letting in ledgers for floor joist.

Started by PEG688, April 10, 2006, 10:08:42 PM

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manhattan42

#25
PEG688 said:

"Heres a few more photos Manhatten , and by the book you are right ,1 3/8" is the max let in for a 5 1/2" framing member.  Even on that is 3 1/2" in thickness  as in a post.  

 This project with the engineering goes beyond the basic "book" code .  I wonder why they chose 1 3/8"  Knowing that would rule out dementional lumber ,IE , 2x4 / 6 for the ledger.  

I think you'd agree that the 1/8 " difference is moot , it would not be  the 1/8th Inch that would break the camels back , so to speak  

 That being said  from a inspector stand point some where you have to have a # / rule to go by . This one seems to me to not make any sense, a 1x4 on paper would work but in application I'd not trust a 1x4 common fir to do the job , so a 2x4 would be the natural next step as 5/4 material is not avaible in framing type stock .  Buying a 5/4 x 4 D. Fir board at about $3.00 a  foot would be a hard sell.    

So either the writers of the new code want to make this type of semi" ballon " framing impractical  for cost and compliance or they just screwed the pooch by going with the hard and fast 25% rule  "


PEG688:

I believe that what has occurred is that the research has indicated that notching bearing studs over 25% of their depth simply weakens their bearing capacity too much and that the 7/8" maximum notch depth for nominal 2x4 or 1 3/8" maximum notch depth for 2x6s is incidental.

I don't think it's correct to look at 1 3/8" and think it's almost 1 1/2" so why not 1 1/2". There's more to it than that.

The problem is that notched studs weaken the structure when done beyond 25% on bearing walls or 40% on non bearing walls.

You also have to understand that especially for exterior bearing walls that have a roof above, the action of loads on the roof is going to be not only to bear down on these studs but also to bear these walls out....and that the weakest point will be at the notch where the studs could literally snapoff or split.

Wood fibers are also not so precise as to flow in perfectly straight lines.

What this means is that with a notch, you have created a starting point at which a split is going to readily occur.

Not so with planed smooth lumber.

What this means is that wherever you have a notch, you also have a greater likelihood of the lumber splitting, and when lumber splits, as you know, it does not do so in a perfectly straight planed line.

It follows the fibers in the lumber which, depending on the cut, are not all that straight.

When you notch studs, they can split outward causing the stud to be sheared in two over the length of the split.

Keeping the notches to less than 25% for bearing walls is the optimal maximum depth the stud should be bored.

And it goes without saying that the most stable stud is one that hasn't been notched at all.

That said and even though the design of your structure has been approved and stamped, it is in my opinion still a very poor one, and there are much better methods to attain essentially the same floor construction without having had to notch any studs.

In this particular case, architectural professor or no, it's a very poorly thought out framing plan, imho.

One could have achieved the same thing without notches by simply placing jacks under each floor joist and ended up with a stronger structure in the process.

Doesn't mean it is 'wrong'. There are many ways to skin the structural cat.

Be that as it may, a maximum notch in a 2x4 at 25% is 1 3/8".

Cut into a nominal 2x6 this leaves you with 4 1/8" of wood left.

One might think, "Well, Gee. That left over 4 1/8" is deeper than an undisturbed 2x4 nominal stud (3 1/2")"....But this notched 2x6 is actually potentially weaker than an undistrubed 2x4 because the potential for splitting is greater with a notch in deeper material than with no notches in narrower lumber.

This is why is some cases a 2x4 #SS of one species can be ultimately stronger than an economy grade of another species at say 2x6 because of all the greater potentials for splitting and failure with the less pristine lumber with more knots.

Anyhow, notching in a ledger board would not be how I would recommend this job to have been designed, but your designers did and got their approvals and will be the one's living with their decisions.

Best regards.

bartholomew

QuoteAmanda_931 asked:

"Is it really a notch if you fill it all up with basically the same material you took out of it (even if it's going th opposite direction)?? "

To my knowledge, it is still a notch and weakens the bearing studs by creating kind of a 'hinge' point at which lateral forces could bend or ultimately snap them.

I'm puzzled by this too. With platform framing you also get a hinge point where the bottom plate is nailed down. But of course you don't depend on those nails to keep the wall from falling over. The wall is usually braced upright by the perpendicular walls. So why would it be any different with balloon framing? If anything, the deeply notched balloon frame wall would be stronger.


peg_688

#27
  [size=12] The thrust of the roof load in wind / snow loaded or earthquake would be pressing on/ out  those notch's , as in the photo's the roof stress/ load is to push out . The lowered joist are in tension  collar ties but lower than "normal" . So the notch is in effect the weakest point , as I see it.    [/size]

 This shot , shows it best I think , althought that wasn't the intent of the photo , I'd show it better if that was what I was shooting for  :)

 

bartholomew

Ah, thanks PEG, that explains the engineering requirements on this structure. I was thinking more generally of a structural ridge or trusses. I also just read Manhattan's description of the increased splitting potential when the notch is in tension, which is definitely relevant to your structure, but I'm not really sure to what degree that applies if you don't have the roof thrust.

manhattan42

bartholomew asked:

"I'm puzzled by this too. With platform framing you also get a hinge point where the bottom plate is nailed down. But of course you don't depend on those nails to keep the wall from falling over. The wall is usually braced upright by the perpendicular walls. So why would it be any different with balloon framing? If anything, the deeply notched balloon frame wall would be stronger."

Again, notching weakens the stud, joist or rafter by decreasing it's designed ability to resist loads.

This weakened point and the fact that it is notched will also encourage splitting at the notch.

Any because of lumber fiber orientation, does not mean the split is going to be clean.

This gives a notched stud of say a 2x6 a weaker status in life than say a 2x4 un-notched because it has no particular weak spot along its length and no point at which splitting is prone to occur.

True, joints between floors in a platform frame are also 'hinged' points, but they are less weak than notched studs.

Placing fasteners into the notches through ledger boards face nailing can futher weaken the notched point.

The fact that there will be a 'cathedral ceiling' above this lot further weakens this entire roof system and adds considerably more load pushing out against these notched studs than normallywould be extered in platform framing.

I've pretty much exhausted my knowledge of 'engineering' here.

We'd have to send out for a structural engineer to get the real skinny on how notching and loads work..

Hope this helps anyhow.


peg_688

#30
Quote

 Best regards.

 [size=12] Jee I had a big , long line by line answer and the computer dumped it  >:(  I'm not redoing it  ;D

  The job is what it is , approved ,stamped , permited  etc .

   I don't designum, I buildum.

  I'm not going to try to tell a  engineer and / or a architect they need to redraw a set of  plans , after the permit is issued  , ain't in my job discription  ;)

 Best back at ya , PEG  
  [/size]

peg_688

#31
  
QuoteWe'd have to send out for a structural engineer to get the real skinny on how notching and loads work..


[size=12]  I'll point out here , again,  the owner has payed for a structural engineer and the math/ engineering  of this place ,as draw and as built ,meets or exceeds that engineers requirements.  

 
  [/size]


Amanda_931

And those are the guys (engineers and architects) who come up with plans for all-glass houses (with a little tiny bathroom/closet core) in seismic areas of Japan.  That presumably work, even in that difficult environment.

(at least I think I've seen one of those from Japan--it may even have been two-story)

Or those strange looking arangements featuring cables with turnbuckles instead of plywood sheathing.  Not for me, but they presumably work too.

And it's why one can sometimes substitute an engineering review for standard practice or codes-mandated.

peg_688

#33
 Heres some engineering ,
 
 

 

 This is a glass eterior wall at SEATAC ,John's MTL seen it . It like , [highlight]approx.[/highlight] 60' to 80' in height and 200' plus long .

It's curved from right to left and bowed from top to bottom held in tension / compression ?? by the cables and turn buckles .

Being a builder this is amazing , to me , how they held it together while building it I have no idea, :-/ way beyond my mind / grasp :o  ::)



manhattan42

PEG688 said:

"Jee I had a big , long line by line answer and the computer dumped it    I'm not redoing it  ..."


Shucks. And here I just thought I left you speechless.... :D

PEG688 said:

"The job is what it is , approved ,stamped , permited  etc .

   I don't designum, I buildum.  

  I'm not going to try to tell a  engineer and / or a architect they need to redraw a set of  plans , after the permit is issued  , ain't in my job discription "


I agree. You should have been involved at the design phase on this one, and there likely wouldn't be all these issues.... :o

jraabe

PEG:

I too have marveled at that huge glazed wall structure at the SeaTac airport. (It no doubt helps to have an hour or two to kill sitting in plastic chairs.)

It is a very elegant structural system with all the tension and compression members so clearly differentiated and expressed. There are cables going into steel ribs going into buttressed pillars. And then the glass panels themselves in their snug rubber mounts. It works together much like a sailing ship with the glass adjusting to the shifting wind loads.

I have no idea what kind of load that wall was designed to meet. I expect it is many times the 85 mph design wind load of our area. I wouldn't be surprised if it wasn't expected to survive a terrorist blast impact (from inside or outside?)

Anyway, thanks for the photos... I think I have some somewhere too.

peg_688


manhattan42

Just did.

The 'notification' for PMs isn't really noticeable.

I would not have even known I had a pm until you asked..


glenn-k

In your profile you can have it do a popup to notify you.  Somehow I still miss the popup once in a while.

Daddymem


glenn-k

Cool Design.  Since it is not included in the codes, I assume that if an Engineer designs it, then it supersedes  the codes.  Is that correct?

Daddymem

I'm no structural here, I do site engineering, but I imagine it would be the same red tape.  When I go outside the rules I have to appeal to some board somewhere asking for a variance.  Our plumbing mob union for example dictates all plumbing issues....all piping up to 10-feet from the foundation.  If I wanted to use a pipe material that is not in the plumbing code, I need to file with them for a variance.  If the variance is signficant enough, the board may decide that revisions to the code are needed.  I imagine the structural guys do the same.

glenn-k

Seems that codes cover materials used -strengths etc but ultimately it is the EOR who takes responsibility for odd designs- basicly odd stick -post and beam --built or structural steel etc.  That may even be the case in the design PEG is working on.   EOR says it will stand - special inspectors and inspectors see to it that it is to his design and accept it.  He is responsible.  I saw a 4 story steel structure falling over because of no lateral bracing but city inspectors approved it.

Amanda_931

QC keeps telling people they can't make it safe.  They can only speak up if they notice something off.

Mind you some of the old hands can be pretty helpful.  If they want to.  But for the most part they're not engineers.  

I assume this goes for the codes people as well.



manhattan42

#45
Building Codes are 'prescriptive' codes, meaning they try to tell you in advance what to and not to do under the most common building conditions.

But simply because alternate materials or designs are not included in the codes does not mean they are not automatically not allowed or can't be used. The code specifically states they can be used, but only under an engineered design.

For instance, trusses and I-joists are not contained in the codes because they are engineered wood products and need to be either designed by engineers or installed by using the manufacturer's engineering/application charts.

For designs as in PEG's case, such must be submitted with stamped and sealed archtiect or engineering plans and show the appropriate calculations as required by the code officials because it goes beyond the allowances of the prescriptive codes.

Ultimately it is the code official, however, who still gets to approve or disapprove even a stamped engineering design.

The degree to which engineered designs get approved depends upon the sophistication of the code official's plans reviewer/plans department.

In small departments where there may be a single code official who does eveyrthing, the trend is to simply approve anything that comes in with an architect's or engineer's stamp or seal...because the code official doesn't have any way to verify or dispute them.

In larger departments, the code office will have their own engineers on staff who can verify the designs and send them back for correction if they spot errors....and errors do occur even with stamped architect or engineer supplied designs.

I was just involved in a small project (as a builder) for an existing home where my customer needed to replace an existing leaking jetted bathtub on the 3nd floor with a new one.

They decided to replace the old 35 gallon tub with a larger 95 gallon unit and that sparked problems.

After we removed the old tub, we found that floor joists that were to bear this larger tub were already too small for the spans and that 1 of the 3 joists that would bear the full 1400lb weight of the tub when fully loaded was notched nearly clean through by previous plumbers to install the 3" waste pipe. This effectively left 2 already undersized joist 32" on center to bear the full loading.

I told the owners I could not proceed without an architect's or engineer's design approval.

Their son actually worked for an architect and they said they would have the architect come by, look at the project and send us his findings.

Well, he did and declared the floor 'safe' to install the tub.

In the mean time, I had provided the structural details to an architect friend of mine who completely disagreed and said the floors would collapse under the weight given the conditions.

When I told the customer that the only way I could proceed was to bring this to the attention of local code official who would then need to see engineering caulculations for the loads and the floors (the proper thing to do), they chose instead to fire me (because all they could see was that it was going to cost them more money to do things correctly), went and got a permit by tricking the code official using only their architect's letter.

In the mean time, I ran these issues past several other structural engineers and architect colleagues who all agreed that the floors would never hold the weight of the tub.

The moral of the story is, that many code officials would accept an architect's letter or stamp on the face of it, even if it is wrong and unsafe...because they have no way to dispute the design or even know potential problems even if they were there.

In my case, I refused to take the liability for an installation I knew to be questionable even though one professional designer would approve it. Ultimately it wasn't his liability, it was mine.

The bottom line was that the people got their jetted tub, and I'm off the hook should the floors give way.

Ultimately, however, it is the code officials who issue permits and grant approvals.

In this case, the code offical got snookered by some less than forthright owner's using what were flawed architectural opinions.

glenn-k

#46
Thanks for taking the time to explain that, Manhattan.  I'm sure if it fell through you would be the one they would try to sue.  I always try to watch for potential liability issues, as in the case here in town where I took own the old warehouse. The owner and one of the building inspectors wanted to just rip it down with an excavator.  I explained that we needed to know exactly how everything connected into the common wall or the second building could go down.  As I suspected, half the roof of the adjoining building was supported on the common wall.  I plainly told the owner of the second building that if something went wrong during the demolition of the warehouse, that half of his building would go down with it.  He ok'ed adding a shoring wall.

I removed 6 joists attached to wrought iron tees in the failing wall, and then cut loose about 40 joists pocketed into the top of the failing common wall to prevent side pressure from being exerted on it as the building fell.  The mortar holding the 2 foot thick 25 foot tall walls was tested at 19 psi strength - little more than wet sand.

The owner of the building being demolished thanked me.  He said if it was not for my insistance on checking everything, he would have owned 2 buildings in town.  I just ignored the little building official with the big cowboy hat spewing tear it down without checking opinions-- it was obvious he didn't have a clue.  The local engineer dropped by and totally agreed with me and offered helpful suggestions on tearing down the canopy.  I'm not sure if he was on a retainer or not but it was nice of him to offer help anyway.

manhattan42

#47
You made the right call Glenn, and I'm glad it worked out for you.

Here's one in my area where the building official screwed up big time.




The house under construction is a two family residential that was being built on an existing foundation after the old structure was demolished.

Although the owner was granted permission through zoning to construct the building so close to the property line, the code official simply ok'd the job without any thought to fire separation distance between the structures.

Under the International Residential Code 2003, structures that are within 3 feet of the property line must have fire rated construction: There must be no window or door openings in these walls, and have fire rated construction for  the walls and all projections (eaves) into the fire separation space. (Sections R302/R317)

This structure ended up being 2 1/2 feet from the property line.

The eaves were not fire rated construction and had perforated soffits.

I counted 6 windows on the wall that was supposed to have none.

Fire rated wall construction is required to prevent one structure from catching another on fire when in such close proximity, and this is a amjor life safety concern.

This building was being constructed in a community near mine and I alerted the code official and their solicitor of the problem.

The solution was to shut down the job until the building's owner could install a sprinkler system.

It turned out that the code official had misread the 2.5' distance from the property line that was listed on the submitted documents as 7.5' and hence neglected to reject the plans based on the fire separation distance.

His error ended up costing the building's owners many thousands of dollar for the sprinkler system.

So as in Glenn's case with the demolition project, the building official's errors could have or did result in unnecessary costs to the owners.


glenn-k

#48
Does the sprinkler system reduce the clearance per code?  

Most of our town burned down in 1866 with the building I just demo'ed being one of a couple left standing due to their fireproof construction including steel firedoors.

manhattan42

Glenn asked:


"Does the sprinkler system reduce the clearance per code?"  


Well, no, not really. But it will provide equal or better protection given that the building is within the fire separation distance....

But if you think about it, the sprinkler system will only protect the older building to the left if the new house catches on fire, but won't do anything to protect the new building on the right if the old building catches on fire....then begins to burn the new house.

Why? Because the new building will have to have caught on fire from the old building and be pretty fully engulfed before its sprinkler systems go off to suppress the flames.

Again, the code is prescriptive telling you what you should do in most instances, but the Code allows for alternative methods that meet or exceed the minimums the code prescribes.

In plain English, the Code tells you the easiest way to comply but does not tell you that is the only way you can comply.

The sprinkler system becomes an engineered alternative that meets or exceeds the requirements.

But in this case, is still not a complete answer to the problem and not something I would have allowed. I don't think it meets or exceeds the requirements of a fire rated wall design which protects both structures equally.

But that municipality's code offical and insurers did....

So that's all that counts.