Cladding University
Founded 1997
Engineering 101 - Calculation Organization and Numbering
Instructor: Steve Strebel, ALT Cladding and Design
Too many engineers overlook the importance of presentation in their calculations.
Without a good presentation, the technical content suffers greatly and the value of
the calculations is diminished. The first element to any calculation
presentation is the organization of the calculations and the first element of
presentation is a logical numbering system.
Class Syllabus
The Importance of Readability
The Computer
Numbering
Consistency
Summary
The Importance of Readability
If no one can understand your calculations, the calculations are ineffective. No matter how technically brilliant you are, no matter how error-free the calculations, in today's world, calculations that cannot be understood are of little value to anyone.
Calculations are technical communication. Years ago, calculations were something done in-house by the curtain wall contractor to do a rough check on the main parts of the wall. The calculations were all done by hand and, for the most part, were only intended to be read by the engineer who made them. Today, however, the calculations are a vital part of the documentation and paperwork of the project.
The calculations must document the structural integrity of the cladding. This is why they are done. Many different people must be able to interpret and understand the calculations. Calculations may be presented to any or all of the following.
- In-house departments. The calculations should provide a means for conveying structural design changes to the drafting department. Too often the engineer makes a change and the drafting department misses the change and the wall is built incorrectly. A good calculation layout provides a means for the draftsman to quickly page through the calculations and find any changes required for structural reasons.
- In-house engineers. Often, other engineers must pickup and revise your calculations. If they cannot understand what was done, time and money are wasted. The only thing sadder than an engineer not understanding his company's calculations, is an engineer not understanding his own calculations. I've seen this happen too often. Clearly, if you cannot understand what you did three months ago, no one else will either.
- Owner's consultant. The consultant is probably the only person looking into the details of the calculations on a consistent basis. Hard to understand calculations make verifying the design harder and therefore makes approval less likely.
- Civil Building Department. Usually, an engineer for the city of the building must approve the calculations. If the calculations are poorly presented, it is likely the engineer cannot understand them and approval may be delayed.
The Computer
Don't fool yourself. Nicely typed, computer calculations are no guarantee that your calculations are good, or even understandable. Some of the most useless, impossible to understand, and error filled calculations I have seen were perfectly typed Word documents. Unfortunately, these calculations superficially look great. But to someone who needs to actually understand what is going on in the numbers, a logical orderly, and well-referenced hand written set is far, far superior to strings of numbers churned out on a spreadsheet.
This is not to say that typed calculations cannot be done properly. They can. But it is not uncommon for an engineer to think that a stack of laser printed sheets is enough to make a super presentation.
There is one other important issue. If your handwriting is a disaster, type it.
Sheet Numbering
Simple sheet numbering is the first step in a logical and orderly presentation. Without an organized approach to sheet numbering, tracking, revising, and understanding the calculations all suffer.
Good calculation sheet numbering schemes have several basic rules. These are listed below.
- Logical groupings. The simplest way to number calculations is to start with number 1 and count upward as high as required. Sadly, this is also one of the poorest methods. Good presentations are logically grouped. Grouping can be by the type of calculation (i.e., mullions, anchorages, connections, etc.) but this tends to make verifying the thoroughness of the calculations hard to do. A better grouping method is by wall area or type, such as typical wall, starter areas, storefront, etc.
- Unique numbering. Each sheet should be uniquely numbered. Sheet numbers must not be repeated. Just as your drafting department would not issue two drawings with the same sheet numbers, you should not issue two different calculations with the same sheet numbers. The exception to this is when calculations are submitted in "books" or "packages". In this case, there may be repeated sheet numbers among the books as long as each sheet is clearly part of a specific grouping.
- Consistent numbering. Sheet numbers must be maintained between submissions. In other words, if four sheets are added in-between sheets 10 and 11 of the first submission, sheet 11 does NOT become sheet 15 in the second submission. A numbering method such as 10, 10A, 10B, 10C, 10D, 11 should be used. DO NOT RENUMBER YOUR SHEETS WITH EVERY SUBMISSION.
- Note revisions. Just like the drawings, revised sheets should be noted as revised. The date and revision number must be noted on the revised sheets. The reason for the revision should be noted in the calculation set or on the revised sheet. A revised calculation sheet would be numbered along the lines of Sheet 10, Revision A.
Section numbers are the best way to number your sheets. In this type of organization, each sheet begins with a section number. So, sheets for the design criteria might be numbered 1-xx (where 1 is the section number and xx is the sequential sheet number of the sheets in this section.) Then the typical wall might be numbered 2-xx, the top of the building 3-xx, the stone wall 4-xx, and so on.
The beginning of each section should include a sketch of the wall area being checked. Text descriptions are just too obtuse to be helpful. Sketches can, and should, be supplemented with text, but starting the section with a sketch of the area being checked is the best way.
Be sure and use appendices when appropriate. Some calculation sheets are referred to frequently within the scope of the entire job. In this case, it makes sense to include these sheets as appendices. Examples of these types of sheets include section properties, bolt allowable loads, and manufacturers data sheets. So, section property sheets might be numbered A-1, A-2, A-3, and so on.
With a little planning, appendix sheets can be shared for all jobs. This saves the effort of preparing these sheets for each job. For example, sheets for common bolt allowable shears and tensions could be prepared and always used as Appendix B in your calculations.
Consistency in Presentation
Consistency is a main element of any presentation. Your submission calculations should not appear to have been sequenced by dropping the stack on the floor and picking them up in random order. Most calculations have a few basic components that should always occur in the same order. These components, listed below, should be sequenced the same whether you are checking a mullion or an anchor.
- Title. Sadly, I have seen calculation checks simply start with equations. There is nothing that actually says what is being checked. Always title the check so it is clear what is being checked.
- Drawing reference. Add a reference to a drawing that shows the element being checked.
- Sketch or free body. Sketches take very little time and improve the quality of the presentation immensely. With few exceptions, a calculation check should always include a meaningful sketch.
- Imposed and/or factored loads. Clearly show, or reference, where the loads come from. This can be a reference to a computer output, or a simple "R = WL * Area" calculation. Numbers should NEVER appear without the origin of the number being clear.
- Analysis. This is where the loading and geometry are put into equations to find moments, bolt shears and tensions, and the like.
- Stresses and deflections. This is where the results of the Analysis are combined with the section properties of the element being checked.
- Allowable stresses, loads, and deflections. The allowable stresses must be calculated or the origins of the values used in the calculations must be referenced.
- Stress or Deflection Ratio. Always compare the imposed stress to the allowable stress. In general, it is worth the extra effort to also calculate the percentage of stress to allowable stress.
In general, each check will include all of the components above. They will, of course, vary for each type of check, but the same general approach should be used over and over. How you get to the answer is as important as the actual answer.
Summary
Even the most brilliant, technically awesome engineer is only as good as his presentation. A good presentation forces you to think logically through your calculations. You will end up better set of calculations and you will actually understand what you did much better than if you just slap a bunch of numbers on pages.
Technical people tend to focus too much on the numbers because this is what they are good at. By paying a little attention to the presentation, you enhance both your calculations and yourself. You are a professional, you should produce professional documents.