one thing I'd be careful about not believing in the Devil. We already know he's in the details...pipejester

RE: Door Penetration for Concrete Shearwall of Historic Building

That is a fantastic summary! Nicely done Kootk... I've been learning, looking up, refreshing, and correcting my memory a great deal during out debates. It makes me very, very happy. I absolutely adore being wrong in conversations and on paper (which does not bear my seal). There is no better way to learn and to hone our art & science of structural engineering!!!

RE: Door Penetration for Concrete Shearwall of Historic Building

(OP)
CEL and KootK, thank you so much for your insight. I've learned a great deal from this thread and feel more confident tackling this job.

KootK, I have one question. Can you please explain your previous comment (above): I've seen a lot of "reinforcement" that probably does more harm than good from a strength perspective. Thank you for you sharing your knowledge - I appreciate it.

RE: Door Penetration for Concrete Shearwall of Historic Building

@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.

RE: Door Penetration for Concrete Shearwall of Historic Building

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

RE: Door Penetration for Concrete Shearwall of Historic Building

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.

RE: Door Penetration for Concrete Shearwall of Historic Building

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.

RE: Door Penetration for Concrete Shearwall of Historic Building

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.

RE: Door Penetration for Concrete Shearwall of Historic Building

Quote (CELinOttowa)

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.

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