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-   -   1950's Roof Framing Strength (http://www.diychatroom.com/f19/1950s-roof-framing-strength-2722/)

KempLN 06-10-2006 10:28 AM

1950's Roof Framing Strength
 
I have what I think is a very well built house/garage that was built in the mid 1950's ranch style. Below is a specific example of my garage and the issue at hand.

The Ceiling joists are true 2x12's connected to 2x6 Rafters. I want to hang storage shelves and a Lift system (Pull no more than 500lbs) using Lag Screws directly into the bottom of each 2x12. Images are below for a better idea.

http://img108.imageshack.us/img108/1...oist0012pi.jpg


http://img108.imageshack.us/img108/2...300wide8lw.jpg
The debate centers around 2 things.

1. The Ceiling Joist to Rafter junction is compromised since the Joists are reduced down to about 4 inches at the end tip of the Joist at most outside portion the wall plate and cannot handle ANY additional load beyond the roof system and the 2 existing Garage Door openers.

2. 7/16" x 3" inch Lag bolts would pull out of the 2x12 Ceiling joists with the 500 lbs lift system. There are 12 Lag Screws attaching 4 Steel Unistrut Rails (3 each) in this system now.

My position is that this construction is far superior to current framing I see today but I would also like some sort of idea of loading abilities of this construction system.

The Unistrut Rails tie in the 3 Ceiling Joists and share the 500lb load. With 12 Lag Srews, each is only taking on about 41.5 lbs each or 125 lbs per ceiling joist, Right?

The Ceiling Joists are 16" on center with a 20ft span. The lift system is about 4 ft from the outside wall of the Garage.

I make no pretenses that I have any knowledge about engineering and would greatly appreciate input from experienced framing expertise to help decide to move forward or pull down the lift system and not use it?

:confused:

joasis 06-10-2006 12:02 PM

I don't see that you have a problem...I would go right ahead with what you are doing.

KempLN 06-10-2006 02:22 PM

Roof Strength
 
Thanks for the encouragement. Do you have any idea about the load range for the Ceiling Joist just so I can get a quantified idea how to respond to my buddies?

fettycan 06-10-2006 06:09 PM

why not hang some steele on top of joists less chance of 2x12 breaking than the lages working loose

manhattan42 06-10-2006 07:35 PM

Spans, Spieces, Spacing
 
The ability of sawn lumber to bear loads is based on the distance the lumber must span, the speicies of lumber and the on center spacing among other things.

2x12s of even some of the poorer grades of lumber for common lumber speicies should be able to span these distances with little problem and include some 20lb per square foot light storage if a subfloor was installed on top of the joists and the loads spread over the entire floor, but for these concentrated loads of 500lbs or more in a few square feet.......I wouldn't count on it.

First, you should never trust concentrated loads to bearing points like screws alone. Planning to support 500lb or greater loads suspended from screws alone is serious injury or death waiting to happen.

fettycan has the right idea. The loads should be spread over several joists by placing your mounting bars straddling on top of several joists.

Screws mounted from below can fail with disastrous results.

But even with your loads spread over the tops of the joists, 500lbs can be well above these joists to handle given other unknown factors.

Only way you will get a definitive answer to your questions is to consult a structural engineer. Anything less is merely guesswork and you will be left in the lurch if the guesses are wrong.

You will need to know the species of lumber, the exact size of the lumber, the spans, and grade of the lumber. The engineer can then calculate the load bearing ability of your joists for the given concentrated loads...then give you a green or red light and make suggested upgrades if necessary to make what you wish to do work.

Notches, cuts, cracks and other imperfections can compromise your joists' ability to carry loads and contribute to failure.

Have a professional assess and design your system.

joasis 06-10-2006 08:45 PM

Quote:

Originally Posted by manhattan42
The ability of sawn lumber to bear loads is based on the distance the lumber must span, the speicies of lumber and the on center spacing among other things.

Very correct..this is grade lumber obviously on 16 centers.

2x12s of even some of the poorer grades of lumber for common lumber speicies should be able to span these distances with little problem and include some 20lb per square foot light storage if a subfloor was installed on top of the joists and the loads spread over the entire floor, but for these concentrated loads of 500lbs or more in a few square feet.......I wouldn't count on it.

The idea of "concentrating the loads" in a few feet doesn't really work this way. The horizontle joist will "carry" the load to the bearing wall. The amount it will carry is based on "load deflection". Due to the spacing from the wall and your correct assumption of 50# or so per load point, you are way safe unless you see a structural defect. Anytime you think about dead loads and have a number of say 20# per sq/ft, that does not mean you can't safely walk around since when you lift one foot, you are concentrating your entire weight in a concentrated load point. Load bearing has a lot to do with averages..averaging a load, worst case load position, ect. I have worked with engineering when doing mezzanine decks in malls and have seen loads set at 150# live load...and everytime I walk in a mall upper floor and feel the floor shake, I wonder.

First, you should never trust concentrated loads to bearing points like screws alone. Planning to support 500lb or greater loads suspended from screws alone is serious injury or death waiting to happen.

Lags are not screws...lag bolts have the ability to hold great loads, providing they are installed correctly.

fettycan has the right idea. The loads should be spread over several joists by placing your mounting bars straddling on top of several joists.

Anytime you can spread the load, it of course, would be safer. However, if you were to lay 8 foot 2 x 4 on top, you would still be loading only the 2 joists on either side of the load, since the bending moment of the 2x would "lift" it on the ends. Best case is cutting a load in half.

Screws mounted from below can fail with disastrous results.

We mount garage door openers with small lags in a concentrated area all the time, I have yet to see one fail, unless improperly installed.

But even with your loads spread over the tops of the joists, 500lbs can be well above these joists to handle given other unknown factors.

Unknown factors like ? Roof structures are designed to support the weight of the structure, decking, shingles, ceiling, attic load if it has one, and snow loads, ect. They also must support live loads, like wind pressure, uplift, and the weight of workers.

Only way you will get a definitive answer to your questions is to consult a structural engineer. Anything less is merely guesswork and you will be left in the lurch if the guesses are wrong.

Sure, if you can find one.

You will need to know the species of lumber, the exact size of the lumber, the spans, and grade of the lumber. The engineer can then calculate the load bearing ability of your joists for the given concentrated loads...then give you a green or red light and make suggested upgrades if necessary to make what you wish to do work.

If you really want to know, you can find all the calculations online, or have your local library get a copy of "Engineering for Architechts and Builders", a bible of what works.

Notches, cuts, cracks and other imperfections can compromise your joists' ability to carry loads and contribute to failure.

As stated...common sense would preclude placing a lag screw next to a cracked joist.

Have a professional assess and design your system.

The system you purchased should have information on correct installation...read the product literature carefully and follow the guidelines...they would not make it if it was a sure fire way to get hurt. They will have disclaimers, but also parameters for safe installation.

Bonus 06-11-2006 01:23 AM

I would like to see the load carried above the joists not below, is there some way to utilise u-bolts to get the load on top of the joist? I'd like to see blocking between the joists to stiffen them, and I'd want to take a careful look at the joists for defects, knots, cracks, insect damage etc. Then I'd go for it, and have no problem parking my truck under it.

I don't think the rafter/joist connection is an issue, you have 3-1/2" inches (4"?) of bearing on the joist, that's good.

KempLN 06-11-2006 09:48 AM

1950's Construction
 
Thanks for all the input here! It appears the same debates are panning out here, but with more of an objective view and with people that have more experience of how this system works participating.

I think there is still quite a bit of concern with the use of the 3 inch by 7/16 Lag on the bottom of joist. That is why I posted this. The company that sells the comercial (warehousing rack) hanging from the ceiling has included in its instructions to attach each of the 4 hangers with 2 inch Lag Screws (Bolts) 2 each at each hanger. That puts only 8 Lag Screws (Bolts) holding the 500 lbs load with no known cross bracing of the joists. I would think this company has employed the engineers, lawyers and has the necessary liability insurance (probably over $2M per incident) to make this available at the retail level. They do have disclaimers to suggest that they are not someone without industrial & retail experience.

The 500 lbs lift system is from an idea I found on another forum. The Unistrut is constructed with an inside lip that allows the bottom rails to attach and slide. It does not slide on rollers so the entire rail takes the load and spreads it along its length. Each Unistrut will accept a 1700 lbs load per 2ft length according to their website. My thought is that they do tie the load bearing to each of the Ceiling Joists.

I do also understand that the joists & Rafters would accept a large load from the top more so than pulling down. It would seem very difficult to find or to make the 13 inch long "U" Bolts but it would make sense that it would be safer.

So, what I think I am gathering here is:

1. Based on my image of the Garage Ceiling Joist to Rafters connection, the system can handle the load. It must be that the reduction of the ceiling joist down to 4 inches on the wall top plate is not a problem due to how the rafter system works. With the 2x6 rafter, sharing the load on the wall plate since of course they are nailed together, right?

2. An Engineer would have to sign off on the use of the Lag Screws, but in general it probably is not a good idea. Even though the lumber that was used appears to be top grade fur without a crack or knot anywhere to be seen, the results could be hazardous with lesser wood construction!

3. To improve my lift system, then either "U" bolts that put the load on top of the beam or using a bolt, nut washer system with a hole drilled in the side of the beam to access the bolt end would be safer. Understanding that small holes in the center of the joist is safe the same way plumbing an electrical lines are ran, not too large and centered is best. I will at least do the later. It's simple, cheap and safer for the long term.

4. This system worked for me but it may not for everyone.

joasis 06-11-2006 10:09 AM

Let me give you guys a little insight on how engineering works at times. When I was working as an estimator with a steel building manufacturer, we would spec buildings for all kinds of apllications. One in particular would be a gymnasium type building, with suspended heaters, hung from the roof purlins. We would look up the heaviest heater made, and place it in the general area specified, at the worst case loading...usually as far from a main frame or rafter as possible. We would then spec I beams to carry the load back to the main frames...and quote the building. If it sold, the engineers would get the exact load and location, and look at the parameters, and take out the I beams, and hang them from purlins that also shared the roof load. We would over engineer and they would minimize to remove the excess material. Building sells for X dollars for X number of pounds of steel...then take out x number of pounds of steel as un-needed, and increase profits. That my friends is what engineering can and will do. Most homes are over built, and rightly so. I alwasy run to the over side, then under side. I have seen (and I bet many of you have as well) a shade tree mechanic will remove an engine from a car hanging the hoist ftom a joist in the garage.....now it will deflect, and is not a safe practice, but it will work if the joists are strong and good.

Back to the lag bolts. If a pilot hole is drilled at a size slightly less then the pitch diameter of the lag, this nearly removes any possibility of splitting the joist. The pull out strength of the lag is incredible, much greater then the 42 or even 50# it would be supporting. Ideally, a 2x4 plate could be layed on top of the joist and an all thread could be used, and would be better, but really unneeded. If any of you really doubt the hold of a lag, get a 2 or 3 inch x3/8 lag screw and pilot a hole, screw it in, and leave enough shoulder for a large wrecking bar to get under. See how much force it takes to pull it out. Bet you money, no one will strip it out unless the wood is flawed. And a wrecking bar can put a great amount of force on the lag screw.

Bonus 06-11-2006 11:08 AM

Agreed, Joasis, I'm sure you're correct. Who knows how much weight is gonna get hung off this thing once its built?

Had another one: How about a steel strap that hooks over the top of the joist and has an 'ell' at the bottom for the unit to bolt onto?

__
| |
|
|
|
_|

Couple screws into the side of the joist to keep it fixed vertical.

Or just do it. :D

Edit: for some reason the 'drawing ' doesn't read the way I draw it , imagine it with the bottom 'ell' one step to the left.

manhattan42 06-11-2006 08:29 PM

The bottom line is, no matter who engineers this thing....the manufacturer of system who is selling a product at retail or a site engineer who must design a 'home made' system, it must still be approved by the building code official based on stamped and approved design plans.

So all the 'we did this' and 'it works this way' patronizing does nothing to help in this case.

Suspending a 500lb concentrated load requires engineering to get a permit to install the assembly. Plain and simple.

And trusting anonymous sources on an anonymous forum on the internet is about as meaningful to the assurance of your safety and the legality of your installation as is using a Magic 8 Ball to predict next Tuesday's weather.

Call a local engineer or other structural design professional who can assess your structure and make recommendations based on the observed findings and load calculations.

Then present the findings to your code office for permit approval.

Anything less is simple foolishness.

joasis 06-11-2006 08:54 PM

Great point, manhatten...It would be interesting to see if anyone who purchased one of the lift systems from HD or Lowes actually would go secure a permit to install one. I can't even imagine filling one out, and I fill them out frequently. The point of a forum is an exchange of ideas, opinions, and interests in a given topic. Sure, we can post an opinion or idea that ignorant, or meaningful, with no liability to ourselves. For someone to seek an opinion on the lift system shows me someone is thinking about safety, and my opinion is based on my experience and knowledge of the subject. If KempLN really feels that an engineer should be consulted ( here in Oklahoma, this would be minimum $500), he will make the call. If his local municipal government issues permits for this "alteration", then he needs to get one. Just for conversations sake, which part of the IRC 2003 or 2006 would you guess he would need to consult?

I need to remember this before we install garage door openers unless we get an engineer and a permit meeting code...we obviously have been doing it wrong since they are concentrated loads held up by insignificant lag screws.

manhattan42 06-13-2006 07:45 AM

Quote:

"Just for conversations sake, which part of the IRC 2003 or 2006 would you guess he would need to consult?"
Section R104.11, for one, of either the 2003 or 2006 IRC which states:

ALTERNATE MATERIALS, DESIGN AND METHODS OF CONSTRUCTION AND EQUIPMENT. The provisions of this code are not intended to prevent the installation of any material or to prohibit the design or method of construction not specifically prescribed by this code, provided that any such alternate has been approved.

An alternative material, design or method of construction shall be approved where the building official finds that the proposed design is satisfactory and complies with the provisions of this code, and that the material, method or work offered is, for the purpose intended, at least the equivalent of that prescribed in this code."


R104.11.1:

TESTS. "Wherever there is insufficient evidence of compliance with the provisions of this code, or evidence that a material or method does not conform to the requirements of this code, or in order to substantiate the claims for alternative materials or methods, the building official shall have the authority to require tests as evidence of compliance..."


Just to name a few....

The problem with this home-made lift design in the initial photo is that it is outside of the prescribed elements of the Code because it uses 'alternative materials, design and methods of construction and equipment.' It is required by Code, therefore, to be engineered.

By Code, it cannot be installed without an engineer or other design professional's stamp and the approval of the building code official.

It is a life safety/public health issue.

For a manufactured shelf system as illustrated by photo #2, it will probably be fine to install provided that the manufacturer's installation instructions are presented to the code office for approval. The manufacturer is the one who is providing the engineering in such a case.

Same thing would apply for a garage door opener.

Code requires such electrical appliances to be installed in accordance with the manufacturer's installation instructions.

The major issue here is this 'home made' lift system.

As a builder and a Code Official I would never approve such an installation and would require an engineer's stamped design before allowing it or installing it.

I'll see if I can find the photo from another forum posted by a fellow who did just what our original poster is proposing with his lift. It illustrates the collapsed joists and rafters and what was left of his antique car after the lift and roof system gave way....all because he simply 'guessed' at home much weight his lift system and roof could handle.

It cost him tens of thousands of dollars to repair and replace.

That's many times more costly than a $500 or less consultation fee by a design professional.

That God no one was under his roof when the lift gave way....:eek:

joasis 06-13-2006 05:29 PM

A builder and code official....hmmmm...sounds like a conflict of interest. I agree with the passage cited, but do not agree with it in application. So rather then play word games, i will stand on the principals involved, you stand behind the blue books and they will stop speeding bullits I hear, and let the guys who use the forum as intended glean the information they choose to follow.

The building codes are not the holy grail, the divine inspiration, or the law of the land. They are a guide of standards, minimums, and recomended practices. I think use of the codes is neccessary, but more importantly, it is the "code official" or building inspector who makes the system work. One who cites the codes like the ten commandments would not be a welcome person on any building site...one who sees a problem, and assists the contractor before weilding the mighty hand of authority will have far more respect. I have had 3, yes 3, serious conflicts with a building inspector. I have won all 3, I have successfully sued the inspector and won a claim from his insurance carrier, forfeiture of his bond, surrender of his license, and termination of his contract. This "by the book" guy tried to prove he knew it all...he caved right in during a deposition when questioned by a lawyer, and I have had no more problems since. I respect our state inspectors, and a good building inspector, applying the IRC codes in a constructive method is welcome to my job sites.

Remember, this is a DIY forum....most people looking here wouldn't even have an idea of the IBC codes...

I had to add this as a final word. I hope all of us in construction build to greater standards then codes require. I know I do....I rely on sound engineering and design, as well as experience...not to cheat the process, but to build the best homes and buildings I can. I enjoy reading the posts and offering advice...but it is what it is...advice. I will not offer technical explanations with the threat of the building codes as the end all.

Bonus 06-13-2006 11:50 PM

Right on, Joasis.


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