# In IRC is a Beam a Girder?



## Uncle Bob (Feb 22, 2013)

2009 IRC, R502.5  Allowable girder spans.

1.  When beams (girders?) are used for purlin braces do you use Tables R502.5(1) and (2)?

2. I cannot find a span chart for "beams".  Does this mean that all beams used for purlin and other roof bracing must be Engineered?

I have attempted to find the differences between beams and girders on the internet and there were as many different opinions as answers.

From what I've read most believe that beams carry only loads directly on top and girders carry side loads; this being the difference.

If a beam only provides resistance from the top (bending); then shouldn't only vertical loads that apply weight straight down be used on beams?

Pulin braces are an important part of the support system.  Because purlins normally are at an angle to the top of the beam/girder?; it seems to me they cause stress to the sides of the beam/girder?; which makes them a girder.

Hope I phrased the above correctly.

Thanks,

Uncle Bob


----------



## DRP (Feb 23, 2013)

A beam is a member that resists load in bending. A girder is a beam that resists loads delivered to it by other beams. a girder may have loads on top of it (a dropped girder) or the loads may hang from the sides ( a flush girder), or the loads could be dangling from it. If the term is confusing don't use it, a girder is just as properly called a beam. You can call my ride a dodge if you want to show your superior knowledge but it just as correct to call it a truck.

Generally we check a beam oriented upright and loaded vertically. The load flows through the beam and is resisted by the geometry of the section in its' strong direction. Lay that same member flat as a walkboard and the geometry of the resisting section changes considerably... a 2x12 isn't going to take the same load flatways that it can if upright. If the beam takes the load through the section at something between vertical and horizontal look at that section modulus, it'll be something between the resisting ability of vertical vs flatways.

Not sure if it would help or confuse, your call, I can show you math for an identical timber and load with the load pushing in each of those 3 directions. But... remember we deal in assemblies, the ceiling surface or some form of lateral bracing can very often redirect the flow of load to a more favorable orientation. A purlin is a beam, you are calling a purlin post (or brace) a purlin, don't confuse them. The purlin post is prescriptive to 45* for this reason.


----------



## DRP (Feb 23, 2013)

This page is not finished, but it might help with some of the concepts, comments always welcome.

beam_design1


----------



## Uncle Bob (Feb 23, 2013)

DRP said:
			
		

> A beam is a member that resists load in bending. A girder is a beam that resists loads delivered to it by other beams. a girder may have loads on top of it (a dropped girder) or the loads may hang from the sides ( a flush girder), or the loads could be dangling from it. If the term is confusing don't use it, a girder is just as properly called a beam. You can call my ride a dodge if you want to show your superior knowledge but it just as correct to call it a truck.Generally we check a beam oriented upright and loaded vertically. The load flows through the beam and is resisted by the geometry of the section in its' strong direction. Lay that same member flat as a walkboard and the geometry of the resisting section changes considerably... a 2x12 isn't going to take the same load flatways that it can if upright. If the beam takes the load through the section at something between vertical and horizontal look at that section modulus, it'll be something between the resisting ability of vertical vs flatways.
> 
> Not sure if it would help or confuse, your call, I can show you math for an identical timber and load with the load pushing in each of those 3 directions. But... remember we deal in assemblies, the ceiling surface or some form of lateral bracing can very often redirect the flow of load to a more favorable orientation. A purlin is a beam, you are calling a purlin post (or brace) a purlin, don't confuse them. The purlin post is prescriptive to 45* for this reason.


DPR,

The reason for my question is that the 2009 IRC tables (R502.5(1) and (2) are for Girders and headers (none for the word beams), so I'm seeking to find out if the Tables for girders and headers are to be used for beams/girders that support purlin braces.

Can anyone answer that question?  Do you use the above tables to determine the span for beams (girders?).  If not is engineering required?  There is no table for anything called a "beam".

2009 IRC, R502.5  Allowable girder spans.

1.  When beams (girders?) are used for purlin braces do you use Tables R502.5(1) and (2)?

2. I cannot find a span chart for "beams".  Does this mean that all beams used for purlin and other roof bracing must be Engineered?

I have attempted to find the differences between beams and girders on the internet and there were as many different opinions as answers.

From what I've read most believe that beams carry only loads directly on top and girders carry side loads; this being the difference.

If a beam only provides resistance from the top (bending); then shouldn't only vertical loads that apply weight straight down be used on beams?

Pulin braces are an important part of the support system.  Because purlins normally are at an angle to the top of the beam/girder?; it seems to me they cause stress to the sides of the beam/girder?; which makes them a girder.

Thanks,

Uncle Bob


----------



## ICE (Feb 23, 2013)

Purlin braces can land on a bearing wall or an engineered support.  Walls are self explanatory.  Beams are engineered to the degree that the load and span will vary.  I recently had a building with 2x6 ceiling joists and a purlin brace to a 2x on top of three ceiling joists.  I let them make suggestions and when they got to three 2x8 sandwiched, I said OK.

This website is informative and if you read what's there, you will gain a better understanding of beams.

http://theownerbuiltcabin.com/articles/beamdesign/beamdesign1.html


----------



## DRP (Feb 23, 2013)

Lose the term girder or header and use the term "beam". I know its hard for a bureaucrat to not parse semantics, it's hanging you up.

You can take apart your own logic by considering that the terms header and girder are used in that table, the top/side argument falls apart right there.

A beam can carry a load from it's top, its' side, it's bottom... none of that matters to what a beam is. I pull my engines from a beam, the load is dangling. If I sit the engine on top of the beam or if I slung a chain over the beam and dangle the engine underneath, does the beam care? not one wit. A beam is a member that resists loads in bending. Is a beams ability to resist load dependent on orientation and direction of load? Sure.

The table is for uniformly loaded beams. Are your purlin braces spaced frequently or are there very few concentrating point loads on the beam?

Is the beam laterally stabilized by a ceiling or some other means? If so consider the load to be vertical as long as the brace is steeper than a 45.

For the english part we'll end up in france.

In timberframing the massive beam running down the center of a room with joists framed into it from each side is called a "summer beam". Its a girder, a beam that carries other beams. Where in the heck did that name come from? Sumpter, a pack animal, a heavy hauler. And in old french from whence that came "sommier" our summer beam.

Actually the old cash ox should be calling it the summer table  

Bressummer - Wikipedia, the free encyclopedia


----------



## mark handler (Feb 23, 2013)

See previous thread

http://www.thebuildingcodeforum.com/forum/residential-framing-codes/10144-girder-beam.html


----------

