# Rafter thrust control for flush beam



## jar546 (Oct 5, 2013)

I found this on a renovation, post fire job where they changed the layout.  They will need straps across this for control of rafter thrust.

View attachment 1961


View attachment 1961


/monthly_2013_10/image.jpg.5a78b969fa554172613bee912c6e3998.jpg


----------



## fatboy (Oct 6, 2013)

How long? Or does the beam actually carry the ridge?


----------



## pwood (Oct 6, 2013)

is there a question here?


----------



## mark handler (Oct 6, 2013)

Have them provide calcs or straps, to show compliance with thrust requirements


----------



## MASSDRIVER (Oct 6, 2013)

If the beam calcs, it seems they could "purlin" that ridge, maybe double 2 x 6 up to it every 4 feet. Straps are going to give the drywallers fits, and probably require some sistering of 2x4's on the joists for fastening purposes.

Brent.


----------



## RJJ (Oct 6, 2013)

It looks like the ridge is off center to the beam? Got more photos?


----------



## mark handler (Oct 6, 2013)

RJJ said:
			
		

> It looks like the ridge is off center to the beam?


Does that make a structural difference?


----------



## RJJ (Oct 6, 2013)

only if the beam is intended to support the ridge.


----------



## mark handler (Oct 6, 2013)

RJJ said:
			
		

> only if the beam is intended to support the ridge.


Kickers can be angled....


----------



## jwilly3879 (Oct 6, 2013)

They can put the straps on top of the joists


----------



## mark handler (Oct 6, 2013)

jar546 said:
			
		

> They will need straps across this for control of rafter thrust.





			
				jwilly3879 said:
			
		

> They can put the straps on top of the joists


Great minds ....

Or they could justify the withdrawal of the fasteners in the hangers....


----------



## MASSDRIVER (Oct 6, 2013)

jwilly3879 said:
			
		

> They can put the straps on top of the joists


Beam is probably larger than the joist, you just can't see it from the camera angle. doubt seriously you can strap on top.

Great minds and all...  

Brent.


----------



## mark handler (Oct 6, 2013)

Straps can be placed on the "bottom" of the joist, though is creates issues with gyp bd nailing

Or add ties







There are always options....


----------



## jar546 (Oct 7, 2013)

I have no idea why the beam is there because they could have replaced all of the ceiling joists to make that span but they gutted this house and wanted an open floor plan.  I believe the used the beam as flush to replace the wall that was there being used as the splice point for the ceiling joists.  The CJ and the ridge have zero relevance to each other since the ridge is a board as you can see and not a ridge beam at which point my photo and post would not be relevant.

I have required strapping to ensure rafter thrust control.  The drywaller can bitch all he wants.


----------



## jar546 (Oct 7, 2013)

MASSDRIVER said:
			
		

> Beam is probably larger than the joist, you just can't see it from the camera angle. doubt seriously you can strap on top.Great minds and all...
> 
> Brent.


You are correct oh wise one Brent.  They must strap the bottom to please the drywall guy..


----------



## MASSDRIVER (Oct 7, 2013)

jar546 said:
			
		

> You are correct oh wise one Brent.  They must strap the bottom to please the drywall guy..


Hey don't get me wrong. Never pass up an opportunity to screw with the drywaller. Fun is fun.  But you have to give him a CHANCE at least.

If you strap it (depending on length) you are  creating an area he simply cannot fasten to, unless you sister on some nailing, or let him drill into straps. Or tension ties.

Brent.

Looks like they tried what we did here. Our ridge is directly over the beam and supported by it.


----------



## 4thorns (Oct 9, 2013)

> I have required strapping to ensure rafter thrust control.


Just curious.

How long do the straps have to be?

What are the fasteners required for the straps?

What is the loading used to determine the above questions?


----------



## jar546 (Oct 9, 2013)

4thorns said:
			
		

> Just curious. How long do the straps have to be?
> 
> What are the fasteners required for the straps?
> 
> What is the loading used to determine the above questions?


There are many calculators available for this and equations available to anyone.  Once we know the span, spacing, pitch, snow and dead load the thrust can be determined.  Simpson has specifications on appropriate strapping for this instance.  There are some that have determined the strapping equivalent to the ceiling joist connection nailing requirement too.  It is much easier than people think thanks to engineered products.  That saves us from sending a framer to an engineer all the time.


----------



## Rider Rick (Oct 9, 2013)

jar546 said:
			
		

> There are many calculators available for this and equations available to anyone.  Once we know the span, spacing, pitch, snow and dead load the thrust can be determined.  Simpson has specifications on appropriate strapping for this instance.  There are some that have determined the strapping equivalent to the ceiling joist connection nailing requirement too.  It is much easier than people think thanks to engineered products.  That saves us from sending a framer to an engineer all the time.


Jeff,

Which Simpson strap is called out?


----------



## brudgers (Oct 9, 2013)

..........


----------



## MASSDRIVER (Oct 9, 2013)

brudgers said:
			
		

> ..........


----------



## RJJ (Oct 9, 2013)

I would go for the ties!


----------



## jar546 (Oct 9, 2013)

Rider Rick said:
			
		

> Jeff,Which Simpson strap is called out?


Using nails, an LSTA18 works best unless the holes line up with the LSTA12 since it has to span the beam width and joist hangar bottoms.  Anything from the LSTA12 on up works with nails.  With simpson screws the numbers are even better


----------



## 4thorns (Oct 9, 2013)

> There are many calculators available for this and equations available to anyone. Once we know the span, spacing, pitch, snow and dead load the thrust can be determined.


Who is responsible for determining this information and the required connection? Is this something you would determine and inform the builder?


----------



## jar546 (Oct 9, 2013)

4thorns said:
			
		

> Who is responsible for determining this information and the required connection? Is this something you would determine and inform the builder?


It is the builder's responsibility to provide this information and our job to check it.  We are not designers.


----------



## fatboy (Oct 9, 2013)

"It is the builder's responsibility to provide this information and our job to check it.  We are not designers."

Agree.........


----------



## 4thorns (Oct 10, 2013)

OK. Let's say that the builder has provided you with all the info and has chosen a strap that he feels is adequate.

Would you be willing to explain to me how you would prescriptively determine whether or

not his fix is acceptable?


----------



## jar546 (Oct 10, 2013)

By the manufacturer's rating of the strap and the information that he provides.  There are plenty of easy calculations that can be performed to verify this.  In addition, through conversations with structural engineers in the past, we have an idea what works in our area(snow load may vary) based on the size of the roof and the direct comparison to the nailing requirement VS the specifications of the strap from the manufacturer.

We can always tell them to go to a PE and get the non-prescriptive fix specified.  Please don't forget that not everything is in the IRC and we are sent to the IBC and NDS which has much of this information prescriptively.


----------



## ICE (Oct 10, 2013)

4thorns said:
			
		

> OK. Let's say that the builder has provided you with all the info and has chosen a strap that he feels is adequate. Would you be willing to explain to me how you would prescriptively determine whether or
> 
> not his fix is acceptable?


It is called flying by the seat of your pants.


----------



## mark handler (Oct 10, 2013)

jar546 said:
			
		

> It is the builder's responsibility to provide this information and our job to check it.  We are not designers.


You could offer guidance, everyone needs to learn.


----------



## DRP (Oct 10, 2013)

Simply provide equivalent to table R802.5.1(9) footnote e.


----------



## jar546 (Oct 10, 2013)

DRP said:
			
		

> Simply provide equivalent to table R802.5.1(9) footnote e.


Yes, absolutely.  It is much simpler than I am explaining it and that table is the entire key to prescriptive compliance.


----------



## RJJ (Oct 10, 2013)

Yes it is simple and 802.5.1(9) footnote e. is spot on.


----------



## RJJ (Oct 10, 2013)

Was this beam on the original plan? I believe you stated a wall was removed thus the need for the beam CJ connection?


----------



## jar546 (Oct 10, 2013)

Beam was not expected to be there.  It was a post fire renovation that they decided to change the layout for and a surprise to us.


----------



## RJJ (Oct 10, 2013)

surprises happen all the time.


----------



## steveray (Oct 10, 2013)

MASSDRIVER said:
			
		

> Beam is probably larger than the joist, you just can't see it from the camera angle. doubt seriously you can strap on top.Great minds and all...
> 
> Brent.


couldn't they use something like a simpson VB brace over the top? I have not seen it done or looked much into it, but have seen it in the catalog and seemed reasonable.......


----------



## Rio (Oct 15, 2013)

steveray said:
			
		

> couldn't they use something like a simpson VB brace over the top? I have not seen it done or looked much into it, but have seen it in the catalog and seemed reasonable.......


That was my thought............

VB Knee Braces


----------



## Rio (Oct 15, 2013)

I just read the catalog page again and the VB brace requires the members being connected to be a minimum of 2-1/2" so that's out if those are 2x joists; strapping on the bottom and blocking to pick up where the gyp. bd. can't be nailed as suggested above would be quick, good, and easy.


----------



## mtlogcabin (Oct 15, 2013)

Would make the drywall guy happy. Probably only needed every 2nd or 3rd rafter


----------



## jar546 (Oct 15, 2013)

mtlogcabin said:
			
		

> Would make the drywall guy happy. Probably only needed every 2nd or 3rd rafter


Don't let Glenn see that.  His eyes will burn and catch on fire.


----------



## DRP (Oct 15, 2013)

The main problem with the VB brace is that when thrust is applied it would tend to lift the joists as they seperate from the beam unless there were something like a ledger or upside down hanger above the joists to keep the joists from rising. Used as simpson specifies the hanger is in the correct orientation for the load to pull the joist down into the hanger.


----------



## fatboy (Oct 15, 2013)

"Don't let Glenn see that.  His eyes will burn and catch on fire.  :wink:"

For sure.................


----------



## Rider Rick (Oct 16, 2013)

I would build it with rafter ties like Mark said in post #13 that's old school.


----------



## mark handler (Oct 16, 2013)

Rafter Thrust


----------



## 4thorns (Oct 16, 2013)

> I would build it with rafter ties like Mark said in post #13 that's old school.


Have to agree in this case. You have existing joist hangers. Lay another piece of steel over them and now you have a huge bump

in the ceiling that you will never hide without a lot of work by guys that may be P@#$ed off!

I noticed that the ceiling joists don't line up from one side of the main girder to the other. Strange. Rafters appear to line up at the ridge.



> The main problem with the VB brace is that when thrust is applied it would tend to lift the joists as they seperate from the beam unless there were something like a ledger or upside down hanger above the joists to keep the joists from rising.
> 
> Used as simpson specifies the hanger is in the correct orientation for the load to pull the joist down into the hanger.


Is it possible to prescriptively determine whether or not the fasteners in the hangers are adequate to resist the uplift created by

Knee Braces?


----------



## jar546 (Oct 16, 2013)

4thorns said:
			
		

> Is it possible to prescriptively determine whether or not the fasteners in the hangers are adequate to resist the uplift created by
> 
> Knee Braces?


Most joist hangars are rated for uplift in pounds.  Uplift is not the issue, however.


----------



## 4thorns (Oct 17, 2013)

I agree that uplift is not an issue as it sits. I was referring to DRP's post:



> The main problem with the VB brace is that when thrust is applied it would tend to lift the joists as they separate from the beam unless there were something like a ledger or upside down hanger above the joists to keep the joists from rising. Used as simpson specifies
> 
> the hanger is in the correct orientation for the load to pull the joist down into the hanger.


If the uplift created by the Simpson Knee Brace can be restrained by the existing ceiling joist hanger fasteners then this detail is a viable option.

If not, a ledger above the joist could be installed (per DRP) above the joists. A little more work for the guy swinging a hammer as

opposed to a lot more work for the hanging the rock. In the end it should come down to a satisfied home owner. You're gettin paid either way.


----------



## DRP (Oct 17, 2013)

This is not the way I would want to try to solve the issue but here's a stab.

Figuring the uplift is a trig problem but it's late and my brain says do it the graphical way, by looking at the proportions of the triangle formed. The sides of which are; the top of the joist, the vertical distance of the beam above the joist, and the hypotenuese of the triangle is formed by the strap. For example, if the tension force being resisted was 1000 lbs, the beam were 2" above the top of the CJ and the strap was connected 12" from the beam the uplift force would be 2/12 of 1000 lbs, 167 lbs. By that method I'm coming up with 1014 lbs in the strap and its' connection. the numbers are not for this situation, just an example.


----------



## steveray (Oct 17, 2013)

"without a lot of work by guys that may be P@#$ed off!"

Do you know what the difference between a taper and a puppy is?.....The puppy stops crying after 8 weeks!


----------

