Removing a support from basement wood beam 2

[Ipetu]

Existing 5.5"x5.5" x 15.5' long wood beam supported at the ends and an intermediate timber post at about 9' from one end. The intermediate post is to be removed. The existing beam has to stay in place and appropriate strengthening has to take place with steel sections (2 C channels) with depth less than or equal to 6" (152 mm). The beam supports two floors in a residential occupancy. Tributary width of the existing beam is 10' (3.0 m). The following loads were used 1.0 kPa for dead load and 1.9 kPa for live load. I assumed that the new channel sections will take all of the load. From a strength point standard channels will not work. I had to select a MC 150x22.5. However this resulted in deflections of L/170 for total load and L/259 for live load. Code limits for live load is L/360. So I chose a MC 150x26.8 (the largest 6" deep MC available). Now my deflections are L/203 for total load and L/308 for live load. Still not satisfying the L/360 code requirement.Questions:

1) Attributing some amount of load sharing to the timber beam will satisfy the L/360 code requirement. Is this correct?

2) During the actual construction I was planning on pushing the channels right up against the floor joists to ensure that the compression flange is laterally supported. However I read on a post sometime back the following "Make sure the beams are as high as they can go but do not try to
force the C Channel up to contact the floor joists becaue when you release the force the C channel will just pull the floor joists away. Just sung it up there as best as you can." What does the author mean exactly? Is there a real danger in pushing the channels to contact the joists?

3)The usual bolting for this type of work is to use 5/8" diameter threaded rod at 1' interval in a top and bottom staggered pattern. What is the science behind this? i.e. how is this determined?

4) Will the bolting mentioned in 3) above ensure that the compression flange is fully supported? If not how can support for the compression flange be achieved?

Thanks very much.

 

[msquared48]

Rather than do what you describe as you seem to be fighting the problem in an up[hill battle here, why don't you shore the joists, remove the existing 6X6 beam, and install a new glulam flush beam that could be the depth of the floor joists plus the original 6X6.

If that doesn't work, then try a parallalam. You might be able to get a GL 5.125 X 12 or 13.5 to work (24FV4), depending on the depth of the existing joists. How much head hieight do you have below the 6X6 presently? 6 feet?

The existing joists can be hung off the new wood beam with metal joist hangers.

Mike McCann
MMC Engineering

 

[Lion06]

1) It may or it may not. You'll need to do a transformed section analysis and see how much load the wood takes and if it takes enough load away from the steel to get the deflections under the L/360 limit.

2) Pushing the channels against the joists is not the mechanism that will brace the compression flange. The bolting to the original 6x6 will brace the compression flange (at the 2' intervals that the top bolts occur at per your assumption).

3) I believe that's just a standard. I would size the bolts based on the amount of load that the beam has to transfer into the steel. This will be a plf quantity. Space the bolts to satisfy the yield limit equations of the NDS. This will be critical if you do go down the loadsharing route in question 1. It's not as critical if you don't. Provide enough bolts at the end to get the load from the channels back into wood beam for bearing. This is critical regardless of whether you go down the loadsharing route of question 1 or not, unless you provide an adequate connection from the channels to the support (e.g. full bearing). I actually have a paper on this from an old edition of Structure magazine. I'll post it later.

4) The bolting will brace the compression flange. Only the top bolts will achieve this.


A couple other things I would suggest. You may, or may not, have already done these.

1) Check the foundations for the new reactions without the existing post in place.

2) Check vibrations. If you are having a hard time getting deflections to work, vibrations could be an issue.

3) If you don't provide a positive connection from the channels to the support (e.g. full bearing), make sure you check the wood beam for the new shear at the ends. It will be significantly higher.

 

[Lion06]

Here is the article

 

[dik]

If I reinforce using steel, I generally ignore the wood capacity since the relative stiffness of steel is so much greater. I generally use the existing beam to provide stability for the steel only.

Dik

 

[dhengr]

I’m kinda new at this forum thing, but not so new at the engineering thing and I’m a bit perplexed by the quick and dirty, fast and loose, of some of the threads. From what I’ve seen in many threads, a question is asked by someone who doesn’t have the foggiest idea of the whole concept surrounding his problem, he’s not asking for a simple clarification of one aspect of the problem, he wants a bachelors degree in engineering from the thread. He already has a computer and a canned analysis program which he doesn’t understand, so he’s already half way to a PhD. The lack of basic understanding is further exemplified by the lack of enough background info. so that an intelligent exchange could possibly take place, and then the profound advice ensues.
My compliments to all of you: good question, because enough thought was put into the problem before asking, then enough info. provided so that meaningful exchange can take place; the responses are darn thoughtful too, all good points.
Only things I might suggest considering:
1. Have the channels pre-drilled, clamp to wood beam, and drill wood from both sides, straight and level please.
2. The jst. to bm. bearings are working fine, don’t mess with them, set the MC 1/4" lower than the bot. of the jsts.
3. Jack the wood beam up 3/8" at its third points (not my magic number, but probably pretty close, you think about it), do this slowly, and watch and listen for what’s starting to crack above. Remove the post, clamp, drill and bolt the MC’s (not threaded rods, i.e. threads out of bearing areas). The idea is to camber a bit more than any existing deflection, but less than the anticipated delection.
4. Release the jacks gradually, most of the 3/8's will go away, in bringing the whole system into play, the bolts bearing in over sized holes in the wood.
5. Bearing stress will very likely be a problem, beef the jambs (posts) up by adding bearing under the MC’s, they are actually carrying the load now, and by the suggested end bolting pattern, which will be problematic on a 6x6. Check the found., but likely not much of a problem.
6. If you remove the existing 6x6, LVL’s might be best and most easily obtained, deflection and end bearing will the critical design considerations. Will the fl. sheathing span the LVL’s, cut jst. end to cut jst. end? If there is a brg. wall above, you must press the LVL’s up to the sheathing similar to #3 abv. Otherwise, you may want to leave a gap btwn. the top of the LVL’s and the sheathing because the jsts. will settle into the hangers later.
7. Shore the jsts. an 1/8" or 3/16" higher than the top of the LVL’s, use and read (suppliers literature) about the right sized jst. hangers and use the right nails. Nail the jst. hangers to the LVL’s first with the jsts. seated by hand pressure. Jst. hangers set to a snapped line don’t account for variations in the jsts. and tend to lead to an uneven fl. Lower the jsts. part way to really seat them in the hangers, and nail hangers off to the jsts. Most of the 1/8" should disappear. Shim the rest under a bearing wall. It’s kind of a finicky job if you want to do it right.

 

[BAretired]

1) Attributing some amount of load sharing to the timber beam will satisfy the L/360 code requirement. Is this correct?

Probably not. It will help a bit, but a 6 x 6 will not help very much. Your channels are too shallow for the span.

2) During the actual construction I was planning on pushing the channels right up against the floor joists to ensure that the compression flange is laterally supported. However I read on a post sometime back the following "Make sure the beams are as high as they can go but do not try to force the C Channel up to contact the floor joists because when you release the force the C channel will just pull the floor joists away. Just sung it up there as best as you can." What does the author mean exactly? Is there a real danger in pushing the channels to contact the joists?

Place the channels up until they make contact with the joists but do not force them up. If some joists are not making contact, you can fill the gap with shims.

3)The usual bolting for this type of work is to use 5/8" diameter threaded rod at 1' interval in a top and bottom staggered pattern. What is the science behind this? i.e. how is this determined?

Spacing of the top bolts should be governed by the unsupported length of the channel unless you are leaving a gap between channel and joists in which case the bolts have to be capable of transferring all of the load into the channels. Spacing of the bottom bolts is nominal.

4) Will the bolting mentioned in 3) above ensure that the compression flange is fully supported? If not how can support for the compression flange be achieved?

Bolting through the wood beam near the top of the channels is enough to provide lateral support to the compression flange.

It would be markedly better structurally if you could install somewhat deeper channels, perhaps 7" or 8" deep.


BA

 

[MiketheEngineer]

I agree with Dik - just the size the C-channels to take the load and deflection. Any help from existing wood beam is just frosting on the cake. Put a couple of bolts every 4-5' (check Lu) and you are good to go. You can even use nails or lag screws to "pin" the top flanges against the existing wood beam - no thru bolts required

Shoring the floor and removing the existing beam and installing a new Lam beam is prettier but a lot of work. Also - watch out for squash blocks carrying columns from above and shear from possible wall above.

 

[BAretired]

Personally, I prefer thru bolts over lag screws, but the holes must be aligned perfectly and drilled from each side.

In addition to bolt holes drilled in the channels, it is helpful to get the fabricator to drill a few small nail holes. That way you can hold the channels in place with nails while using them as a template for drilling the wood beam.

BA

 

[msquared48]

You just need a good jig for the drilling here.

As Will Smith would say, it's time to get jiggy!

Mike McCann
MMC Engineering

 

[nuche1973]

I have one question: Will the owner let you tear out the floor? If not, then installing a new beam with face hangers may not work, no room to swing a hammer. You could use a pneumatic nail gun, but you'll need to check the capacity of the nails. Most hanger manufacturers will have reduction factors for nails not supplied with their product.

 

[Ipetu]

Turns out the span is 13.8' and not 15.5'. The 13.8' certainly helps to keep the live load deflections at L/425 (which satisfies the L/360 limit). We will use 2 MC 150x26.8's and use 5/8" diameter through bolts at 1' intervals. Thanks for all the input. The information provided will be considered as we embark on this little project.

 

[paddingtongreen]

Whoa, hold the phones. We have a two span beam that is doing its job, it's fully loaded, and a column that is also doing its job. We want to replace the function performed by the column, supporting the beam in the middle.

Fabricate a "U" shaped saddle with the opening to fit the beam except the sides should be say 1/2" short. This saddle to be drilled for some countersunk screws to be driven from the outside, and have some countersunk bolts, welded in place, with the threaded ends sticking outward.

Provide pilasters or other support at the walls for the channels, they will be carrying only their selfweight and the column load.


Jack the beam close to the column only until the interface opens by the slightest margin ( we don't want to crack any plaster walls above). Remove the column and replace it with the "U" assembly. Jack the assembly hard against the bottom of the beam, drive the wood screws perhaps two on each side to hold the assembly in place. Bring up the channels, pre-drilled to fit the studs on the saddle, and at the maximum intervals for top flange stability. Put spacers in between the channels and the beam and drive screws though the stabilization holes. These should not be large, an old conservative rule of thumb said use 2% of the flange compression, and we don't want them transferring vertical load, drive shims between the channels and the pilasters and remove the jack.

In this design, the channels only carry the column load, the beam continues to do its job as though nothing had happened, and we don't tear it to shreds with big through holes at close intervals. Those through bolts are problematical anyway, unless they are drilled through the steel and wood, in one go, "in situ"; otherwise the fabrication clearances make for a sloppy fit with doubt as to whether they are all in bearing at the same time.

Timing has a lot to do with the outcome of a rain dance.