Footing Loads

FYI - Check footing loads on a house this week for a foundation repair project in Columbia, MO and thought someone would like the information:

  • 50’x28’ one story all brick house on a full basement (Engineered Trusses)
  • Footing load on long wall 2100 lbs/ft
  • Footing load on short wall 1500 lbs/ft
  • Concentrated load at end of beam on short wall 5200 lbs

Same house with a stick built roof:

  • 50’x28’ one story all brick house on a full basement (Engineered Trusses)
  • Footing load on long wall 1900 lbs/ft
  • Footing load on short wall 1500 lbs/ft
  • Concentrated load at end of beam on short wall 8100 lbs

I don’t undertand. Both conventional and truss roofs bear on the long exterior walls. Conventional roof framing will transfer a minor amount of weight to the short (gabled end) walls. The beam in the basement supporting the first floor framing should not be affected by the roof framing unless purlin braces oin a conventionally-framed roof bear on walls that transfer the load to the beam (basement girder).
Trusses are typically designed to bear on exterior walls only and should transfer no weight at all to interior walls and the basement girder. What am I missing here, Randy?

Ken

The ceiling joists on stick-built roofs are supported by the outside wall and a load bearing wall in the center of the house. This puts 1/4 of the total ceiling joist load on the outer walls and 1/2 the ceiling joist load on the center wall. Most stick-built roofs have some vertical supports and/or some mid-spam rafter supports all terminating on the center support wall. Some very narrow houses may not have any interior braces, possibly collar ties only. In that case only 1/2 the ceiling joist loads are supported by the center load bearing wall. In many cases around here carpenters have no clue how to brace a stick-built roof and you find these internal braces terminating on a ceiling joist mid-span with no support wall which causes the ceiling to sag.

So the footing load on the outside wall is greater with trusses when some of the roof load on a conventionally framed roof is transferred to interior bearing walls by purin braces?

Are you saying three is better than two because of the transfer ratio. 1/4 on each wall and 1/2 in the middle wall transfering to foundation.

Same house with a stick built roof:

  • 50’x28’ one story all brick house on a full basement (Engineered Trusses)
  • Footing load on long wall 1900 lbs/ft
  • Footing load on short wall 1500 lbs/ft
  • Concentrated load at end of beam on short wall 8100 lbs

Contractors don’t understand what is needed so the ceiling starts to sag.

Are the rafter/purlin braces adequate if they are sloped less than 45 degrees from horizontal?

http://www.nachi.org/forum/attachments/f23/49274d1319961282-footing-loads-roofframe.jpg

Those rafter knee braces are a really bad idea, just like adding 2x4 ridge posts at a center bearing wall (more common in my area), unless the house was designed that way.

It transfers a significant roof load from the outer walls to the center bearing wall/beam and can lead to settlement of the beam post footings.

My original point was to show footing loads, but everyone so far has picked up on the differences in truss loads. Not to preach to the choir but all stick built roofs are installed after all the walls are built, including interior walls. Then the ceiling joist are placed, supported by the exterior & interior walls. Typically these houses are built with the main load bearing wall running down the center of the house, which sits directly over the main beam in the basement or crawlspace. This configuration will place 1/4 the total ceiling load on the outer walls and 1/2 the total ceiling load on the center support wall. (see diagram)

Now the the roof rafter loads on a stick built roof have multiple support configurations some are good and some are sloppy. If the rafters have no purlin supports or braces and only collar ties then all the rafter load will go to the outer walls. When you see vertical supports, purlins and diagonal bracing then some rafter loads are being transferred down which is being picked up by interior walls that transfers the load to the floor which transfers some of the load to the main support beam and so on.

The first picture shows a stick built roof with a hallway down the middle and braces termination on both hallway walls. The second photo is a large new house with purlins and braces scattered all over. (note: some of the purlins were under sized and bowing under the load) The third photo shows a purlin/brace system terminating on an interior wall, which could work if the floor joist under that interior wall is doubled or tripled depending upon the load. In most cases nobody though about how the loads would be transferred to the footing, i.e. the load path, and the ceiling and/or floors start to sag over time.

Back to my original post… The point I wanted to show is the footing loads on a typical house are not equal and can vary by a large amount. You can see over time the footings with the larger loads will tend to settle more than the others. If the walls and footings are adequately reinforced you will not likely see the actual settlement differences, but you may see some stress cracks develop in the walls. In theory you would vary the footing widths to provide a even pressure to the soil so the building settles evenly, but this is only done on commercial buildings that are custom engineered.

I hope this helps clarify the footing load issue…

simple spans.jpg

DSC01623_640x480.jpg

Clear as mud! Thanks for explaining this for newer HI’s.

…and for the CMI’s that think they know it all.:wink:

LOL, OK, so homes with trusses may have foundation cracks from differential load bearing if the footings have not been custom designed to allow for the difference.
I’m trying to picture where those cracks would most likely be located and what they would look like. At first I thought vertical cracks at the corners, but if the weight on the long walls force the corners of the short walls down, would that create a bending force in the middle of the short wall that would cause through cracks wider at the top?

LOL, some kinds of knowledge you have to force into your head with a big C-clamp.
OK, so homes with trusses may have foundation cracks from differential load bearing if the footings have not been custom designed to allow for the difference.
I’m trying to picture where those cracks would most likely be located and what they would look like. At first I thought vertical cracks at the corners, but if the weight on the long walls force the corners of the short walls down, would that create a bending force in the middle of the short wall that would cause through cracks wider at the top?

Ken

Now I have your creative juices flowing… If soil was truly a homogeneous material and the compaction prior to placing the footing was uniform your assessment is good. However soil composition varies as well as true compaction. That’s why I tell others dealing with concrete and steel is a science and working with soil is more an art then science. Unless you did a complete floor level survey and recorded all the footing elevations when the house was built it would be pretty hard to prove what caused all the cracks found in a foundation ten years later. If everyone now has a better understanding of how building loads and load path effects the foundation, I have done my part.

Thank you, Randy! This is the sort of information that is very tough to find online. It’s especially helpful to be able to ask you questions directly. Your participation on the boards is greatly appreciated.

Yes it is very much appreciated!

very good.

GREAT INFO !!! Thanks for sharing Randy.

Still the loads in both cases exceed 2000 psf… the problem for the most part is soil bearing capacity and/or in NC are the expansive soils we have here…add to the fact that very few homes are properly drained…the results are foundations cracks etc.

I love stick built roofs…often cheaper and greater versatility.

I am currently building a large custom home with a very complex roof system…LVL’s everywhere including the ridges. Will have to pics and post.

Jeffery

That footing load is per linear foot not square foot. If your footing is 2 foot wide then the soil load would be 2000/2 = 1000 psf

Im saying that soil bearing capacity is measured at per square foot.
As stated earlier, many times its the soil and improper drainage around same.

If memory serves me, most clay soils are around 2000 psf, sandy gravel around 5000 psf and bedrock over 10,000 psf.

Still with expansive clay soils it becomes problematic…

Understandable why a simple carpenter told the story about a wise man building his house upon a rock… :slight_smile: