Bearing capacities of Steel Angles? 1

[HB2U]

HI. I'll apologize in advance for the long post.....
I am an Environmental Engineer by trade, so i do not know a lot about this structural stuff, but here's my problem.

I have a REALLY old house. It's basic construction is Four stone walls with a single large beam spanning from gable end to gable end to act as the main support for each floor(story). From this main beam there are Joists runing to the outer stone walls to support the floors of each storey of the building. One of these joists (that supports a divider wall on the first floor) has cracked at the point where a smaller member is mortised into it causing the wall above to sag, throwing the doorway through the wall out of kilter. I want to jack up the beam around the crack and install a steel angle along its length to support it. I plan to install steel posts set on footers to support each end of the steel angle and then bolt the cracked timber to the steel.

The wood joist that cracked has dimensions of 3" x 8" x 7 feet long. I planned to use an angle that is "L" shaped - Long side 5 to 8 inches and short side 3 to 5 inches. Minimum of .25 inches thick.

Problem is I do not know if any standard sized steel angles fall within this range. Nor do I know if this thickness steel angle will be as strong as the 3 x 8 timber that it's replacing/fixing.

Any help is greatly appreciated, even if you can only supply me with the standard dimensions of steel angles available along with their load bearing capacities when supported by a column at each end.
I should mention that rotational forces on the steel angle should be negated by the fact that the wooden beam that it will be sistered to is mortised into the main beam and inserted into a Pocket in the stone wall, both of which should hold it upright.

Thanks again.

 

[LPPE]

L8x6, L8x4, L7x4, L6x6, L6x4, L6x3.5, L5x5, L5x3.5, L5x3 are all standard sizes.

 

[mjohan]

I am curious as to why you would choose steel angles and footings if you only have a cracked floor joist?

Is it possible to jack up the cracked joist and sister some new 2X joists? Most likely 2x8 joists.

 

[HB2U]

Mjohan: I don't believe that I can conventionally sister the beam for the following reasons:

In an attempt to help you understand the layout a bit better, the house is of Georgian architecture. Stone construction of the outer walls with timber framing to support the interior structure (walls floors and stairs)

The house has a center beam that runs the entire width of the house from gable end to gable end and is seated at each end within the stone wall of the structure.It is like this for each floor of the house.(First, second and attic)

The House is set up so that each floor has a center hall/stairwell that runs perpendicular to the main beam with one room on either side. (Both the attic and basement are large undivded rooms. The stairs just enter the middle of each.)

The floor joists for the ROOMS are set perpendicular to the main beam. They are attached to the beam via a mortise and tennon joint and are nestled in the stone wall at their other end.

When you are in the basement looking up at the rafters - In the area of the center hall/Stairwell there are OVERSIZED joists (At least 3 x 8 inches rough cut lumber, maybe more I haven't actually taken a ruler to it yet) that frame out the area of the center hall in such a way that the dividing walls in the rooms above are situated directly above them.

The floor boards in the hallway run PARALLEL to the gable ends of the structure (Perpendicular to the main beam); therefore there are several smaller timbers joined to these oversized joists via pegged mortise and tennon joints to support the hall floor over the lengths of its floorboards.

It is at one of these joints that one of the oversized joists is cracked through. Since the dividing wall for the room upstairs rests directly on center of this joist,and there are cross members joined to one face of this joist, I would only be able to sister it on one side. This means that the load being supported by the Sister would be offset by at least 1.5 inches, contributing to torque around the new joist. And then there's the question of how to join its ends to the stone wall and main beam.....So I'd need the posts and footers anyway. Besides, I don't think that even (2) 2 x 8 pieces of lumber could support the weight of the walls above since they're the old horsehair plaster over hand split wood lath types of walls.(very heavy)

I would just put an I beam directly below the cracked joist and then support that at both ends, but head space in the basement is at a premium!

I thought that the steel Angle wouldn't take much headroom away, but yet it would provide solid support of the walls that will be resting on it.

Hope you have a better picture of this in your mind now.

Any other sugestions? Am I wrong in planning to fix it this way?

Thanks.

 

[mjohan]

I see your point. I take it your beams and joists are also open.

Angles which pylco has mentioned are all angles which might interest you. Each will have a minimum standard thickness, 1/4" to 1/2". They will all, most likely work for your purpose. You might want to use 1/4" or bigger lag screws @ 2' on center to also attach the angle to the cracked joist. If it is 7' long, use four lags. Only a suggestion. Good luck.

 

[mjohan]

HB2U,

I wanted to let you know that a calculation assuming 500 plf on an angle 5x3x1/4" thick, assuming laterally restrained in the vertical plane, would calc. slightly overstressed according to AISC.

It sounded like your joist is also acting as a beam. Without telling you what size, bigger is usually better. Take care.

 

[HB2U]

Thanks for the information! I spoke to the local steel place and they have 6 x 3.5 available in 3/8" thickness so I'm going to go with that.

Again, thanks for all of the information.

 

[mjohan]

Your welcome,
One last thought... Check out simpson hangers to replace that steel column you are suggesting. It does not seem necessary to install a column and it might cause you more problems.

I am actually going to take a column out of my basement and design a flitch plate to enable a further span of my main beam.

 

[VoyageofDiscovery]

HB2u,

What is the span between supports of this angle beam you wish to install.

VOD

 

[HB2U]

VoyageOfDiscovery: There will be one post at either end of the steel angle. They will be 7 +/- feet apart. There is a doorway centered along the seven foot span that is about 32 inches wide. loading will look like this...

W W D D D W W
S T L A N G L
P P
O O
S S
T T
Where "W" is the wall, "D" is the door, "STLANGL"
is the steel angle.

Thanks for any input that you have.

 

[VoyageofDiscovery]

Hi HB2u,

According to the March 1981 Edition of Civil Engineering ASCE a single angle lintel 6 x 3.5 x 3/8 with 7ft of clear span should be good for 1314 lbs point load situated at midspan or 375 lbs/ft UDL with deflection of L/921 and L/806 respectively.

Your doorway may be transferring load from above, directly down to the supporting beam beneath it, so include the loads above this doorway as well when figuring your loads.

Hope this helps.

VOD

 

[mjohan]

Voyage,
The allowable loads you have given are for an angle able to bend in any direction. I believe by lagging the angle to the beam would prevent bending in the vertical plane, yielding much lower "actual" stresses (fb=M/Sx). Should this not be assumed?

 

[VoyageofDiscovery]

mjohan,

I believe actual stresses will be the same, however the stability of the angle will be greatly improved. The article states that the assumption for its derived capacity is that the upstanding leg acts as a thin plate supported laterally along its lower edge. Thus the assumption would still be valid.

Regards

VOD

 

[mjohan]

I am a bit confused as to why the code stated the angle is supported laterally along it lower edge. Brick does not typically get tied into the angle.
The angle, however, was, and should be, considered free to bend in any direction. The allowable load of 375 plf for a seven foot simple span are assuming an angle capable of bending in "any" direction. I use a great book by Charles G. Salmon and John E. Johnson. "Steel Structures, Design and Behavior, Second Edition". Section 7.8 "General Flexural Theory" talks about unsymmetrical bending and restraints from bending. The following is a quote cited in this book on pg. 378...
"If attached construction constrains an unsymmetrical section to bend in the vertical plane, the restraining moment capacity can be computed by using EQ. 7.8.5." Fb=My/I
Regarding HB2U's original question as to what size angle he should use for his joist repair, I believe my original advice was correct.

Take Care,
Mark

 

[VoyageofDiscovery]

Hi Mjohan,

I see your confusion and now that you mention it I am also a bit bewildered by the statement of lateral support along its lower edge. However, I cannot discount the formulae. Respectfully, dismissing the published conservative formula goes against my better judgement unless I am shown it is in error.

I still believe the stress that you alluded to on Feb 26th is the same whatever the lateral support conditions, unless they act compositely.

Regards

VOD

 

[GStruct]

I think the "bottom laterally supported" means that the bottom edge of the vertical angle leg is laterally supported by the horizontal angle leg -- not that the entire angle is laterally supported. I'm not familiar with this article could you let me know specifically in what journal it was published?

 

[mjohan]

Hi voyage,

I was wondering if you could tell me if the charts that you have cover a 20'8" span. If so, could you tell me the size angle for a uniform load of 450 plf?

Thanks so much,
Mark

 

[VoyageofDiscovery]

Hi mjohan,

I'm afraid not, the chart only goes up to 11 ft.

Regards

VOD