Highlights of the ASCE 7-10 Wind Provision

Posted on February 13, 2012 by admin

The release of the new ASCE 7-10 Minimum Design Loads for Buildings and Other Structures comes with major changes in the wind design that are of the interest of the structural engineer especially in the wind debris and hurricane prone regions. The top of these changes is the introduction of the 2012 International Building Code “ultimate wind speed maps” as the source of new wind speed data per each risk category. Among other changes are the reinstatement of exposure “D” in hurricane prone regions, revision of ASD and LRFD load factors and combinations, revision of minimum design wind pressures, revision of wind speeds defining hurricane prone regions, and lastly the omission of the occupancy factor for wind. In this blog I will shed some lights on these changes and contrast them from the older counterparts of ASCE 7-05.

Wind Speed Maps and Wind trigger line

For ASCE 7-10, there are several wind speed maps (one per each risk category). For category II, the wind speed map for Florida is given below for ASCE 7-05 and ASCE 7-10.

Florida Wind Speed Map for ASCE 7-05

In most cases, the wind speed values increase from ASCE 7-05 to ASCE 7-10.

Florida Wind Speed Map for ASCE 7-10

Not only do the ASCE 7-10 wind speed maps define new wind speed data, they also delineate a new boundary for hurricane prone regions and wind-borne debris regions

Wind Equation

Both old and new wind provisions use the same wind pressure equation:

q=0.00256K_z K_zt K_d V² I₀₅.

where:

q = velocity pressure evaluated at mean roof height (psf)

Kz = velocity pressure exposure coefficient

Kzt = topographic factor

Kd = wind directionality factor

V = basic wind speed (mph) from ASCE 7-05 or new ASCE 7-10 maps

I₀₅ = Importance factor (1.0 for Category II buildings, 1.15 for Category III and IV buildings) only for ASCE 7-05.

The use of speed map per each risk category and the incorporation of uniform recurrence interval wind speed contours eliminate the importance factor from the wind velocity pressure equation given by ASCE 7=05.
The ASCE 7-10 uses the same wind velocity pressure equation with two changes: New wind velocity values per the new wind speed map, and the removal of the importance factor term.

ASD/LRFD factors

Wind loads produced by the velocity pressure equation are factored by ASD/LRFD new factors and combinations. The new values are adjusted to counteract the new wind speeds, resulting in design velocity pressures similar to the old values in hurricane non prone regions and are lower than the old values for hurricane prone regions. The old values are design wind pressures calculated for exposure “C” or lower for category risks II, III, and IV.

Reinstatement of Exposure “D”

A new development in the simulation of strong storm winds over open water shows higher wind speed in the proximity of the storm eye and slower wind speed over the larger extent of storm resulting in a change of exposure “D” definition allowing for its reinstatement in the wind calculations. The ASD/LRFD load factors and combinations are adjusted for the hurricane prone regions to allow for close to 100% similarity between Exposure “C”ASCE 7-05 and Exposure “D” ASCE 7-10.

Minimum Design wind loads

ASCE 7-10 prescribes a new minimum wind loads have changed for design of main wind force resisting systems (MWFRS) under both directional and envelop procedures. The new values of 16 psf and 8 psf for wall and vertically project roof respectively are different from the universal 10 psf value prescribed for wall and roof by the ASCE 7-05. The primary observation here is that the ASCE 7-10 wall minimum load of 16 psf is the LRFD factored load of the ASCE 7-05 corresponding value. Given the resulting wind loads based on the new wind speed maps, it is very unlikely that the new prescribed minimum wind loads to be governing load values.

Wind provisions are constantly updated by the ASCE due to the associated uncertainties of many parameters. While trades such as building inspection are not directly linked to wind calculations, many building failures can be explained if proper wind calculations with the most up to date provisions can explain many of such failures.

The Factor of Safety, is it really about safety or about our ignorance?

Posted on January 11, 2012 by admin

In previous blogs we discussed the factor of safety as a leeway for potential money saving on the expense of building’s wellbeing. If people continue to save money by reducing the factor of safety while maintaining a “safe” structure, why should we believe the factor of safety? Are we missing something? Is the factor of safety really about safety, or is it an opportunity for some to save on building materials and hence compromising the safety rating that go undetected in a building inspection? Unfortunately, many perceive the factory of safety as a “structural overdesign”by “academic” engineers.

To understand how the factor of safety is derived, we first define it in simple terms and then briefly (and informally) explain its basic components. In its simplest definition, a factor of safety for an element is the ratio between how strong a structure is and the loads that may be imposed on it. Material strength, structural system, loading, stresses, structure’s environment and use are the main areas affecting the factor of safety.

“Material strength” is a significant engineering area where thousands of studies were conducted to provide the user with valuable information regarding material ultimate strength (ductile: http://en.wikipedia.org/wiki/Ultimate_tensile_strength, or brittle :http://en.wikipedia.org/wiki/Brittleness). The bottom line of these studies is that each material has prevailing characteristics with margin of uncertainties.

“Loading” is the calculation of worst loads of different types, different patterns,and the worst combinations during the life span of a structure such as Live load, Dead load, Wind, Earthquake, Ocean waves, Storm Events, …etc. The derivation of these loads and the load combinations provides provide values with uncertainties.

analysis”. The end result of this analysis is how much the structure is stressed due to the acting loads. The structural analysis theories, equations, and numerical solutions rely on assumptions and parameters with great uncertainties.

“Stresses” is the calculation of how the structure reacts to the “loading”. This is the core of structural engineering “design and analysis”. The end result of this analysis is how much the structure is stressed due to the acting loads. The structural analysis theories, equations, and numerical solutions rely on assumptions and parameters with great uncertainties.

“Structure’s environment and use” refers to conditions such as environmental exposures (radial, chemical, weather, .. etc.), expected misuse of the structure, the cost and consequences of failure, political sensitivity, ….etc. Providing quantitative measuresfor these conditions is an art that is subjective and highly uncertain.

All the above factors (with their uncertainties) are processed to produce an overall uncertainty. This process is complex and is not intended for this blog. Building codes such as (FBC, BOCA, UBC, ..etc.)and agencies such as (AISC, ACI, ASME, NEPA, NCMA, …etc.) perform their own statistical (and often stochastic) analysis of these uncertainties to estimate and recommend a factor of safety.While uncertainty is a reflection of lack of knowledge, high variability and theory shortcomings; its quantification is complicated and is a sensitive subject in structural engineering. It haseconomic implications on project budget and is often subject to criticism by the layman.

The Structural Engineer, the Contractor and the Factor of Safety

Posted on December 23, 2011 by admin

In our previous blog, we covered the designer/contractor conflict of interests as one of two main risks ofa Design-Build contract. Factor of safety erosion is the second risk of a D-B contract and it is the focus of this posting. The factor of safety, a significant subject in Structural Engineering, is an added “cushion” of building materials beyond what is effectively required to resist all applied forces.The applied forces, the effective requirements, and the factor of safety are all parts of the structural design and are prescribed by building codes, calculations and design procedures. Even though the design methods and principles are based on a sound science, there are significant uncertainties in the design procedures, material strengths, and applied force calculationsthat preclude the designer from obtaining an accurate estimate of building materials that assures building safety.Building codes recognize this limitation and prescribe procedures to calculate the factor of safety based on extensive studies. A designer with poor training can unknowingly underestimate the factor of safety driven by the desire to save on building materials.

Any building with a reduced factor of safety is considered noncompliant.If my building design has such a low (eroded) factor of safety, it does not meanfailure is imminent. The building is simply more risky and is more prone to adverse conditions and construction defects. Factor of safety noncompliance is not easily detected by building inspection. You usually do not notice such a noncompliance, until the building is exposed to unusual conditions resulting in failure. This failure ranges from minor cracks all the way to catastrophic collapse. The low factor of safety is all profits to the contractor with the hope that nothing of such unusual conditions would occur. Examples of such conditions are CAT 5 Hurricane or a tornado, material or construction defects, current or future adverse soil conditions. A significant occurrence of one or more of these conditions is less likely at early stage of building life. The lower the factor of safety the faster these conditions could occur and hence the higher the risk.

This posting and the previous posting have purposely emphasized the disadvantages of the Design-Build contract. In the next blog, I will explain to you how to take advantage of the D-B contract while protecting yourself from the disadvantages. I wish you all happy holidays.

Thanks

Al Ali, PhD, PE, PMP, CGC

What you need to know before you go for aDesign-Build contract.

Posted on December 16, 2011 by admin

Many Engineering and Construction firms, including ours, offer a Design-Build option and a one stop shop experience for their clients. In this process, the architect, the consulting engineers, and the contractor work together under one integrated firm. Check out how we execute this process. If you think this process is all advantageous, then think again.

Under a D-B contract, you will enjoy a headache free process prior and during construction where one firm is offered both the project design and the project construction. This firm may be a General Contractor who hires a consulting engineer/architect to prepare a design package; or it may be a structural engineer/architect who is typically a licensed general contractor. While hiring a design firm may not go through a bidding process due to the relatively low design cost, the selection of a construction firm can greatly impact the project budget and hence should go through some form of bidding and competition. By a direct assignment of a construction firm, the owner looses all the benefits of bidding competition and allows for a designer/contractor conflict of interests during design (e.g., makes the design more construction friendly on the expense of functionality) and during construction (e.g., building inspection performed by the contractor’s own people) putting the client at an increasing risk and vulnerability.

There are two categories of risks associated with a D-B contract. The first risk is the increase of the overall cost (direct cost) due to the elimination of competition. Such a lack of competition relieves the contractor from any pressure to provide competitive prices resulting in an increasing possibility of a “too high” overall project cost. In some situations, the owner bids out the project (once the design document is completed) and then asks the D-B contractor to match (or negotiate around) the lowest bid. The fact that the D-B contractor is involved in the design phase (directly if he/she is the designer or indirectly if he/she is the main contractor) gives him/her the freedom to influence the bidding process in many ways such as leaving out key design details and specifications resulting in bidders shooting up their prices for lack of information. The second risk is the liberty of the contractor (or the designated consulting engineer/architect) to manipulate the design document to alter material quantities for his/her favor while maintaining changes in structure functionality invisible. Due to the complicated and critical nature of this risk category, I prefer to defer it to my next blog. Please be aware, I am not accusing any contractor of corruption, I am just talking about the risk.

Thanks

Al Ali, PhD, PE, PMP, CGC

Is it a good idea to rely on a structural engineer alone?

Posted on December 9, 2011 by admin

It depends on how competent the engineer is in the areas of practice. A deciding factor is the project scope and complexity. While I firmly believe in specialization, I believe a successful structural engineer should be well rounded in terms of education (formal and informal) and experience to be able to address incidental work from other engineering areas. There are a lot of engineering activities that are acquired by experience alone. Building inspection is a good example for such an experience. How do you pick a good engineer from a bunch of consulting engineers?

Structural Engineer Versus Architect

Posted on November 14, 2011 by admin

Hello,

This is Dr. AL ALI, the CEO of Universal Engineering Inc. Welcome to my Blog, my window to the world, where I will do my best to be informative and useful for both consumers and professionals. My topic today is to briefly address the difference between an Architect and a Structural engineer. In the construction world, the terms Architect and Engineer are used interchangeably by consumers. Unless you are a contractor, the chances are you do not know the difference. It is commonly known that the Architect designs the building while the Structural Engineer builds it. This is erroneous understanding or at best less than accurate.

To understand the difference, let’s look into the process of planning a house. You need an Architect to help you prepare drawings where he/she puts together your functional requirements such as the number, type, and size of rooms and their interrelationships, colors, windows, stairs, cabins, trims, facades, Balconies, ..etc. Once the architectural drawings are completed, the building must be “Engineered” to be capable to stand the worst case scenario load conditions during its life span. Examples of these loads are furnitures, people, hurricanes, earthquakes and ocean waves as well as the building own weight. So what does “Engineering a building” mean?. Every element of the building such as roof, walls, beams above openings, windows, floors, columns, and foundation must be “designed” to enable the entire building to function coherently as intended by the Architect. The process of calculating, sizing, prescribing, and drawing the building bearing elements is called Structural Engineering design performed by a Structural Engineer. The product of structural design is a set of drawings and often a calculation report to be added to the architectural set. The more demanding and complex the architectural requirements, the more challenging the structural engineering design. Examples of such complexities are:

Owners of Ocean front properties would like to have open spaces with floor to ceiling ocean views. This requires the elimination of structural beams which are very important element for building rigidity against Hurricanes
A less restricted Architect’s imagination often leads to upper floors not aligned with lower floors bearing elements resulting in complications of structural calculations.
circular stairs and unsupported mid floor landing stairs are also sources of complexities.
architectural requirement of a column that ends at any floor level rather than foundation is another big structural challenge.

So can the Structural Engineer do the Architect’s job or can the Architect do the Engineer’s job?. Well, it really depends on the size and level of complexity of the job. A Florida Professional Engineer can perform architectural work if it is deemed incidental to Engineer’s work. In the same way, a Florida Licensed Architect can perform engineering work if it is deemed incidental to Architect’s work.

So whom would you hire for your project. It is a decision for you to make. If the architectural requirements are dominant and not clear to you then an Architect may be your first choice. On the other hand, if the architectural requirements are straight forward but the structural requirements are critical (e.g., a second floor addition to an old building) then a structural engineer is your first choice.

AL ALI

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