Tag Archives: wind loads

What an Engineer of Record Does for a Post Frame Building Part II

Continued from yesterday’s blog, an article by Jesse Lohse in SBC Magazine:

 

System Design

  • Understand Load Path
    • Gravity
    • Lateral
    • Uplift
    • MEP conflicts
  • Initial Designs
    • Roof System
    • Walls
    • Floor System(s)
    • Foundation
  • Broad Analysis for construction documents

System Design

Once an initial conceptual design is complete, an engineer will turn their attention to system design in a top down manner. An understanding of the structure’s load path is imperative with specific considerations given to gravity loads, lateral loads, and uplift on the various elements within the structure. Once the engineer has a general idea of the structure’s load path, they will begin initial designs of various structural systems. Working from the top down, engineers will produce initial designs first for the roof system. Then the walls including the gravity and lateral force resisting systems and any required beams and columns will be designed followed by the floor systems and repeated as many times as necessary, dependent on the number of levels and different unit types in the structure. Once the roof, walls and floor system has been designed attention will turn to the foundation and footings, leveraging information from the soil report derived in conceptual design. This broad analysis information is compiled into initial structural construction plans.

Element Engineering

  • Accurate dimensions
  • Specific member analysis
  • Coordinate geometry defined in CAD
  • Analysis model created (SAP 2000, STAAD, RAM, etc)
    • Dead loads
    • Live loads
    • Wind loads
    • Seismic loads
  • Internal forces
    • Axial forces
    • Bending moments
    • Shear force
    • Drag force
    • Combined forces
  • Initial Member Sizing

Element Engineering

The element engineering phase begins with the engineer ensuring accurate dimensions for the various portions of the construction project through geometry coordination as defined in 2D or 3D CAD software. Trusting the dimensions are accurate, the engineer will begin specific member analysis for the defined spans, such as calculating roof loads that are transferred to exterior and interior bearing walls. The lateral force resisting systems generally require the most engineering time combine with window and door perforations that require headers and beams. This analysis combines gravity, wind uplift and lateral loads paths. This load path analysis will also be applied to the floor system and foundations, giving the engineer a general idea of the variety of loads within the structural elements and where additional attention will be required in subsequent design phases. To aid in this analysis, engineers will use specific software applications geared towards structural design such as SAP 2000, STAAD and/or RAM. This analysis software will allow the engineer to apply a variety of loads including dead, live, wind and seismic. As a function of this analysis, the engineer will be able to determine axial forces, bending moments, shear force, drag force and the combined forces. Once the forces have been determined, the initial member sizing can commence, allowing the engineer to establish a ‘rough draft’ to further refine in downstream design steps.

Iterative Design

  • Design to code
  • Redesign Analysis model
    • Incorporate more accurate load paths
  • Fine Tune Final Designs

Drafting

  • Create structural plans
  • Fully detailed

Iterative Design and Drafting

Engineer sealed pole barnEngineers use an iterative process to fine tune the various elements into final structural element designs. Think of this as repetitive in nature working toward the ultimate goal of an efficient design that meets the variety of requirements the structure’s configuration places on the path that the applied load will need to take to get to the ground. The engineer starts with a broad understanding of the loads on individual elements and narrows the focus until each element and ultimately the entire structure is designed to safely transfer all loads, meet code requirements and provide an acceptable solution that can be signed and sealed. Through this process the load paths are accurate, specific and reliable. With the accurate load paths, drafting can be completed with fully detailed structural plans available for construction. 

Construction Administration

  • Review submittals
  • Obtain approvals
  • Prepare schedules
  • Monitor construction
  • Perform site inspections

Construction Administration

Further in the construction process, the engineer is often called upon to review RFI and deferred submittals, obtain code approvals or prepare construction schedules. Certain products, such as roof trusses, are considered a deferred submittal. This means the engineer allows the designs to be created by others and sealed by a specialty or delegated engineer.  Those sealed designs are reviewed by the EOR and either approved for manufacture or returned for revisions. Beyond review of conformance to the structural design, engineers will also monitor construction progress on behalf of the client and will often perform site inspections to make sure the construction process is progressing and installation of products is without errors. 

 

Mike the Pole Barn Guru comments:

Whew! That’s a lot the Engineer of Record does in the design of a post frame building. This whole process takes time and sometimes even I sometimes get impatient while waiting for building plans to be produced and signed by the Engineer. But I know given adequate time the plans will be accurate and result in a beautiful post frame building.

Optimum Aspect Ratio of Length and Width

I suppose I inherently knew the answer to the optimum aspect ratio of length and width for post frame construction, but never really sat down to write about it. Well, reader JEREMY in EFFING has an inquiring mind and wants to know:

“In general terms is there an “optimum” aspect ratio to gain the best strength and minimize costs of construction? I.E.: if you are looking at around 3,200 square foot of space is it “better” to 24′ X 136′, 30′ X 104′, 40′ x 80′, 48′ x 64′, or 54′ X60′. I assume the 24′ width may require a division wall at 68′ to help carry the long wall wind loads so I doubt it is economical, and the 54′ building would require heavier trusses and posts.”

Let’s begin with what is usually the most cost effective dimension for length – multiples of 12 feet (24, 36, 48, 60, etc.) as well as width (multiples of 6 feet).

Arena BuildingWorking from the 3200 square foot benchmark this would give 24’ x 132’; 30’ x 108’; 36’ x 84’; 42’ x 72’; 48’ x 72’ and 54’ x 60’ as the ones which should be the most cost effective dimensions. The closer width and length are to each other, the lower the shear forces which must be carried by the roof.

At 5.5:1 the 24’ x 132’ building will, in all probability, require interior shear provisions – knocking it out of contention. Depending upon wall and roof height, wind speed and exposure at 3.6:1 the 30’ x 108’ might have some of the same challenges as well. When buildings become long, tall and narrow, high loading conditions can result in the need to reinforce sections of the roof and endwalls with structural sheathing besides the traditional steel.

The assumption the “54’ building would require heavier trusses and posts” is only partially correct.

Yes, the trusses may be fabricated from higher grade or larger dimension lumber and have more internal webbing. However, in many cases the added cost of each individual truss being ‘heavier’ is less than having to invest in a greater quantity of smaller span trusses.

Building columns happen to be very strong in compression (ability to withstand downwards forces). The larger span truss will probably not require larger columns due to roof snow and dead loads, and the stiffness of the roof in a small aspect ratio may offset the wind load placed on the extra height of the truss.

About Hansen BuildingsThe real answer here is we are talking about differences of cents per square foot, not dollars. Plan your new post frame building dimensions to best meet your needs for how you will be utilizing the building not only today, but potentially in future years. Good planning will always end up being the optimum for the dollars invested!

Tornado! What We Didn’t Learn in Moore

Tornado! What We Didn’t Learn in Moore

Moore OK Tornado RadarOn the afternoon of May 20, 2013, an EF5 tornado, with peak winds estimated at 210 miles per hour (mph), struck Moore, Oklahoma, and adjacent areas, killing 23 and injuring 377 others. The tornado was part of a larger weather system which had produced several other tornadoes over the previous two days. The tornado touched down west of Newcastle, OK staying on the ground for 39 minutes over a 17-mile path, crossing through a heavily populated section of Moore. The tornado was 1.3 miles wide at its peak. Despite the tornado following a roughly similar track to the even deadlier 1999 Bridge Creek-Moore tornado, very few homes and neither of the stricken schools had purpose-built storm shelters

Between 12,000 and 13,000 homes were destroyed or damaged, and 33,000 people were affected. Most areas in the path of the storm suffered catastrophic damage. Entire subdivisions were obliterated, and houses were flattened in a large swath of the city. The majority of a neighborhood just west of the Moore Medical Center was destroyed. Witnesses said the tornado more closely resembled “a giant black wall of destruction” than a typical twister.

The Oklahoma Department of Insurance said the insurance claims for damage would likely be more than $1 billion. Total damage costs have been estimated as high as $2 billion.

How did all of this happen?

Home builders protest the estimated cost of $2,000 to $6,000 for home tornado shelters would make houses unaffordable.  How much is a human life worth? If it is one of my loved ones, a whole bunch more than this.

Some of the fault might be placed upon local government, as well as the Building Departments which provide recommendations for design wind loads for structures. The design wind speed for the Moore area, by Code? 90 mph or roughly 20.736 psf (pounds per square foot). Consider 210 mph is over 107 psf – more than FIVE TIMES the design wind load!!

Anyone wonder why the photos were as astonishing as they were?

Not trying to be confusing….the values we use for wood design are based upon 40% of a 5th percentile figure. As an example, if 100 random pieces of lumber were tested for strength, and the values plotted on a curve, take the 5th lowest value from the bottom, and use 40% of this value. This does afford a certain amount of cushion for safety in designing wood structures.

At the least, I’d recommend increasing the required design wind speed to something in the range of 150 mph (57.6 psf) to 170 mph (74 psf). While this would not eliminate all destruction, it would certainly tend to be better than what exists currently.

In examining a fairly significantly sized full enclosed pole building kit package (40 feet wide by 60 feet long and 14 foot tall), the price increase from 90 to 150 mph design wind speeds was just over 10% (not much more than $1200).

Post frame construction lends itself naturally to being resistant to extreme wind loads. With columns embedded in concrete backfill, there is no weak point at ground level, as found in conventional stick frame construction, or manufactured housing.

With a minimal number of mechanical (nails, etc.) connections, as compared to stud wall buildings, pole buildings run a far lower risk of compromising these crucial joints. Over the years when I have examined “why” buildings fail (either pole buildings or stick framed), it was most often the connections which failed.

Play it safe – play it in a post frame building designed to actually support the types of loads with which it could be hit.