Tag Archives: ASCE

Required Bracing or Not?

When he was just a tyke (I considered adding in ‘little’ however he was 3’6” tall on his second birthday) my son Brent asked me in all seriousness what it was like to watch space aliens building pyramids. While I am not quite as old as Brent may have thought I was then, it has been awhile since I was muddling through architecture school.

Architecture school, back in my day, was more about drawing pretty pictures than it was about how to make sure those pretty pictures would actually stand up. I do foggily remember having an eight a.m. structural design class at Montana State University (who in their right mind would schedule this class mid-Winter in Bozeman?).

ASCE (American Society of Civil Engineers) News had a summer guest contributor (Kyle Vansice) in 2012 who wrote about some of famous architect Frank Lloyd Wright’s work. Some excerpts include:

“However, I believe that Mr. Wright more likely stretched the existing construction techniques past their previous limits, possibly to a degree that diminished the factor of safety.”

“Unfortunately, Mr. Wright’s genius in architectural design was not matched by his understanding of engineering principles.”

“I believe the engineering behind his designs did not meet the exacting standards typical of a Frank Lloyd Wright space. Mr. Wright believed that the structures of his day were large, bulky, and overly engineered. He felt that it was possible to challenge conventional wisdom and beat back some of the conservatism in structural design. I think it is also true to say that while he knew existing limits were antiquated, he did not fully appreciate the actual physical constraints that his designs surpassed.”

“A good example of the need for structural remediation can be found in Mr. Wright’s riverside masterpiece, Falling Water. The iconic cantilevered portions of the home had at one point deflected close to seven-inches over their fifteen foot span, and analysis of the structure has revealed that the as-built design had placed these cantilevers dangerously close to their failure limits. Post-tensioning was eventually required to restore these portions back to their intended elevation and to prevent the sort of catastrophic collapse deemed inevitable (Tyler Meek, Fallingwater: Restoration and Structural Reinforcement).”

Falling Water

“Other well-known examples of Wright’s imprudence in engineering include the Guggenheim in New York and Taliesin; both of which encountered significant structural rehabilitation in order to restore serviceability and prevent failure.”

Mike the Pole Barn Guru comments;
Hansen Pole Buildings’ Design Studio Manager, Caleb Johnson posed this to me earlier this year:

“Have you ever heard of a county that requires knee bracing or corner bracing via comment below from one of my clients:

“Architects and structural engineers are telling me that all buildings need knee bracing, corner bracing, etc., which in a traditional stick build is the norm”.”

If one is to venture into a Building Department and not have engineer sealed building plans, it is fair license for them to add in any structural members they desire (whether structurally necessary or adequate). These hard working folks are generally not engineers.

Most architects (even our iconic Mr. Wright) have a limited grasp of how to structurally make things work and rely upon engineers to come up with designs to prevent failures. When it comes to fully grasping nuances of post frame design, there are a limited few engineers who have done research necessary to achieve practical design within limits of safety.

Knee bracing, for one, would be a rare feature of traditional stick building and when applied to post frame construction could be detrimental. (https://www.hansenpolebuildings.com/2012/01/post-frame-construction-knee-braces/).

Corner bracing (also referred to as diagonal bracing) in most instances is a redundant member, should an engineer actually understand principles of utilizing shear strength provided by steel skin (roofing and siding). Extended reading on diagonal bracing can be found here: https://www.hansenpolebuildings.com/2016/03/diagonal-bracing/.

There is a moral to this story – unless you are building using prescriptive tables within building codes (IRC or IBC), you are best to only build from fully engineered building plans, both for safety and cost effectiveness. If you are hiring an engineer to produce your plans, look for a NFBA member engineer (www.NFBA.org) with extensive post frame experience. Or, even better, invest in a complete building package including engineer sealed plans specific to your building, at your site and including extensive step-by-step instructions and unlimited free technical support from those with actual post frame building experience.

Design Wind Speed Changes

Design Wind Speed Changes with Building Code Editions

Every three years a new version of the International Building Code (IBC) is printed, which brings with it the latest and greatest information for building design as approved by Code Officials. State and local permit issuing jurisdictions then can either adopt or amend the Code as they best see fit.

Even though the Code is updated on a three year cycle, some jurisdictions opt to continue to utilize earlier versions of the Code.

Provisions for design loads are set forth in Chapter 16 of the IBC.

There are significant changes to the design wind load requirements for fenestration between the 2009 IBC and the 2012 editions of the same code. These are due to significant changes to the wind load provision of ASCE (American Society® of Civil Engineers) 7 between the 2005 and 2010 edition.

The design wind load provisions of the 2005 and earlier editions of ASCE 7 were based upon allowable stress design of building components. The intent of this method was to provide loads to which the building components had a fairly high likelihood of being exposed during the service life of the building. The building components were then designed to remain serviceable (i.e. not require replacement) when subjected to this load.

The 2010 edition of ASCE 7 provides design wind load provisions which are based upon strength design of building components. This method provides loads which have a lower likelihood of occurring during the service life of the building. The building components are then designed not to fail (rupture) when subjected to this load.

This change in methodology results in higher design wind speeds and pressures. At first glance, this might give the appearance of requiring higher DP (Design Pressure) ratings. In actuality, the 2012 IBC contains provisions to multiply this new, higher load by a factor of 0.6 for the purpose of conversion to the more traditional method of determining the design wind pressure based upon allowable stress design. It is very important the builder, code official, manufacturer and anyone else involved in choosing or approving the structural building design for a particular application understand the higher design wind pressure provided by the 2012 IBC must be multiplied by this 0.6 conversion factor.

In most, but not all, cases this conversion results in required design pressure ratings which are roughly comparable to the more traditionally determined values.

ASCE7-10 also provides three different design wind speed maps. The different maps are based upon the assigned Risk Category of the building being designed.

  1. There is one map for buildings whose collapse would present a low risk to human life, such as barns and storage facilities.
  2. There is a second map for buildings whose collapse is considered to be a moderate hazard to human life. Most buildings fall within this category.
  3. There is a third map for buildings whose collapse is considered a high threat to human life, and for those which are considered essential facilities. The former includes assembly or education buildings designed to house groups of 250 or more people, some medical care facilities and any other buildings designed to house 5,000 people or more. Essential facilities include occupancies such as hospitals and police and fire stations, which are essential during emergency response situations.

The new maps result in higher design wind loads for buildings of moderate hazard to human life than for those of lower hazard. The highest design wind loads are given by the third map for buildings of high hazard to human life and essential facilities. Previous editions of ASCE 7 and the IBC also required these types of buildings to be designed to higher design loads, but the actual increase was applied in a different manner.

Considering a new post frame (pole) building? If you are looking at a building which is NOT designed by a registered design professional (RDP – engineer or architect) then there is an excellent chance the person or persons involved in the design do not understand the changes brought about by the newer editions of the Code and you could end up with an under designed building.

Under design can result in catastrophic failure – or even death. Don’t take the risk, demand an engineered building.

Your life or the lives of your loved ones could be at stake.