DO HOUSE FOUNDATIONS NEED STRUCTURAL ENGINEERING?
by Patrick R. McCormick, S.E., P.E.
BRANDER CONSTRUCTION TECHNOLOGY, INC.
Introduction
When structural engineers begin their practice, they dream of designing large complicated structures that support or ornate landmark buildings and bridges. Structures associated with man’s endeavor to create and to defy Mother Nature’s constant battle to reclaim building materials to their most basic components.
In most cases though, structural engineers work their whole careers designing what most people would consider normal, everyday structures that, while not “breath-taking”, are the everyday structures on which we all travel and in which we all live and work. Unfortunately, one “everyday” structure that is sometimes forgotten when it comes to structural engineering is the house in which we grow-up, raise our kids, and grow old. When a new house is built, little thought is given to the loading or load resistance of the foundation system supporting the house. The primary focus of the owner and builder is on floor plan, landscaping, and overall esthetics. Typically, the foundation that supports the house receives little or no structural engineering input and unfortunately, some houses could really use it.
This paper will review investigations of three separate failures that have occurred in foundations supporting typical single family residences, explore the cause for the failures, and list ways those failures could have been avoided with the services of a structural engineer.
Typical Scenario
For my presentation today, I will be referring primarily to the One and Two Family Dwelling portion of the Wisconsin Administrative Code, which generally parallels the International One and Two Family Dwelling Code, and most other One and Two Family Dwelling codes.
What happens when a person or family decides that they want to build a new house. First, they look at lots or places they would like to live. Then they start thinking about the house it self. How big, how many rooms, and an important one; how many garage stalls. Very little thought is given to the foundation on which their house will be built, short of deciding what depth is needed for headroom in the basement.
After deciding they want to build and what kind of house they want, the owner hires a homebuilder that was either recommended or that they have seen advertisements for. Most homebuilders advertise design services for homes or can hook the potential homeowner up with a local “designer”. What is a designer? In Wisconsin, according to the Wisconsin Administrative Code, homes can be designed without the aid of a registered professional, like an engineer or architect, so the owner ends up with a drafter employed by the contractor or a local lumber supplier that can “plan” their house. In reality, all they are doing is drafting the plan. This is where many of the hassles start. Who is evaluating the foundation requirements? What level of protection or assurance can a potential homeowner count on when it comes to the structural integrity of their house?
Getting back to my scenario; a floor plan for the house is finalized, a bill of materials is developed, and a cost is established. Everything is moving ahead nicely. The builder takes care of constructing everything from the foundation, to the building, to the landscaping.
The owner has the lot, a plan, a builder, and eventually gets the house of their dreams. This is the way that the vast majority of home building projects go, and SHOULD GO!
Now let’s throw in some variables. The lot the owner has selected is sloped, or adjacent to a river, the soils of the lot have low bearing capacities or have a high water table. What about bad weather during construction? What construction practices will be followed and who will be in charge? Typically, these questions are not asked by an average homeowner because they are being taken care of by their BUILDER! Who is responsible or better yet liable for the application of the code in home design for a house that is safe and will perform as expected?
Here are a few situations that some of the variables just mentioned cropped up, they were not addressed by the individuals involved, and subsequently, damage occurred to the foundations of homes being built.
This first one involves a sloping lot with reasonably stable soil. The owner planned the house himself and then hired a Home Builder to build it. The builder then had the plans drafted by the lumber supplier and subcontracted a foundation contractor to build the foundation for the house. The plans put together by the lumber companies’ drafter gave no special attention or direction to the foundation contractor, on how to accommodate the sloping lot, so he did pretty much what he normally does for typical footings and unreinforced foundation walls. Now the builder has the foundation walls, and needs to place a slab-on-grade on top. Therefore, after getting a deal at a local quarry, he back-fills inside the foundation walls with clean stone. Now this in-its-self is not completely out of the question, but one other factor was not addressed, the drain tile for the house extends around the inside of this foundation wall, through the clean stone. So as water is collected by the drain tile with the intent that it flows to the sump pump and is pumped away, the water flows through the drain tile and fills the clean stone behind the wall. Soon after the slab-on-grade was installed and the house was built above, cracks appeared in the foundation wall, tile on the slab on grade cracked, and the exterior masonry façade cracked. Ground water that typically runs through the perforated drain tile to the sump pit, drained into the clean stone. Nobody considered the lateral loads on the foundation walls from the stone let alone the water from the drain tile emptying into the stone.
The next situation actually involves two separate foundations that had the same problem. During construction who is best qualified to evaluate and address loads, whether temporary or permanent? Well on these home projects, nobody was qualified or even competent and both ended up as failures. For some reason the excavator decided to backfill around the exterior walls of these foundations before the floor framing was installed. As with most house foundation walls in Wisconsin, these were not reinforced, so it did not take much to cause the failures you see here. Again, no one qualified to understand structures was involved with these projects to provide direction on stabilization requirements for the walls.
The third situation involves a concrete masonry foundation. It is unique in that the failure occurred soon after construction in 1991 but wasn’t observed or addressed until just recently. This project involved an architect. Unfortunately, the foundation was shown on the drawings as an 8″ block wall with no vertical or horizontal reinforcing. The drawings did show pilasters, but did not call for them to be reinforced or grouted. The mason built the wall according to the drawings provided and the wall was destined for failure. During the investigation of this failure, it was determined that within a month or so after the original construction was completed, the mason was called back to address cracks that appeared in the walls. He addressed the cracks by filling them with a “repair” mortar. That was it. No one looked at why the cracks appeared. About 9 years later, heavy machinery for a near-by road construction project was driven on a near by roadway causing the foundation to vibrate, and soon after the owner found what he thought were new cracks in his foundation. The inadvertent sins of the past were soon discovered and the owner realized that he had a big problem.
What Controls the Construction of One and Two Family Dwellings
There are codes that pertain to the construction of one and two family dwellings. In Wisconsin there is Chapters 20 thru 25 of the Wisconsin Administrative Code which address:
1. Administration
2. Construction Standards
3. Energy Conservation
4. HVAC
5. Electrical Standards
6. Plumbing
There is also the International One and Two Family Dwelling Code, which address many of the same issues as the Wisconsin Code. These codes have pages of information on design criteria, excavations, footings, foundations, floors, walls, roofs and ceilings, etc. Specifically for foundations, information such as soil bearing capacities for common soils, lateral soil loads for backfill material, concrete strengths, and reinforcing bar yield strengths is included. These and the other requirements that each dwelling must meet are ultimately the owner’s responsibility.
I have talked to a number of homeowners that have had houses built and at best, they knew of the code but did not know what was in it. How does an owner take responsibility for something that they know nothing about?
Let’s look specifically at one part of the Wisconsin Administrative Code that is of interest today:
Section 21.02(2) — METHODS OF DESIGN — All dwellings shall be designed by the method of structural analysis or the method of accepted practice specified in each part of this code.
We all know what structural analysis is. It appears that the material information mentioned before is included in the code for use if structural analyses are performed. But what is accepted practice?
According to Webster: Practice — to do or engage in frequently or usually; make a habit or custom of….
Now in the case of the sloping lot or a lateral load on the foundation wall, the code is very vague on the “accepted practice”. The code leaves a lot up to interpretation. Looking a little further, the Wisconsin Administrative Code gives minimum concrete wall thickness based on height of the wall and the unbalanced fill supported then goes on the say that:
21.18 Foundations: (1)(a) Design — Foundation walls shall be designed and constructed to support the vertical loads of the dwelling, lateral soil pressure, and other loads without exceeding the allowable stresses of the materials of which the foundations are constructed.
So what does the foundation contractor do? He knows how to build concrete foundation walls like he has done in the past but he does not know how to evaluate stress levels. When applied to foundation construction, “accepted practice” would mean that if a certain technique or procedure had been used for a given circumstance in the past and was successful, that it would be reasonable to believe that it would work in similar circumstances. Makes sense. But when typical techniques and procedures are used in circumstances that have different variables, who makes the decision on what to do?
In the case of the sloping lot, the foundation contractor used what he considered “accepted practice” and built the wall like all other house foundation walls he had built in the past. Once the wall was in, the homebuilder followed what he considered “accepted practice” by backfilling with clean stone and completing the remaining construction.
In both cases of failed foundation walls, the excavators used what they considered “accepted practice” and backfilled the foundations before the floors were built, causing the failures.
And in the case of the failed block foundation wall, the mason kind of had an out with the drawings prepared by the Architect, which did not show any reinforcing in either the walls or the pilasters. Unfortunately, if the contractor had followed the general guidelines in the code this failure would likely not have happened.
Unfortunately, no one involved in any of these situations evaluated or were qualified to evaluate the loads on the foundation walls that ultimately caused the failures.
Liability and Expectations under the Current System
When a potential homeowner decides that they would like to have a new home built, what are their expectations regarding the work product, such as:
1. Levelness
2. Cracks
3. Plumbness
What assurance does the current system provide that owners will achieve those expectations? Many situations arise where the results do not match the owner’s expectations, even when those results are in accordance with standard or accepted practice. The Wisconsin code doesn’t provide definitions for such things as Level, Acceptable Crack Widths, or Plumb. Therefore, if the owners expectations are not met with the finished product, the only recourse is litigation and that is usually an uphill battle. We have had many owners that went to litigation in an attempt to hold contractors liable for such things as:
1. Drywall Cracks
2. Unlevel Floors
3. Foundation Wall Cracks
Unfortunately, their attempts usually come up short because the contractors followed “acceptable practice” or there is no established industry standard to judge the acceptability of the distress they have (i.e. cracks, un-level floors).
Many of the undesirable situations occur when the Standard or Accepted Practice does not cover the particular variables of a project. When applying “accepted practice” and standard procedures, who is liable when those practices or procedures don’t work? Typically the owner points to the “builder” who “designed” the house as the responsible party and who is liable when failures occur or expectations are not met.
Let’s look at the definition of Liable from Webster’s:
LIABLE: legally bound or obligated, as to make good any loss or damage that occurs in a transaction.
Is it reasonable to expect a building contractor to interpret the code and make decisions regarding the structural adequacy of code previsions for a given project? What exactly is the contractor legally bound to provide to the owner in a typical house-building project? One way of determining what an individual entity is bound to provide or liable for is what they have or can have insurance for. For homebuilders, insurance coverage usually includes General Liability and other labor related Insurances. Work product is not covered by insurance because there is not liability for the builder if he does not deviate from accepted practice. Which means, if the builder followed what would be considered “accepted practices” and a failure occurs, their insurance would cover only the damaged property but not the failed materials.
Most homebuilders say that they will be responsible for the entire project. But that responsibility or liability ends with their insurance policies. Many homebuilders don’t realize that when they run into unique situations, application of sound engineering principles by qualified individuals, can avoid most building hassles. So why don’t more homebuilders come looking for SE’s? Probably for some of the same reasons SE’s don’t pursue homebuilders, lack of knowledge about the profession.
So now we’ve got the stage set, there is definitely a need:
* Homeowners could use Structural Engineering for their foundations when more than Standard or Accepted practices are needed.
* Seeing could eliminate potential foundation hassles experienced by homebuilders.
So the system as it is now, does not make it clear to potential homeowners; who is liable for and who is really looking after their best interest when it comes to the design of their new home.
Why Aren’t S.E.’s More Involved?
So when it comes to designing average homes in which the majority of us grow-up, raise our kids, and grow old, why aren’t structural engineers more involved? As shown in the examples earlier, there are times when the services of a Structural Engineer could be used. A couple of reasons that S.E.’s have not been more involved in home design are:
No experience in the market
We don’t know who to market, should it be the owners or the contractors. Do you advertise home design, or market homebuilders at trade shows?
Lack of knowledge of “Acceptable Practice”
S.E.’s are typically well versed in the code requirements for commercial and industrial applications but how do we integrate with the code dictated “acceptable practices”?
How many of us would design a concrete foundation wall with no reinforcing at all? This table is in the Wisconsin Administrative Code, which gives minimum thicknesses for plain concrete walls.
TABLE 21.18-B
CONCRETE WALL THICKNESSES
====================================================================================
Nominal Thickness Maximum Height of Unbalanced Fill for Material of Wall Being Supported
Type of Concrete (Inches) (Wood Frame — Feet)
3000 PSI Unreinforced Concrete 8 8
10 9
12 10
14 11.5
This makes it confusing:
If I were a contractor and I needed an 8′ high foundation wall I would pour an unreinforced 8″ thick wall and conform to the code. In most cases this will work, it has a very good track record.
As a S.E., if I needed an 8′ high wall, I would first have to evaluate the vertical and lateral loads on the wall, then I would design the wall, based on the design loads, to meet the building code and requirements of ACI.
When it comes to designing for strength, stability, and durability of structures, structural engineers are qualified. Interpreting the Code, calculating stressed, designing load resisting structures are what we are trained for. Building codes for multifamily, commercial, and industrial construction require that a qualified design professionals be involved that are trained to deal with strength, stability, and durability. But this same requirement is not present in the Codes for One & Two Family Dwellings. So to become involved, the value of our services must be shown to (potential) homeowners and contractors alike. Value can come in a number of different forms such as:
* Assurance that they are going to get a home that performs to their expectations.
* Actual and potential savings in construction costs and in future maintenance costs.
So when it comes to designing average homes in which the majority of us grow-up, raise our kids, and grow old, why aren’t structural engineers more involved? A couple of reasons are:
The only problem with it is that the average homebuilder is not qualified to make decisions based on that kind of information. So that means there is a good chance that the home is being built based only on the experiences of the contractors involved. How does a homeowner judge if that level of performance will meet their expectations?
I am going to review the Wisconsin Administrative Code requirements briefly and how they pertain to the design and construction of one and two family houses.
When building a house, is the average owner aware of how decisions are made and who is really qualified to make those decisions regarding the construction of their house. Looking further into the Wisconsin Administrative Code, you realize that if you are building a Multi-Family Dwelling, three or more families, that a design professional is required, and with that, are held to a higher standard of assurance and protection than one and two family dwellings.
In addition to the owner is the average home builder aware that structural engineers can and do design for the unique circumstances that they run into, which their experience won’t cover. Looking back at the situation I described earlier, when the foundation contractor was confronted with sloped foundation walls and he just applied what he normally does for foundation walls on a level lot. Then when needing to fill the space, the contractor used clean stone thinking it was better than other fill. Adding to that, the fact that the drain tile ran through the stone, causing a hydraulic load on the wall that was not considered. None of these were considered until after the failure occurred.
Liability
Now lets look at the ramifications of hassles with one and two family dwelling foundations.
What does an owner do when a failure of their foundation occurs?
Building Code
The failures being discussed all occurred in Wisconsin, which has its own building code for uniform dwellings included in the Wisconsin Administrative Code. The Wisconsin code does not require that a structural engineer must design house foundations. It gives basic guidelines for standard soil types, bearing capacities, building material requirements, and expected lateral loads, but fails to call for input from someone either versed in using that data or trained to recognize that the standard guidelines no longer apply.
Failure Review
The failures encountered included a foundation with cracked and displaced walls that occurred soon after construction, a complete wall collapse during construction, and a wall failure that was thought to have been repaired, only to be discovered some 9 years later. Investigations of the failures included field observations and measurements of the foundations, evaluation of the loading conditions, and review of the delivery process used for each project.
Investigations determined that the loads to which the walls were exposed were not extraordinary. They were typical, easily predicable loads that foundations designed using standard techniques and procedures are capable of resisting. The building materials used were either concrete or masonry, both of which are adequate when properly applied. Each foundation failure resulted in either major repair, such as reinforcement of walls and footings or total replacement. In each case, structural engineering services were not used originally to design the walls for loads imposed both during construction and during the service life of the walls.
Each of the future owners retained the services of a homebuilder to design and construct his house. This builder provided a “design”, usually developed at the local lumberyard, for approval by the owner. The design included a detailed floor plan, foundation plan, and basic building section. None of the drawings reviewed during the investigations included details for the foundation system and none required more than a couple of bars at the top and bottom of the walls. The unfortunate thing is that this is not in violation with the current building code. The owners were not aware that their foundations were not designed because none of the owners were qualified to judge if their “design” was adequate or complete. They assumed that their builder would provide the necessary expertise to design and build their home. They were wrong.
Once approved by the local jurisdiction, the builder began construction and the foundation installation was started. The construction techniques used were based on the foundation contractor’s experience, not an accepted standard.
A unique situation was encountered during construction of one of the house foundations. The site was sloped and required that the foundation step down to accommodate a 10′ difference in elevation from the front to the back of the house. Not having encountered this situation before, the contractor simply backfilled behind the wall, never considering the additional load imposed on the wall because of the added height. Subsequently the foundation wall cracked, bowed, and displaced and significant bracing and partial replacement was required.
Conclusion
Each of the failures investigated could have been avoided if the services of a structural engineer had been used. Utilizing standard techniques and procedures, the structural engineer would have accounted for the loads and conditions encountered in each project to provide the owner with an engineered foundation for an “everyday” home in which they can live, raise kids, and grow old.