buildup beam and loft floor posts (question)

[beckhamk]

I would like to get some feedback on the beam build up and construction of  the center beam and joists with regards to using 6x6 for additional support for the loft floor.

I am researching taking the 20x30 1 ½ story plans and making the cottage wider 24x30 plan to have engineer review and have trusses designed for this.    We would be going with a block crawlspace and have several block pillars running down the center of the crawlspace to support the center beam.  For this example, I am just using 2x10's.  Looking to do a build of beam of 3  x  2x10's and ½ plywood which would run down the center of the cottage. Each end of the beam would be sitting in a pocket in the block and the top of the beam would be flush with the stop of the sill plate.   Use 2x10's for the rim joists and joists themselves, having  2x10 joists overlapping on the center beam.   Here is the meat of my question:  We are planning to use 2 or 3 6x6 post beams to support the loft floor, when doing this would one want to double up the floor joists at the locations where the 6x6 posts are going to sit?  Or would it be better (seems wasteful) to do another build up beam on top of the center beam and then use hangers to attach the joists to this instead of overlapping them on the lower beam?

Thanks in advance for your wise insight.

[rwanders]

Others here can provide more technically rigorous info. However, i built a 24x24 1 1/2 story somewhat similar to your plan. Three or even two 6x6 posts seem excessive to support a loft unless it will be full width and depth. I framed a 10x24 loft using 2x12s running from the exterior wall to joist hangers on a 4x12x24' glulam which was supported near the center by a 4x4 post buried in a partition wall and in the exterior wall framing at each end. I wanted to avoid posts in the open areas as much as possible.  Just an alternative design that may work for you.

[beckhamk]

Thanks for the info, which is good to know from the sub floor upwards.  But what did you do for your center beam on your main subflood?

[Davegmc]

Rwanders: Does the mid-span support (4x4) for your loft Glulam go all the way down to a footing? Or is it sitting on a mid-span floor support beam or similiar?

Dave

[Squirl]

You can find a walk through of the ICC girder sizing charts for outside bearing walls here:

http://countryplans.com/smf/index.php?topic=10511.0

You can use these charts to interpolate the proper size to number of posts necessary for support for a certain weight.  Per code guidelines.  These codes are designed for outside bearing walls so, approximately half the weight of the designated area, which may be slightly overbuilt if you are going with trusses, but it won't hurt to overbuild and you will know the sizing and spacing are safe.

[rwanders]

The 4x4 midspan support for the loft glulam is supported by a mid span foundation beam and thence down to the concrete columns and footers. It does require proper sizing of the glulam and foundation beams.

[beckhamk]

Thank you all for your posts, this is all good stuff and added some additional checks.   

I feel i have the loft floor figured out, but i think everyone thinks my question was about the joists for the loft. but really my question is about is about the build up of the 1st floor center beam. If I have 3 x 2x10's to form a center beam sitting on block pillars that is flush with the sill plate, would there be 1 x 2x10's on top of the 3 x 2x10 build up beam that the floor joists are connected to with hangers  - then would the 4x4 post for the loft be centered on top of this single 1 x 2x10?  Or is there another set of 3 x 2x10's that tha joists are attached to so that the 4x4 will have a wider set of joist to distribute the weight?  if needed maybe i need to come up with a pic.

[Don_P]

It looks like your post sits directly over the built up girder and over a pier... which would be correct. I'm not understanding the single 2x10 over the girder, the joists typically lap over the girder from each side. A plan view of the floor system would help. I would put triple blocking between the joists under the post. If it's standing alone I'd also use a 6x6 or larger.

[rwanders]

Quote;

"Thanks for the info, which is good to know from the sub floor upwards.  But what did you do for your center beam on your main subflood?"

The 24x24 cabin sets on three 4x12 beams which are setting on 12" concrete piers on footers set about 5' deep plus another 4x12 on piers perpendicular to the other beams which supports a set of double-decked 10x24 covered porches.

[Squirl]

Thank you all for your posts, this is all good stuff and added some additional checks.   

I feel i have the loft floor figured out, but i think everyone thinks my question was about the joists for the loft. but really my question is about is about the build up of the 1st floor center beam.

Actually we were referring to Girders, not joists.  The 3-2x10's spanning from the block wall, and touching each block pillar to the other block wall going down the center of the building that you are calling a center beam is usually called a girder.  As with joists, girders have sizing charts based upon span, width of the building, and weight.  You can find a complete guide to how to use these charts in the original post I put above.

From the picture question with arrows, it looks like you have the idea to put another beam down the center on top of the girder.  The ICC requirement is for the joists to be sistered, and there not to be a beam down the center.  Although, I suppose you could put one girder over top the other, it would be a massive waste of material and a large amount of work cutting each joist.

Also what you are calling a "loft floor joist beam" is also known as a girder.  From your picture, the amount of weight you are proposing to span is pretty extreme for the loft.

[MountainDon]

Take the below as friendly criticism, something we should all think of when we ask questions as well as when we read responses to the questions of others.  The answer to one specific situation may only be correct if all the components are exactly the same. Change one and the correct engineered answer could be different.

From "Pete", the PE (professional engineer)...

The IRC table does not include or account for concentrated loads or continuous beam action, so be very careful how you interpolate, and use their tabulated beam sizes. And, in fact, as an example, if you put 1000# on a simple span beam as a concentrated load at center span, the bending stress will twice as high as if you spread that 1000# out as a uniform load on the same beam. And, of course, if the beam already has a uniform load on it and you put an added post load on it, you must add the stresses from the two load conditions. The first step in designing any beam (ultimately selecting a size) is to correctly determine all the loads on that beam, and their locations. And remember, the lumber yard or big box store won't do that for you, you must do that correctly. Read their fine print when you rely on them for sizing, all they are doing is selling material, they assume no responsibility except for the quality of material, or lack thereof.

The IRC table which is referred to in the "Allowable Girder Spans" thread, assumes some conservative uniform weight and loading on an exterior bearing wall fitting all the conditions of that table, and sizes a built-up beam for that. It is for you to figure out what that loading was and if that actually fits your beam and load condition, it may or may not. Therein lies the danger of some simple interpolation without understanding how the table was actually developed.

My contribution to the earlier thread  "Allowable Girder Spans" is intended to caution everyone against assuming this is a simple problem and that you can use the IRC tables indiscriminately beyond the limits of their intended usage. Simple beams and continuous beams are a relatively simple concept, but not necessarily a simple problem, in their complete and proper design. The comment, (5:36:11AM, 15APR11, reply #9) "You can find a complete guide to how to use these charts in the original post I put above," is certainly not the phrase I would use; because my attempt to be cautionary, in that earlier thread has already morphed into a "complete guide." I am not trying to throw cold water on your projects, I am not telling you what to do, I don't want any part of that.

I am trying to explain construction areas where everyone should be very careful, maybe get some professional help. I am trying to explain how various structural members work, what some of their limitations are, and to explain why some details are good or bad, and why another might be better. I am trying to alert owner builders to areas and details I see which might be under designed or dangerous, but this does not mean they will fail the next day, or under half their intended design loads. I am trying to help everyone develop some basic construction intuition, without the need for a Ph.D. in engineering.

I understand that everyone is trying to save money on their building project by providing most of the labor, and that you hope to be proud of the work you have done. But, at the same time you are spending a fair amount of money on the project and a considerable amount of time too, so don't be penny wise and pound foolish; thank you RWanders. If you can make it stronger (increase the Factor of Safety, where there is some uncertainly in loads, soil conditions, quality of details, etc.) by spending a few percent more on materials, or by improving a detail or load condition, etc., but not add appreciably more in labor, I think that would be a wise thing to do. Irrespective of it being a modest 16x20' weekend cabin, or a MacMansion, you are building something you intend to last for a long time, not a tree fort that might blow apart in the next breeze.

I'm not trying to squelch anyone's enthusiasm or excitement, but I do caution that you work and design carefully, to account for all the likely load conditions, for a successful project which does not turn into a continuous maintenance headache, or worse. As you design, you should be sketching, sketching, sketching.... what does that detail look like, how does it really go together; what is the floor plan, the framing plan, what are the loads on that beam or column, how are they supported, and all of this should have dimensions and notes on it, and on and on. You should have most of this stuff worked out before you start pounding nails, you should certainly have some appreciation for a good or the best way to do a certain detail. There should be some good books out there on building basics, and elementary house construction. Having gone through the vetting process of hard copy publishing, and having been written by a recognized author, they may be a bit more reliable than some of what you get on the internet.

By the way, in a built-up beam consisting of 3 - 2x's, adding 2 pieces of .5" plywood between the three 2x's, only adds width to the beam, pads it out to 5.5" wide. The plywood doesn't really add anything to the beam's strength or deflection characteristics, in my mind. And many times, with a little good planning, the loft floor framing can be mostly supported on partitions below. This assumes that there may be a kitchen and a couple more private rooms under the loft anyway, since a 24' wide bldg. is certainly wide enough for this. You should always try to take heavy loads from above all the way to the foundation with posts or multiple studs, rather than from beam to post to beam to pier to footing. As direct a load path as is possible is almost always the best solution. Loads do not go away just because you forget to consider them, they come back to haunt you later.

Regards.

[beckhamk]

Thanks everyone this information helps.  ahhh "Girder" lol... I should have picked up my house framing book again before creating this thread. I just get an idea and about something and had to ask and see how others have done it.  After reviewing what has been posted here and the house framing book, I am not on the straight path!

[Don_P]

I had a little time to sketch this from memory over the weekend, kinda late but it does show some of what I consider to be prescriptive areas of a similar design and also some engineered sections. I've left out walls to make it a bit easier to see. The foundation, girders and main floor, posts and loft floor system and tied rafters over the loft are prescriptive, right out of the codebook IMO. The right side of the roof being supported by a ridgebeam and the tall gable wall with that tall post supporting the ridgebeam and long studs resisting wind loads are to be engineered by code. In my area on this house that would be a 2x8 wall. The left end support of the ridgebeam, down to the foundation pier can slip through by code but should be checked. There are certainly more caveats but I don't believe your ideas are far off. I do agree review by a qualified design pro is usually money well spent. A good engineer can often do with a dollar what anyone else would do with two.

[Davegmc]

Don_P: I'm looking at a slightly different loft approach. Thinking of framing main floor walls with 10' studs and letting-in the loft floor joists at 8' (to give more loft headroom). The loft floor joists would run opposite direction to your sketch and I'd like to overhang them as much as possible (past the wall below where the lady is standing). This will mean my ridge beam will need to run the full length of the cabin. Do you see any big issues or show stoppers with this approach? Also a question on your pictured ridge beam posts. Those are long 6x6 select beams! Can you segment the post at the floor bottom plate and use another post directly below floor down to footing? Your sketch seems to show the support posts going all the way from the glulam to the footing? Is that the typical way ridge beams are supported?  Is there a prescriptive method to support the post to prevent buckling?

[Don_P]

The foundation and 1st floor system I drew is basically what is shown in figure R502.2 showing a perimeter foundation with internal piers supporting a midspan girder which in turn supports the floor joists. The cantilevered joists are prescriptive if you use table R502.3.3(2) which would be conservative here.

What you are describing is largely engineered design. Nothing wrong with figuring it yourself for general knowledge and preliminary design but it should be checked by an engineer. Section R407 is to my knowledge the only mention and provides very little guidance on columns, it is interpreted many ways but go very carefully. There is an engineered method of calculating the stresses and comparing them to the allowable stresses in the code referenced NDS but there is no prescriptive column section.

The tall post on the gable end has more issues than might appear at first glance. It is tall, slender, has a top load AND simultaneously might have a substantial side load from the wind. It is designed as a beam/column with compression and bending. Imagine holding a stick between thumb and forefinger, squeezing those fingers together and then pushing on it from the side. The combination of both directions of force creates a much greater chance of buckling than from either force acting alone. The math formula for checking this interaction is 2 lines long in the NDS and has 2 pages of instructions... On the building I drew above, in my climate, a quadruple 2x8 post under the ridge with 2x8 studs in the gable wall worked for my preliminary design. These would all be full height members from the top of the floor deck to the top plate of the wall. The post would be better extended down to the sill but usually it rests on the bottom plate and there is blocking from the sill to the underside of floor sheathing. I have had the posts run through the floor system and up to the ridge on a timberframe.

My interior column under the ridge is typically run from main subfloor to underside of second floor girder and from the top of the loft floor to the underside of the ridge. Lateral support for the post by the second floor allows the post to be interrupted at the loft floor level and platform framed. This lateral support reduces the span length of the inboard column and there is no side load. So as a simpler and shorter pair of stacked columns, 6x6's worked for my preliminary design. All floor levels would be blocked to provide a continuous load path from ridge to foundation.