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Trusses1
Instructions: Insert paragraphs where needed in the following text.
A truss is a structure generally formed of straight members to form a
number of triangles with the connections arranged so that the stresses in
the members are either tension or compression. Trusses are very efficient
structures and are used as an economical method to span long
distances. Typical depth-to-span ratios range from 1:10 to 1:20, with flat
trusses requiring less overall depth than pitched trusses. Spans
generally range from 40 feet to 200 feet. However, some wood-trussed
rafters are used to span shorter distances. Some typical truss types are
shown in Figure 1. Generally, roof loads on a truss are transferred from
the decking to purlins, which are attached to the truss at the panel
points to avoid putting any bending stresses in the top chord of the
truss. If concentrated loads are placed between panel points, or uniform
loads are applied directly to the top chords, the member must be
designed for the axial loading as well as for bending. Trusses act much
like beams in that there is usually compression in the top chords and
tension in the bottom chords, with the web members being either in
compression or tension depending on the loading and type of truss used.
As with a beam, the forces in a parallel chord truss increase toward the
center. In a bowstring truss, on the other hand, the chord forces remain
fairly constant because the truss depth varies from a minimum at the
supports to a maximum in the center. In a trussed-roof design, trusses
are placed from 10 to 40 feet on centers, depending on the loads and the
spanning capabilities of the purlins. In residential and light commercial
construction, truss rafters made of 2” x 4” or 2” x 6” members are often
placed 2 feet on center. Open-web steel joists are usually placed 2 to 3
feet on center for floor construction and 4 to 6 feet on center for roof
construction, depending on the spanning capabilities of the roof deck.
Since trusses are thin and deep and subject to buckling, they must be
laterally supported with bridging along the bottom chord. In some cases,
1
Taken from David Rigby, Technical Document Basics: For Engineering Technicians and Technologists
(Upper Saddle River, NJ: Prentice Hall, 2001), 170.
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diagonal bracing is required along the top chords of pitched roofs if the
roof deck is not adequate to act as a diaphragm.
Drafting2
Instructions: Comment on the diction and sentence structure in this passage.
The use of computers is affecting every form of engineering,
including architectural engineering. Changes in technological
advancements created an enormous impact in the transition from
manual labor to the use of more sophisticated machines and computers.
The concept of design and drawing documentation has been greatly
affected by these changes. This revolution is largely the product of the
technological breakthrough created by CAD (Computer-Aided
Design/Drafting).
The CAD system is a computer adaptation of the skills of a manual
drafter. It is a tool that helps the drafter or designer be more efficient and
productive. Although computer-aided drafting is becoming commonplace,
and in some cases necessary, in engineering and architectural firms,
manual drafting is still valuable to all drafters regardless of their field on
interest. In structural engineering and architectural environments,
drafting by hand is still relevant in preliminary planning, site
developments, sketches, and other situations where the use of CAD is
not particularly practical.
Manual drafting and design used to be the sole aid in transmitting
graphic ideas and concepts in the technical drawing business. Because
of higher demands, expensive production costs, and tight deadlines,
design professionals sought better ways of producing drawings. Then,
specialized drafting equipment and tools emerged to help the designer
and draftspersons be more productive and efficient. Unfortunately, these
tools made the drawings appear “manufactured” in the sense that they
were consistent throughout.
The advantages of the CAD system are quite obvious to any
architectural firm. The CAD system allows a drafter to retrieve a detail
from the memory bank, display it on the monitor, and make changes for
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any specific job. The system provides tremendous resources in cost
savings and in the production of quality drawings. A big advantage of
using computer drafting is speed. Many architectural and engineering
firms that use CAD claim the result has been a great increase in
production and substantial cost savings.
Memory3
Instructions: Following the lead of Christensen as well as Gopen & Swan, comment on
the sentence order (the sequencing or the paragraph structure) in the
following passage.
There are basically two types of memories: ROM (read-only memory and
RAM (random-access memory.) For RAM, in which the data is
permanently stored, the data can be altered by special methods, but
there is no “write operation.” In ROM, data are permanently stored and
can be read from the memory at any time, but there is no write
operation, because specified data are either manufactured into the device
or programmed into the device by the user. The data cannot be altered.
ROM is nonvolatile memory; that is, the stored data are retained when
power is removed from the device. Examples of ROM applications are
look-up tables, conversions, and programmed instructions.
RAM is a read/write random-access memory. In RAM, if the power
is lost, the data is lost. Figure 1 shows the concepts of read and write
operations. The term random access means that all locations in the
memory are equally accessible and they do not require sequential access.
RAM semiconductors are manufactured with bipolar or MOS
technologies. The bipolar section has only SRAM (static random access
memory), but the MOS section has both SRAM and DRAM (dynamic
random access memory.) The following discussion will compare static
and dynamic memories and explain the advantages and disadvantages of
each.
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