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Old House Question

Old House Question

Old House Question

Example Scenario: House is 100+ years old, wood framed, raised floor with crawlspace & pier supported. Engineer A is asked to size a beam for a interior load bearing wall that will be removed for renovations. Engineer A determines loads, sizes beam, and makes modifications to foundation under new beam supports. Engineer A also had a quick look at the structure as a whole and did not see any obvious signs of structural damage or safety concerns.

Engineer B at the city determines that the 100+ year old floor joists, roof rafters, and header beams are not correctly "sized" according to loads from current codes and a free structural software he downloaded online. No work or alterations are planned to be made on these components. Engineer B at the city requests full calculations on these components to determine structural adequacy.

Engineer A runs calculations on these additional components and determines some are adequate but others are inadequate by about 5 to 15%. What would be the best option for Engineer A in this situation?

A) Make an arguments with Engineer B at the city that the new work will not affect the components in question.
B) Ask Engineer B for his calculations to determine any differences from yours.
C)Tell the contractor that he needs to do alot more work and probably lose a client.
D) A & B
E)Do what your told and dont question the authorities :)

RE: Old House Question

Depending on the code you are having to design to, many have provisions specifically for existing buildings. They tend to allow us to look past a 5-15% overstress.

Are you using lumber values from the era it was constructed? That lumber is generally quite a bit stronger than the numbers available now. Especially if you're just using software.

RE: Old House Question

It's really going to depend on your local code.

Some say you only have to verify what you change. Some say you have to verify everything if you change anything. Some allow no evaluation if demand/capacity ratio is increased by <X%.

Find the section in your local code that deals with new loads on existing elements. Depending on what that says, pick one of your options.

RE: Old House Question

Florida Building Code 2014.....I will have a look in their existing building book.

RE: Old House Question

Any good references for old era design values on wood and how to compare them to new ones? Would it be acceptable to use a No. 1 grade or even a select structural?

RE: Old House Question

Engineer B at the city needs to be fired for making everyone's life difficult :> We called that being "grandfathered in" around here. They usually don't raise a stink about it unless there is an obvious issue such as a notch 3/4 of the way thru the joists.
As Jayrod said, the lumber back then was super strong and most problems are only serviceability related.

RE: Old House Question

About strong. I agree. Just have the reviewer try to drive a nail into the joists... He'll bend it every time with little to no penetration.

Mike McCann, PE, SE (WA)

RE: Old House Question

Given that the wood is old growth, it very likely has more strength than what the code allows today, likely enough to make up a 10% to 15% difference. Some codes also allow for a performance based assessment, if they are at least 30 years old. Most city engineers I deal with would be pleased to know that some small portion of the house was being upgraded to current code and leave it at that.

In my situation, my house is over 150 years old. Small log timber roof rafters and 2 ft (or more) width lumber sheathing (random spacing, random and tapering diameter). I ran the numbers a few years ago and 'by Code', it is about 250% overstressed. I have seen it with 3 ft. of snow. Other than a bit of a sag, it stood the test of time before I ever purchased the house and continues to perform satisfactorily. I am not worried and have no plans to modify anytime soon.

RE: Old House Question

Old structures must not be allowed to stand, ever. It must be knocked down and replaced with fully code compliant new building.

My own house was built in the 1860's. The underside of the roof rafters are still covered with newspapers from the early 1860's, used to cover the gaps in the sarking boards = draught excluders.

By any stretch of the imagination it looks dubious structurally, but it is still here. I'm sure the numbers would say it fell down years ago.

RE: Old House Question

Option A
Engineer B is being an arse.

Plus F - Actually read the building code relevant to your area and then tell Engineer B where to find the relevant section which he clearly hasn't bothered to find it.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Old House Question


Engineer B can't just make you do whatever he wants. He has to have justifications for his actions. Is he citing any documents?

Sounds like a level 2 alteration per the International Existing Building Code 2012.

In section 807.3 it states: "The minimum design loads on existing elements of a structure that do not support additional loads as a result of an alternation shall be the loads applicable at the time the building was constructed"

RE: Old House Question

No, engineer A shouldn't be analyzing an existing structure that's been working without distress for 100 years. Maybe the most expedient avenue engineer B would be to check the tables in the Florida Residential Code. TABLE R502.3.1(1) The joists in these tables may not work on paper, but they may be a shortcut to showing acceptability rather than arguing about it.

If you'd rather argue about it so as to not set the precedent of having to submit calculations for any whim of the building official, you may have to dig into Florida Existing Building Code, too:

2017 Florida Building Code - Existing Building

The code grants the official broad authority, however

401.2.1 Existing materials.
Materials already in use in a building in compliance with requirements or approvals in
effect at the time of their erection or installation shall be permitted to remain in use
unless determined by the building official to be unsafe

RE: Old House Question

We have a book in our library titled "Wood Structural Design Data" second edition from 1939. It was published by National Lumber Manufacturers Association.

We typically default to a Douglas Fir Use Book published in 1958.

RE: Old House Question

If you decide to (or are forced to) further analyze the joists, I have used the timber valued given in historic versions of the Carnegie Pocket Companion for reference. It may give you an idea of allowable loads used at the time of construction (there is a 1923 version and a 1903 version available for download at http://www.slideruleera.net/miscellaneous.html - these documents are archived and provided by SlideRuleEra) Note the 50% stress increase from tabulated values for building structures.

It varies based on lumber type, but in general it is pretty east to justify #1 or Select structural strength from today's code.

The national parks service also has a document and tool for field grading and analyzing historic lumber that I have found useful. If the building official holds you to today's lumber strength, you may be able to field grade the joists to #1 or SS.

Both of these routes give you a little more to lean on than just "wood used to be stronger, trust me"

RE: Old House Question

Thanks for all the feedback on this issue I am having with Engineer B. Would like to give him a piece of my mind but I went ahead and put together a few quick calcs and referenced some tables from the code. If this starts to be the normal with older houses then I need to reconsider my fees or just not do projects like this.

RE: Old House Question

This should be a change order that the homeowner or contractor is be paying for.

RE: Old House Question

XR250: not the Contractor...


RE: Old House Question


My point really was that the Engineer should not be eating it. If the engineer's contract is with the contractor then the contractor should pay him and get reimbursed accordingly.

RE: Old House Question

I sent an invoice to Engineer B along with my calculations....

J.K! Contractor will get it and probably pass it to the owner.

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