TYPICAL TIMBER SADDLE HANGERS 1

[lextoo]

I WANT TO DESIGN SADDLE HANGERS USING 1/4" THICK PLATE.
WHAT IS THE DESIGN PROCEDURE? IS THERE LITERATURE IN THIS REGARD? I WANT TO TRANSFER BEAM REACTIONS TO GIRDERS.
THE BEAM REACTION IS TRANSFERRED BY BEARING PERPENDICULAR TO THE GRAIN AT THE BOTTOM OF THE HANGER. THE LOAD ON THE HANGER IS THEN TRANSFERRED BY BEARING PERPENDICULAR
TO THE GRAIN THROUGH THE TOP OF THE SADDLE TO THE GIRDER.
HOW ARE THESE BEAM SADDLE CONNECTIONS ANALYZED?

 

[jheidt2543]

Just a thought, why reinvent the wheel? Check out

http://www.simpsonachors.com.

They make the metal joist hangers and have a line of heavy duty hangers too. You may find some readily available for a lot less money than designing and fabricating your own.

 

[lextoo]

WHAT IS THE ANALYSIS PROCEDURE? WHAT ARE THE ASSUMPTIONS?
VISUALLY, THE PREFAB/OFF THE SHELF HANGERS ARE NOT VERY ATTRACTIVE...IN CASES WHERE THE "HANGERS" ARE VISABLE THIS IS A FACTOR.

 

[Trussdoc]

You would have to consider at least the following factors:
1. Loads
a. magnitude
b. duration
c. direction
d. will there be load reversal due to wind load?
e. eccentricity
2. Wood
a. type of wood will determine strength of connectors
b. crushing strength (i.e. compression perp. to grain) will determine minimum area of seats and flanges
3. Steel and Welds

 

[lextoo]

WELL--AS REGARDS THE LOADING--THIS IS A TYPICAL ROOF LOAD
SAY: DL=25PSF AND SNOW LOAD=25PSF---WIND UPLIFT IS NOT A FACTOR.

I AM LOOKING AT A POINT ON THE GIRDER THAT IS SUPPORTING TWO PURLINS-
ONE EACH SIDE --THE PURLIN REACTION IS 6 KIP EACH--SO THAT THE GIRDER IS SUBJECT TO 12 KIPS (AT THAT LOCATION)--NOW HOW DO I DESIGN THE TOP PLATE? DO I ASSUME THE 12 KIPS
IS UNIFORMLY SPREAD UNDER THE TOP PLATE? (IT SEEMS THAT THIS IS, FREQUENTLY, DONE)-- AND DESIGN THE PLATE USING
WL*L/8...THIS REQUIRES A FAIRLY THICK PLATE---IT SEEMS MORE REALISTIC THAT THE PRESSURE PATTERN BENEATH THE PLATE WOULD BE UNEVEN--WITH A PEAK NEAR THE BEAM EDGE...THIS WOULD ALLOW
A THINNER PLATE...WHAT DO YOU THINK?

 

[lextoo]

ALSO, FOR THE SAKE OF THIS DISCUSSION--ASSUME I AM
USING SOUTHERN PINE GLULAM 24F-V3...ALLOWABLE STRESS
PERPENDICULAR TO THE GRAIN = 740 PSI

 

[RockEngineer]

I have designed a lot of custom hangers like you are talking about because I deal with log homes and open beam ceiling combinations that don't fit the Simpson standard profiles.

With 1/4" steel for your brackets with a full saddle (completely across the beam with pockets both sides) you will not stress the steel very much if you keep the loads below the bearing values of the wood. I always assume the load is evenly distributed under the top of the bracket. If you don't you are going to work yourself to death trying to come up with a distribution and it will make very little difference in the end. The weld lengths and bearing lengths on the wood are going to govern your pocket depths and the length of the saddle along your girder. Check your welds in shear and bending. Your 3/16 double fillet gives your 100% efficient weld with 1/4" steel plate. Just to seal things off usually gives more weld than required but work through the calcs. The bolts in your bracket are for uplift and longitudinal pull along your intersecting beams. I ignore them in resisting vertical load if I have a full saddle. If your are not using a full saddle across the top of the beam then you need to determine what portion of the load is distributed to the flange and the bolt. It is conservative to assume one takes the whole load and the other is there for conservation. If you get too involved the cost of engineering for your bracket makes it unrealistic. Extra material is usually cheaper than engineering.

Good Luck

 

[lextoo]

OK....LET ME SEE IF I AM UNDERSTANDING YOU CORRECTLY,
USING A 1/4" PLATE ASSEMBLY FOR THE ENTIRE SADDLE
HANGER BRACKET...AND SIZING THE TOP PLATE ON
THE SADDLE AND THE BEARING PLATE AT THE BOTTOM OF
THE HANGER TO KEEP BELOW THE ALLOWABLE BEARING VALUE
OF THE WOOD (CHKG WELDS FOR SHEAR & BENDING)...YOU ARE SAYING IT IS NOT NECESSARY TO DESIGN THE PLATES
FOR BENDING (WL*L/8)--I HAVE SEEN OTHER DESIGNS WHICH COME UP WITH 3/4" PLATES ON THE TOP BECAUSE OF THIS REQUIREMENT--

I AGREE, WITH YOU --THE 1/4" PLATES WILL DO THE TRICK...
BUT, I'M TRYING TO UNDERSTAND, EXACTLY, WHY...
IS THERE ANY LITERATURE WHICH EXPLAINS WHY THIS WORKS?

 

[lextoo]

OH, I GUESS ANOTHER POINT WOULD THE MAGNITUDE OF THE REACTIONS---SAY 5K TO 10K MIGHT BE OK--BUT WOULD YOU DO THIS FOR 20K TO 30K?

 

[Trussdoc]

With high reactions like that, you have other considerations....such as shear in the supporting beam and significant torsional stresses if the load is unbalanced.

I would use a calculation analogous to that used for column base plates on concrete to account for bending in the top flange on wood that is much softer than the steel. Treating the top of the saddle as a base plate with concentrated loads at the ends and a supporting surface with an fc equal to the fc perp of the glulam.

 

[lextoo]

THANKS TO THE ENGINEERS WHO CONTRIBUTED TO MY
KNOWLEDGE BASE OF METAL TIMBER SADDLE HANGERS...IN PARTICULAR,
I FOUND RockEngineer's COMMENTS EXTREMELY HELPFUL...
ONCE AGAIN THANKS

LEXTOO