New Openings in Existing Masonry Buildings

[preynolds]

OK. It seems we are catching alot of flack lately when designing lintels and jamb columns for new openings in existing CMU buildings. The openings typically vary in size from 14' to 38'. We added wind columns and bearing columns to break up the spans on the long openings. The issue seems to be that contractors/architects can't seem to grasp that the end of a large opening in an ungrouted CMU wall needs some vertical element for a jamb/wind column, be it a new steel column behind or reinforcing in the existing. To try to get the existing wall to check, we would typically utilize up to 32" of a pier element at the opening end to resist the out of plane wind loads. We typically don't have much luck adding reinforcing and grouting solid, hence the introduction of the steel column, which in turn causes complaining about overdesigning, unnecessary, etc, etc. The beauty of this example is that if it was new construction, no one has a fit if we have a 24" jamb column with (6)#6 bars.

So, a few questions....Are we examining the jamb columns correctly? Does anyone utilize more than a 32" max? How do you answer when someone questions the jamb columns?

Thanks for any help.
Pat

 

[Lion06]

Is the opening mid-height of the wall? It would be nice if it could shoot right into the diaphragm and then you never have to think about wind on the supporting columns.

If it's not at the diaphragm, then yes, the columns need to take that load to the diaphragms.

My boss is pretty good at this, so I've taken my approach from him when dealing with clients/contractors who don't understand why we do some of the things that we do. He really goes through every step and explains our reasoning in simplistic terms.

I could imagine this explanation would go something like.............Without the opening, this wall spans from floor to floor with each foot of wall width taking its own load. When we take out the 38' section of wall that you want gone, then the wall alone at the end of the opening isn't adequate to take the load for the adjacent 19' of wall that was removed. Even if you assume that the length of wall effective in transferring this wind load to the diaphragms is 3', you've essentially tripled the load that the wall is seeing. You're taking 3' of wall that used to see 3' of wind load and are now asking it to resist 22' of wind load.

 

[preynolds]

EIT,

We have given that exact explanation. Makes sense to us but of course, everyone else knows better, right? Ha.

This is a question about load path and we feel we have the correct load path established. It is just convincing others that it is required. It is funny how just because a contractor didn't have it in his preliminary budget numbers, the columns aren't required.

How much of the remaining masonry do you use at the jamb to check existing CMU?

 

[Lion06]

Structural engineers are brought on board for a reason, and that reason is that we have an understanding of material and structural behavior that a contractor doesn't. If he doesn't "get" your explanation, tough on him. You're they guy taking the responsibility for the adequacy of this design. Stand your ground and if they want to search for another engineer then let them, but no competent engineer is going to tell them that they don't need to do anything in order to add a 38' opening (or even a 14' opening). Just from a gravity standpoint the CMU probably isn't going to work without grouting it and/or adding reinforcing.

 

[ToadJones]

Out of curiosity, what are you putting in these big openings?

 

[preynolds]

I am on board with telling them tough Sh%t and as much as I want to say it that way, diplomacy will dictate otherwise. ha.

The large openings are broken up into three smaller spans with a new bearing column on the existing foundation wall backed by a new wind column behind the bearing column. This was down to eliminate the need for a new foundation since we can anchor the wind column down to the slab-on-grade since it will not be carrying gravity load.

 

[Lion06]

I hear you on the diplomacy and I would never advocate saying that to a contractor, but on soome level they either understand or they don't. If they understand what you're saying and disagree, well..... there's not too much to be said about that if you're right. If they just don't understand what you're saying, try explaining it differently or get the architect on board to help put a little pressure on. A contractor shouldn't really be bidding a job without your drawings, anyway.

 

[jropert]

I agree with EIT. You as the structural engineer are hired for your expertise in this area. Think about this, the contractor in a similar manner is hired for his "expertise" in constructing the actual project. You shouldn't be telling him how to swing a hammer and he shouldn't be telling you how to design!

I used to work for a structural consulting company in which I was always sent out to do special inspections and observations of the construction (as I was the "young buck"). Even if the contractor has the plans in front of his face (which he even based his bid off of) calling for one thing and he put in something totally contrary to what should be, he will still argue with you saying you're too conservative and he knows more cause he's been doing this for 30+ years.

Ironically I am now working for a construction company as a structural engineer! This is even more challenging as you could imagine since I can be blamed for costing the company money. That's why I try to get involved with our bidding department from the get-go.

I would advise this...Come up with a few different options that can be used for this application. I am assuming the one you first choose might be considered the best option, however, if the contractor sees that you put in some effort on his behalf to give him options, he might relax a bit. There might be options that cost less for materials and more for labor or vice-versa. This might help the contractor see that you selected the most efficient design option that would save him the most money. If they still balk at that, you can go so far as to threaten to take your stamp off the drawings (my old boss used that one before!). Bottom line, your a professional. This is quite cliche' but "kill 'um with kindness"!

 

[preynolds]

we have strayed away a little from my original intent.....are we doing something different with jamb elements than most others? I believe "no" but wanted to hear what my peers are doing to design the jamb elements.

This issue isn't specific to one particular job. just recently, I had a colleauge that implemented wind colums and was somewhat ridiculed at a meeting ("this thing could stand up to nuclear blasts", etc). I understand our obligation and am not swaying from what I believe and if it isn't wind columns, there is certainly something on another job that someone claims is overdesigned (had it happen this morning on a canopp design).

 

[slickdeals]

You cannot design a masonry wall without reinforced concrete jambs/pilasters or "wind columns" for the openings of the size you are referring to. You will need a secondary frame to carry the opening. I am sure you need a pretty hefty header beam in weak axis bending to carry the load to the jambs too. A 38' foot wide opening is major.

We are Virginia Tech
Go HOKIES

 

[preynolds]

again, i am talking in a general sense. i have broken the 38' span into three separate spans with bearing columns and wind columns at 1/3 points of the 38' span.

What if the opening was for a new 12' overheard door in an existing wall? By calculation, you are almost always required to either reinforce the existing wall, provide a steel jamb column or other similar option. We would add the necessary elements, so I ask, is everyone else doing the same/similar thing or are we off in left field?

 

[ToadJones]

Show your client some jambs for vertical lift doors or other large industrial doors that are designed to stand alone.
The on these large doors are often designed to stand alone and not impart forces to the structure. The jambs are sometimes even cantilevered. Some of the jambs I have seen on these are huge....basically 24"-36" welded "I" shapes.
Same principle I do believe.

Also, I got called to look at a large tire shop once. The building had large doors on the front and back (about 20'x20').
On a windy day, they opened the back door all the way and the front door and surrounding masonry wall completely collapsed into the parking lot; Luckily no one was hurt. Some of the problem was wind uplift on the roof but also the lacking masonry design around the door.

 

[preynolds]

toad....that is the perfect example of why we do what we do, unfortunately, we are always too conservative, that is until something happens.

 

[Lion06]

preynolds-
You're not in left field. How else would you get the wind loads where they need to go?

 

[preynolds]

EIT, i am not sure how you would get the loads where they needed to go without implementation of what we described herein.

I get tired of explaining the situation to contractors/architects/etc so I was looking for a sanity check I suppose.

 

[ToadJones]

Tell the contractor to yank a large door header out sometime without supporting the roof. Tell him you don't need the header....same thing, other direction, right?

 

[jropert]

The beauty of this example is that if it was new construction, no one has a fit if we have a 24" jamb column with (6)#6 bars.

I think that is the key. Because you are now changing the structure from what it has been originally designed for, you have to implement the appropriate design. It is justifiable in that sense.

I worked on a project that required new HSS steel wind columns for some hanger doors that each had openings that were about 60 feet wide. Yours is not as large, but steel has been common in similar applications (we've also used steel to re-support gravity loads in openings). I've used concrete jambs poured and doweled into the existing CMU walls to "build back" shearwall strength/stiffness lost from the hole as well as serve as wind/seismic out-of-plane loading on the jambs. This still required the contractor to grout those cells that may be in the same areas as the epoxied dowels.

It appears you are on target.

 

[msquared48]

When the Architect, Client, or Contractor comes to you with changes of that order of magnitude, it is your professional responsibility to give them the most practical, structurally sound and feasible solution you can entertain. BUT...

If they have problems with that, tell them that you cannot change the laws of Physics. You are not licensed to do that, only to use them. However, there is the option of another Architectural concept of their choosing which you will be happy to entertain a structural solution for, using the accepted laws of Physics of course.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Lion06]

Just something else to think about......... I did some large openings in masonry buildings and had to replace a substantial portion of the lateral stiffness that was lost. I know tht we try to never leave something worse off than we found it. So that is something to consider when taking out big chunks of what could be lateral elements.

 

[csd72]

Have you taken into account the additional stiffening effect from the additional axial load at the ends of the opening.

Small but significant.