Tag Archives: engineered building

Prescriptive Structural Requirements for Post Frame Buildings

In a misguided effort to make things “easier” for potential building owners and builders, some Building Departments have prescriptive requirements for non-engineered pole buildings.

This means if someone walks in their Building Department’s door and wants to construct a post frame building, as long as the building owner (or builder) agrees to build to match these prescriptive requirements, they will be issued a structural permit. This is, of course, with a caveat of being able to meet requirements of other departments, such as Planning (https://www.hansenpolebuildings.com/2013/01/planning-department-3/).

WHY IS THIS BAD?
Doesn’t this save a lot of money, not having to pay an engineer?

No.
Prescriptive requirements are often based upon, “we have always done it this way”, rather than having a basis in sound fundamentals of structural design. Every three years a new Building Code version is published, sometimes with sweeping changes in structural design as better research and new technologies (and products) have become available. Many highly qualified design professionals, including engineers, are involved in Building Code revisions.

A classic example of this came when International Building Codes were first adopted in 2000. Prior Codes did not have deflection criteria for wall members in those cases where members did not support a rigid finish (like plaster or gypsum board). New Code limits deflection for all instances. In order to meet these new requirements, in many cases, pole building wall girts can no longer be installed “flat” on wall column exteriors.

Many times materials are included in prescriptive requirements doing nothing but causing more work for whoever is actually doing construction, as well as using unnecessary larger lumber members than what an engineer would have specified.

On occasion, these prescriptive requirements do not actually meet sound structural design! In my spare time, I have challenged more than one of these and gotten Building Departments to make changes, as their prescriptive requirements would have resulted in an under designed building.

Scarily….if you build to prescriptive requirements, and have a collapse, your Building Department is absolved from any structural liability!

THE SOLUTION

If a Building Department has PRESCRIPTIVE REQUIREMENTS for Post Frame Buildings – invest in an engineered building. It is less expensive to pay for engineering and it guarantees a building be designed to sound engineering practice and actually meet building code requirements. Your bonus is those sealed plans are your “insurance’ – your building’s engineer is now liable for both safety and integrity of your new building as long as his or her plans are followed.

Footing Size? A “Reverse Barndominium?” and a Loft Bedroom?

This week the Pole Barn Guru answers questions about the footing size for an open car porch and why a person should use a registered design professional, building a “reverse barndominium” where one build a post frame shell around an existing structure, and if one can build a loft bedroom in a footprint of 20’x 30′.

treated postDEAR POLE BARN GURU: I am building an open car porch, the inside will be connected to another building and on the outside I planning on using 3 – 8 inch x 8 inch x 8 feet posts 12 feet apart. The open car porch area is 24ft x 24ft and the roof is 6 on 12 with 2 x 6 rafters and joists landing on the outside plate. What size footing will I need for each pole? JAY in MORGAN CITY

DEAR JAY: This is a question best answered by the Registered Professional Engineer who designed your building, as he or she will be able to do a complete analysis including soil bearing capacity, design wind speed and wind exposure. With columns only eight feet long, I am guessing you are planning on using wet set brackets into concrete piers https://www.hansenpolebuildings.com/2019/05/sturdi-wall-plus-concrete-brackets/. I would not be surprised to see piers up to three feet deep and two foot diameter in order to adequate resist uplift forces.

DEAR POLE BARN GURU: Are you aware of anyone ever building a “reverse barndominium”? Usually barndominiums are built shell (outside walls) first then the interior, but what about building entirely around an existing structure? I really want to buy this historic house built in 1861. It is currently gutted down to the dirt floors, needs a roof, garage, etc. Why not just enclose the whole thing and DIY the interior without dealing with the outside elements? The primary structure is 19’x38′, but the side structure is an additional 20′ (39′ total wide) with a 6/12 roof. The eave height is 15.5′ and about 20′ at the ridge. The basement is about 4′ deep. I could go 42′ wide with a structure and have the exterior posts completely outside of the current footprint. The lot is 60’x150′ and I’m looking at a 40×80-ish building with a second story.

Is this feasible or have I succumb to the Dunning-Kruger Effect? I have attached an image of my sketchup drawing to give a better idea of my concept.

Thank you great guru. I love your philosophy and transparency throughout your blog posts. I have learned a lot at the cost of otherwise being productive at work. JAMES in WESTON

DEAR JAMES: Thank you very much for your kind words, although I am not as certain your employer would be as happy with me 🙂

Perhaps surprisingly, you would be far from the first person to attempt such a project. Is is entirely doable and actually becomes very similar to what people do with a PEMB (Pre-engineered metal building aka red iron) or a weld up barndominium, where a shell is erected and a building is built inside of a building. You just happen to have your insides prebuilt!

Outside of my loyal readers, most have never heard of the Dunning-Kruger Effect (https://www.hansenpolebuildings.com/2015/01/dunning-kruger-effect/)

 

DEAR POLE BARN GURU: I’m interested in a residential building approximately 20ft x 30ft. How tall would the walls need to be to include a loft bedroom with headspace to approximately 4ft from the sides? JUDE in DUPONT

DEAR JUDE: I will answer your question from a standpoint of you getting best value for your investment – meaning using both floors from wall to wall.

Assuming a concrete slab-on-grade for main level, bottom of framed ceiling would be at 8′ 4-5/8″ this allows for 5/8″ drywall on ceiling and 1/2″ at bottom to be able to account for any variances in your building slab and to keep drywall from soaking up moisture from floor, plus 3-1/2″ for actual thickness of a nominal four inch thick slab.

I would recommend using premanufactured wood floor trusses between floors (https://www.hansenpolebuildings.com/2014/09/floor-trusses/). Plan on a 20 inch thickness, plus 3/4″ for subflooring and 8′ 1-1/8″ putting bottom of roof trusses at 18′ 2-1/2″. In Pennsylvania I would recommend R-60 blown in attic insulation (just under 20 inches thick), resulting in needing a 20 foot eave height.

 

 

Shear Walls

Shear walls are designed to resist lateral forces, such as wind or seismic, and transfer these forces to the component below them, which might be other shear walls, floors, foundation walls,  slabs, footings or embedded columns. Shear walls prevent the roof or upper floors from swaying or moving off their supports as well. Buildings with stiff shear walls suffer less damage under extreme conditions, like in the event of an earthquake.

Shear walls resist shear lateral and uplift forces. These forces are caused by elements like wind, earthquakes and settlement, as well as the weight of the structure and its contents. These combine to create a twisting force which can tear, or shear, a building apart. Including a shear wall, in design, ensures the building will not be affected. Each wall must be supported perpendicularly, either with walls under or perpendicular to them, to ensure stability under forces from all sides. The shear force is transferred over the wall adjoining it, but no further. Uplift forces lift one end of a wall and push the other end down. This can result in a building toppling over. Uplift forces tend to be greater on tall narrow walls and lesser on longer walls.

Shear walls are located on each level of a structure, including any crawlspaces. They’re placed along exterior walls of the building to ensure an effective box structure is created. When a building’s exterior walls cannot provide enough strength, or when the minimum height-to-width ratio for the building is exceeded, shear walls are added to the interior as well. Shear walls function best when they are located so they align with foundation walls or footings vertically.

Shear walls may be made of several materials, although in any given structure these materials are not typically combined. Stucco, when properly installed and reinforced with wire mesh, resists small lateral loads. Lath and plaster construction is also used, although it is not common in new building projects. Plywood was once the material most used for constructing wood shear walls because thickness, grade and nail type and spacing could be combined in a variety of ways to achieve different strengths. With the creation of oriented strand board (OSB), it isn’t used as often. OSB allows the manufacturer to produce the boards in the thickness, proportion and type of wood fiber needed in specific jobs or applications. Steel is used when the forces on the structure are more than any other material can handle.

How do you know if your new building will need shear walls or not?  Unless you are going to hire a registered design professional (engineer), you will not know. To make a determination, calculations need to be done.  It’s not a matter of “eye-balling” it, using height as an indication, or even using “your best judgment”.  I always advise for any building, from “essential facilities” such as fire stations, to commercial and even fully “ag use” buildings – to have an engineer design and seal your building plans.  The lives you save, may be those you love.