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.

 

[Ron]

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

 

[MacGruber22]

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.

 

[CELinOttawa]

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.

 

[cruzinbear]

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?

 

[CELinOttawa]

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.

 

[msquared48]

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)

 

[KootK]

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.

 

[CELinOttawa]

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.

 

[KootK]

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.

 

[CELinOttawa]

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.

 

[KootK]

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.

 

[CELinOttawa]

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.

 

[Archie264]

KootK,

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

 

[KootK]

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.

 

[CELinOttawa]

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.

 

[KootK]

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.

 

[Archie264]

>>>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!

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...

 

[CELinOttawa]

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!!!

 

[cruzinbear]

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.