Delegated Design in Specs for Glazing Systems

We've been asked to review several engineering company’s calculation packages and found engineers
stamping calculations that don’t meet specifications and code requirements.  Please check your calculation packages to be sure.

We found:

1. DL of the frame was not taken into consideration (flexure/axial load interaction) Typical C-Beam Calculations did not include axial load interaction with flexure required by the Aluminum Design Manual
2. Corner location analysis (75% WL acting simultaneously). Bite loss or movement into glazing pocket was not checked. Analysis of corners for 45 degree orthogonal loading was not considered as required by ASCE 7. Important for bite loss at the glazing pocket.
3. No Jam/perimeter mullion deflections to check caulking requirements. Deflections at Jamb perimeter joints were not checked for appropriate joint design. Important for preventing leaks at curtain wall interface to the structure.
4. LL movement was not taken into consideration at all.  Building structural live load movement and system thermal expansion was not accounted for in joint design, splices and glazing pockets. This is required by contract specifications as part of the delegated design.
5. Analysis of local buckling modes and lateral torsional analysis was omitted but is required by the Aluminum Design Manual.

“If undone, you are buying calculations half done                                                and not according to code requirements.”

It may be tempting to overlook it and we understand some contractors will simply say, "It's not worth the effort."  Just keep in mind that these calculations are a part of the “delegated design” required by contract documents. If left undone, glazing contractors, general contractors, AEs and the building owners are buying a design half done and not according to code requirements.

“Half done calculations may result in glass breakage, joint leakage, system failure or other liability.”

Half done calculations may result in damage such as glass breakage, joint leakage, system failure or other liability that quality service providers choose to steer clear of.

Let’s work together to design systems fast, affordable and correct.

Info@ JEIstructural.com 

816-505- 0987

FAST | CLEAR | FRIENDLY | RIGHT

Info@ JEIstructural.com 

816-505- 0987

Codes Changes Creating a Cladding Mess

If you’re a glazing contractor or a building envelop contractor; an estimator or project manager, you’ve probably tried to size your storefront and curtain wall members off of a wind design pressure.  

The wind design criteria are usually given on the contract structural drawings.   Sometimes design wind pressures for components and cladding are given as well.  However, the changes with the 2012 IBC have created much confusion regarding the actual design pressures to be used.

Many times the components and cladding pressures listed on the structural drawings are given in terms of “Ultimate strength design”, which is not used in any of the wind load charts given by manufactures.   Using these pressures will ultimately lead to overly conservative design.

Designers need to be able to calculate the allowable strength design loads from the information given off of the structural drawings.

If you have a question about what is on a structural drawing, give us a call.   

Stewart Jeske

816-734-8345

www.JEIstructural.com

 

Can you break me down the difference in Explosive Weight I and Explosive Weight II?

Explosive Weight I and Explosive Weight II?

Question

Can you break me down the difference in Explosive Weight I and Explosive Weight II?

 Answer

This is the criteria from the 2012 UFC:

Explosive weight I is associated with a larger weight of TNT for a van or truck bomb outside of the base perimeter.   Its based on the assumption that a vehicle bomb will not be able to penetrate the base perimeter and/or will be detected at the gate.  If we have a facility that is less than 200 feet from the base perimeter/fence, then we must consider explosive weight I in addition to explosive weight II and its pressure/impulse may govern over the explosive weight II.

Explosive weight II is associated with a smaller weight of TNT simulating a back-pack or smaller bomb that gets past the base gate.  The standoff for this explosive weight is no longer the 82 feet typical of the 2007 criteria.  Now it is based on the new conventional stand-offs that are listed in the tables based on construction type and load bearing/non-load bearing walls.  The actual stand-off must be used which is from the parking lot to the window/wall in question.

Hope this helps.  Please call if you have any questions.

Stewart P. Jeske, P.E.

JEI Structural - Glazing Systems Engineering

www.jeistructural.com

Updated UFC 4-010-01 (2012) - Impact to Glazing Contractors

Based on a quick review, here are the major changes/impact that I see:

1) The biggest change – Conventional Construction Stand-Off Distances. This is a huge change!

They have re-identified the conventional construction stand-off distances in Table B-2 based on the wall type construction. I anticipate the net effect will be a moving to wall construction which minimizes the construction stand-off due to spacial constraints associated with military bases. So for reinforce masonry and concrete and wood studs w/brick that means about 30 feet and 16 feet and 36 ft respectively! Glazing systems with these requirements will not likely be able to have static equivalent calculations performed – They will be forced to dynamic analysis. I anticipate that these types of installations will start showing up late summer to fall of this year.

2) Another major impact – Testing Requirements. This is also huge! The requirements state that the testing must meet the appropriate pressure and impulse from the applicable standoff and explosive weight. So, given the changes with the Conventional Construction Stand-Off Distances most all of the manufacture’s blast testing (at 82′ Ex weight II and 144′ Ex weight I) will be obsolete! I feel badly for all of the manufacture’s who have invested a great deal of effort and resource to testing in the past 5 years.

3) Dynamic analysis is pushed. Under paragraph B-3.1.3 ASTM F 2248 Design Approach (This is the 3-second static equivalent method) they state, “In order to reduce the conservatism associated with using the ASTM methodology, the window systems may be designed using dynamic analysis or tested …”

4) Framing calculations under the static equivalent method is changed. Framing must now be shown adequate with a design load of 2x the glazing resistance instead of the 3-second equivalent blast load. In some cases (with small glass lites governing – Its usually the case that the Architect is unaware) this will be the difference between 50psf (typical 3-sec load) and 400 – 1200 psf! L/60 is used as a deflection limit now instead of L/160, but I don’t think that will help much – sections will need to be much stronger using this method.

5) Connections and anchors under static equivalent method is changed. The connections and anchors must be designed for either 1x or 2x the glazing resistance instead of the typical loop holes that we have seen 2x the 3-second load (usually 100psf). Anchors and connections at these loads will be difficult if the controlling glazing lite in an elevation is small. I think anchors into masonry and wood and light-gage steel will not be workable with this method.

6) We are now able to Use ultimate strength design instead of allowable stress, but again this helps little in light of the major changes above.

 Stewart Jeske, PE
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Gathering Missing Info on Blast Projects Saves Time.

Many blast project specs do not provide enough information to come up with the equivalent 3-second blast design pressure from ASTM F2248. Glazing contractors can save time by requesting the following information in advance.

• Explosive Weight - (Typically for most projects explosive weight II)
• Standoff Distance - (For most projects it is the Conventional Construction Stand Off = 82 ft)
• Level of Protection - (Typically Low Level of Protection)

This criteria is usually determined during the design phase of a project by the using agency or the design team.

  

For more helpful information on blast projects see JEI Structural Engineering WINGARD, Windas, Window Blast, Blast Mitigation Design

How experience for design calculations saves time and money

Some glazing contractors don’t budget enough for appropriate engineering calculations (structural calcs) on projects. Those who do, understand the value of industry expertise. Some glaziers brag, “I’ve got a guy that does it on the side.” The truth is, a structural engineer with industry experience will save you time and money.

Structural engineers with glazing system experience are familiar with industry manufactures and have worked with their components and framing, so you’ll end up with system components and anchors that will work. An experienced engineer will always look at the boundary joints to make sure that excessive movement will not compromise the joint water barrier. In addition, years of experience usually results in a quick turn around using better software, specifically for aluminum curtain wall and storefront design.

Experts in aluminum code won’t over design your project. Aluminum design is specialized and requires adherence to the complex 2010 Aluminum Design Manual. This 469 page book larger is larger than the steel code! Few engineers ever pick up aluminum design, due to it’s specialization. University curriculums for structural engineers focus on steel, concrete and CMU. Aluminum is never used in the design of building or bridge structures so most structural engineers just don’t come across it in their everyday work. This type of experience is essential if you want your glazed system to work without problems for years to come.

So next time you need design calculations, hire experience. You’ll save money in the end.

Avoid Delays On Recovery Funded Projects

The glazing industry is starting to see the trickle down of government recovery money, but it’s not without a price. These projects are tagged with important features that glaziers will do well to be aware of. Most of the projects are for Department of Defense installations or GSA and have blast resistance requirements.

A significant percentage of the recovery money ends up in the hands of architectural firms responsible for design and construction oversight. The design budgets are nice and fat and many of the architectural firms hire blast load consultants to provide design input, write specifications, and …review your submittals during the construction phase. So now the typical submittal has to jump through an additional hoop. The blast load consultant is usually eager to make their value known, and that translates to a necessity for every “t” being crossed and every “i” dotted.

Submittals which are rejected (for what ever reason) require time, effort and often a tangible dollar amount to be resubmitted. Sometimes the blast load consultants may not be altogether familiar with glazing systems and this results in a nightmare of rejections and resubmittals. Glazing contractors who want to cash in on the recovery money should be prepared for the additional efforts that are being required.

Projects with recovery earmarks are good business when you understand their process.

Stay tuned! Next time we’ll cover some of the items typically required in the submittals for blast resistant glazing.

Antiterrorism: Blast-resistant Glazing Systems

The environment of rapidly evolving antiterrorism codes has left estimators and manufacturers in a whirlwind of confusion asking themselves, “Why do blast resistant requirements seem to be a moving target?” It’s important for us to understand what is driving this part of the glazing industry. A great emphasis has been placed on protecting the inhabitants of government buildings from flying shards of glass due to explosion. The U.S. government will be investing great amounts of capital into protective glazing systems during the next 10 to 15 years to make the changes necessary to their existing buildings and for all new construction. The two major codes driving the changes are GSA/Interagency Security Committee Security Design Criteria and the U.S. Department of Defense Unified Facilities Code UFC 4-010-01, Minimum Antiterrorism Standards for Buildings. The UFC code requires all future DoD programming beginning in 2004 to include blast resistant glazing systems. Over the last couple of years we have begun to see many of these projects entering into construction and it’s essentially the same story for the GSA’s programmed projects.

For more insight on this topic see:

http://www.glassmagazine.com/article/commercial/antiterrorism-blast-resistant-glazing-systems-and-moving-target