Roof Anchorage to Historic Building

[phamENG]

Working on a historic structure, triple wythe brick walls. Brick is in fair shape, will be repaired where it isn't.

Roof at the back of the building is shot. Rafters and ceiling joists rotten at bearing. So we'll be doing repairs. 2018 Virginia Existing Building Code governs, and says the following:

...Repairs to historic buildings need only comply with Chapter 9.

903.1General.

Repairs to any portion of a historic building or structure shall be permitted with original or like materials and original methods of construction, subject to the provisions of this chapter. Hazardous materials, such as asbestos and lead-based paint, shall not be used where the code for new construction would not permit their use in buildings of similar occupancy, purpose and location.
903.2Moved buildings.

Foundations of moved historic buildings and structures shall comply with the VCC. Moved historic buildings shall otherwise be considered historic buildings for the purposes of this code. Moved historic buildings and structures shall be sited so that exterior wall and opening requirements comply with the VCC or with the compliance alternatives of this code.
903.3Replacement.

Replacement of existing or missing features using original materials shall be permitted. Partial replacement for repairs that match the original in configuration, height, and size shall be permitted. Replacement glazing in hazardous locations shall comply with the safety glazing requirements of Chapter 24 of the VCC.

Exception: Glass block walls, louvered windows, and jalousies repaired with like materials.

Built in the 1800s, this building had zero anchorage for the roof. The rafters just sit up there. They were nailed to wood beams elsewhere, but they just sit on the brick wall with nothing but gravity and perhaps some continuity of the lapped framing to hold them down. By my reading of this code, I don't have to do anything. Naturally, that doesn't sit right with me.

So if I'm going to do something, I want to do it right...well I have about a 60plf uplift load at the wall (0.6W+0.6D). Not a lot, but when you consider we can't put unreinforced masonry into direct tension, and the interior and exterior faces of the wall cannot be touched as the historic character can't be damaged (tax credit job with State review)...things get difficult. Based on window spacings, I have to space to my anchors out about 6 or 7 feet (single course of brick over the windows). That gives me a concentrated load of about 335#. To engage a wedge of brick, I'd have to go down over 2ft. Having somebody drill a 3/4" hole over 2ft into the top of a wall 60ft above the ground to put a plate on the bottom in a small cavity that will be refilled sounds absurd from a constructability point of view. And to say "the code technically says you don't have to bother with this, but here's this challenging and potentially very dangerous detail to build, do it anyway" isn't going to go over well.

Anyone have any better ideas for concealed anchorage to historic brick?

 

[dhengr]

PhamENG:
I absolutely agree with KootK’s approach and thoughts, 5OCT,17:38. They always think you are spending way to much, and that that should buy them 10 times what you can possibly deliver, given the circumstances. And, you do have to walk them through the reasoning slowly/carefully. Hopefully, you are not dealing with a builder who got to them first, has their ear, and is telling them that damn engineer is really just spending your money, you don’t need to do this or that, we never do. Of course, he isn’ signing the drawings either, nor did his Grandpa, who thought him how, have an engineering degree.

Edit: What I really started/meant to say..., Add a couple small negative steel (top rebars) over the opening heads, where the bond beam is only 2 or 3 brick courses deep. That way you can assume that some of this header brick work will be lifted too, without breaking up.

 

[KootK]

But there's a difference between understanding that the materials are significantly stronger than we assume (or were, before the fungus ate them) and saying that because a 700 year storm hasn't hit in the last 120 years, we don't have to worry about it.

It will surprise you not at all to know that I disagree:

1) Nobody's saying that you "don't have to worry about it". Rather, you have to worry about it less. There are no absolutes in reliability theory.

2) With each passing year that the 700 year storm does not occur, the chance of it occurring at all within the design life of the building diminishes. It is this diminishment that the Canadian stuff accounts for by lowering the load factor applied to the design environmental event for buildings with history. It's not at all the case that one has to wait 700 yrs to be able to say something meaningful about the appropriateness of the mandated design load. One knows something after the first year and, according to the Canadian code at least, something actionable after the first twenty years. 120 years of history is pretty meaningful in the context of a building that has a 50 year design life or whatever the case may be.

But, still, the lack of any load path at all remains a problem.

When we speak of 700 yr return periods for wind and 2500 years+ for seismic, those are just convenient rearrangements of annual exceeded probabilities aggregated over much shorter building design lives. We do the rearranging because humans prefer round, positive integers on account of our finger counting ancestry. We obviously don't expect our buildings to last 2,500 years.

I know, you've chosen a path and moved on. But, alas, I have not.

 

[skeletron]

For reference with the Canadian Code provisions that are being referenced:
-Commentary L
-based on a set of indices that relate to system behaviour, risk category, and past performance
-the sum of those indices defines the "Reliability Level" which can then be used to reduce load factors
-there are some applicable criteria that almost by default give you a high risk in the category index. One of the criteria is "heritage importance"
-another large factor in the risk is the "risk exposure" based on the occupant density, duration, size of building, etc.
-lots of smaller calculations, but more or less the decision is very binary (...or I guess "pentary" as you have 5 levels)

Middle of the road solutions are fine and necessary in my eyes. Don't make it worse (which belt and suspenders approach can sometimes do). Try to make it better (which isn't always possible) or leave it reasonably the same (as long as collapse is prevented).

I think the truly rule-of-law ethical thing would be to document and notify the client of the risk. Obviously it seems that cost and heritage character are constraining the issue, and I would expect their response to be such. But at least you would satisfy your duty to report. It's not your fault that the constraints do not allow a fully compliant solution.