Expert Answer:Truss Lab Executive Summary Essay

  

Solved by verified expert:This is structure materials Lab test for civil engineering which is Truss lab, the assignment is to write a technical executive summary about the Lab and you give a brief explanation what the lab is. if you read the files I have uploaded you will understand what is needed ( Lab manual, the summary format “how should the summary be”, and my lab data). My triangle angle shape is 45-45-90. if you have any question ask me 🙂
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VI. Lab #3 Measuring Forces in Truss Members Using
Strain Gages
A.
Objectives:
1.
To become familiar with the operation and application of electrical resistance
wire strain gages to measure stresses in simple structures.
2.
To experimentally determine the internal forces in simple structures, and to
compare those results with values obtained analytically.
B.
References:
The chapter on truss analysis in any Structural Analysis (or Statics) textbook
C.
Background:
The elastic stretching or straining of steel is of the order of one to four thousandths of an
inch for each inch of length. The accurate measurement of such strains can be made by
mechanical, optical, or electrical gages. The first strain gages were mechanical, but today
strains are usually measured with electrical strain gages.
An electrical resistance wire device, known as the SR-4 strain gage, consists of loops of
very fine wire cemented to a thin paper strip. The gage is cemented to the specimen with
a firm, tough cement that allows the gage to stretch with the specimen to which it is
attached. The changes in length of the wires alter their electrical resistance, which is
measured and calibrated to indicate the actual strain.
Proper bonding of the gages to the member is essential for obtaining reliable results. Care
must also be taken that the gage does not absorb moisture, since this will cause resistance
changes which affect gage stability.
Strains are determined by placing a wire gage in a four-arm Wheatstone bridge d-c
circuit. When the gage resistance is changed by deformation of the gage, the bridge
circuit is unbalanced. To compensate for strains caused by temperature and humidity
variations, a so-called dummy gage (a duplicate of the active gage) is connected into the
Wheatstone bridge circuit. The active gage measures strain due to stress, plus
deformations due to temperature and humidity effects, while the dummy gage measures
deformation due to temperature and humidity effects only. Although this setup can be
used satisfactorily, it is more convenient to contain the Wheatstone bridge setup within a
specially designed strain indicator. The strain indicator is calibrated to read strains (rather
than resistances), and includes electronic amplification to get a stronger signal.
30
To obtain direct or axial strain averaged from two sides of a tension or compression
member and at the same time to cancel out unwanted bending strains, two active gages
can be mounted back-to-back on opposite sides of the specimen.
In this experiment, strain gages will be used to determine the forces in the members of a
truss. A truss is defined as a structure consisting entirely of straight two-force members
that are pin-connected together at their ends. These connection points are called joints,
and it is further specified that all loads are applied to the truss at the joints, rather than
along the members. When a truss is subjected to an external load, the members develop
internal axial forces which are related to the applied external load by the geometry of the
truss and the magnitude, location, and direction of the applied load.
Figure 1 – Example of a Truss
Trusses may be analyzed by the method of joints, which consists of taking free body
diagrams of joints in the truss and solving the force equilibrium equations for the member
forces. In addition, if the strain in a member is measured, the stress in that member can
be found from Hooke’s Law (stress = Modulus of Elasticity times strain). Once the stress
is determined, the axial force in a member can be found (stress = P/A so P = stress times
area).
D.
Specimens:
Several pin-connected triangular trusses, each instrumented with strain gages, and
constructed from 1018 steel (yield stress = 36,000 psi, E = 30,000,000 psi). Tests will be
conducted on the following truss configurations: 45-45-90 truss; 60-60-60 truss; and 3060-90 truss.
E.
Equipment:
Tinius-Olsen Testing Machine, digital strain equipment, dial calipers
F.
Procedure:
1.
Measure and record the cross-sectional dimensions and length of each member
and pin. Check the strain gages for broken leads, making any necessary repairs.
2.
Theoretically calculate the load to cause yielding of the most severely stressed
member of each truss. Also calculate the force to cause failure in the pin
connections. Through statics, these can be related to the external force being
31
applied to the truss by the Tinius Olsen machine. These calculations will be done
in the lecture prior to the Truss Lab.
3.
Theoretically calculate the critical buckling load for the truss. For a member to
buckle, it must be in compression. From the Euler buckling equation, the force to
elastically buckle a member equals 2 EImin/L2. Through statics, this member
force can be related to the external force on the truss. These calculations will be
done in the lecture prior to the Truss Lab.
4.
Set up a truss in the Tinius-Olsen Testing Machine, making sure that loads and
reactions are applied at truss joints and that they do not interfere with the
members.
5.
Connect the strain gages to the strain indicator and balance and calibrate all gages
per instruction manual.
6.
Apply five loads to the truss starting with the smallest load which gives a
reasonable strain reading. The maximum load applied to the truss should be
based on your calculations of the failure load divided by an appropriate factor of
safety. The load increments should be approximately equally spaced between the
first and fifth readings. Record the applied load and corresponding strain readings
for all members. NOTE: Each truss member has two strain gages mounted on
opposite sides of the member. Record readings from both gages, then average
these readings to determine the strain for that member. Using the average of two
strain gages for a single member eliminates unintentional bending effects.
G.
Calculations:
1.
Find the forces in all truss members corresponding to each experimentally applied
load, by appropriately relating average strain to stress (Hooke’s Law) and stress to
force (force = stress x area).
b.
Calculate all truss member forces analytically using the method of joints.
3
Compare the experimental results with the analytical values by calculating the
absolute errors and the relative errors (%) with respect to the analytical value.
4.
Discuss the results and sources of error and variability.
32
X. Lab #6 Hot Mix Asphalt Superpave Volumetric Design
and Compaction Tests
A.
Objectives:
1. To familiarize the student with the general characteristics of Hot Mix Asphalt
(HMA) concrete and with the Superpave method of HMA mix design, and
laboratory test methods for compacted bituminous specimens.
2. To perform standard laboratory tests on compacted HMA mixtures to determine
the effect of varying design mixes and materials on the degree of compaction and
percent air voids in compacted bituminous samples.
B.
References:
AASHTO T166 Bulk Specific Gravity of Compacted Hot Mix Asphalt Mixtures Using
Saturated Surface-Dry Specimens, American Associated of State Highway and
Transportation Officials
PA Test Method No. 715 Determination of Bulk Specific Gravity of Compacted
Bituminous Mixtures, Pa. Dept. of Transportation, June 2003.
ASTM D6925-07 Standard Test Method for Preparation and Determination of the
Relative Density of Hot Mix Asphalt (HMA) Specimens by Means of the Superpave
Gyratory Compactor
ASTM D2041-03a Standard Test Method for Theoretical Maximum Specific Gravity and
Density of Bituminous Paving Mixtures
ASTM D2726-05a Standard Test Method for Bulk Specific Gravity and Density of NonAbsorptive Compacted Bituminous Mixtures
ASTM D3203-05 Standard Test Method for Percent Air Voids in Compacted Dense and
Open Bituminous Paving Mixtures
C.
Background:
Asphalt pavement is made from aggregates (stone, sand or gravel) using asphalt cement
(a derivative of crude oil refining) as the “glue” or binder. It is produced by heating
asphalt cement and mixing it with aggregates and mineral fillers. The resulting product is
referred to as “hot mix asphalt” or “HMA.” Typical proportions are 94 to 96 percent
aggregate and 4 to 6 percent asphalt cement.
Asphalt pavement is built in layers. The first step is to remove topsoil and compact the
earth. Then, a base that will help to carry the load is placed and compacted. (The base
45
may be constructed solely of stone, or it may include both stone and asphalt.) Two or
more layers of hot mix asphalt are then placed and compacted. Pavement thickness is
chosen based on what kind of stresses the pavement must withstand (trucks vs. cars) and
other factors such as soil conditions and climate. It also depends on the materials used in
the asphalt and what materials might be present in the lower layers of the pavement.
Whenever asphalt pavement is placed and compacted, there will be a certain amount of
air voids, the small pockets of air between the coated aggregate particles. The amount of
air voids is expressed as a percent of the bulk volume of the compacted paving mixture.
Efforts should be made to keep compacted air voids between 3% and 8%. Once voids
reach 8% or higher, the voids become interconnected and allow air and moisture to
permeate the pavement which reduces pavement durability. On the other hand, there must
be sufficient air voids to allow a slight amount of added compaction under traffic loading
without bleeding and loss of stability that would lead to pavement rutting. If air voids fall
below 3%, there will be inadequate room for expansion of the asphalt binder in hot
weather and when the void content drops to 2% or less, the mix becomes plastic and
unstable. Mixes are usually designed for 4% air voids (e.g. 96% compaction) in the lab
and compacted to at least 93% (e.g. less than 7% air voids) in the field. PennDOT
acceptance standards are based on an optimum 4% air voids (96% compaction) in
laboratory specimens for approved mix designs, with an acceptance range of 92% to 97%
compaction (from 8% down to 3% air voids) in field samples. When samples from the
field are tested and fall outside this acceptance range, penalties are imposed on the
contractor (i.e. contractor doesn’t get paid full price for the asphalt that was placed or
contractor may be required to remove and replace the asphalt).
The objective of HMA mix design is to develop an economical blend of aggregates and
asphalt. Historically asphalt mix design has been accomplished using either the Marshall
or the Hveem design method. The most common method was the Marshall, which had
been used by about 75% of the Departments of Transportation (DOTs) throughout the US
and by the Federal Aviation Administration (FAA) for the design of airfields. Then in
1995, the Superpave mix design procedure was introduced by the Federal Highway
Administration. Superpave builds on the knowledge from Marshall and Hveem
procedures. The primary difference between the three procedures is the machine used to
compact the specimens and the tests used to evaluate the mixes. Superpave procedures
are used by DOT’s throughout the US for the design and quality control of HMA
highway projects.
No matter which design procedure is used, the HMA mixture that is placed on the
roadway must meet certain mix requirements:
Sufficient asphalt to ensure a durable, compacted pavement by thoroughly
coating, bonding and waterproofing the aggregate.
Enough stability to satisfy the demands of traffic without displacement or
distortion (rutting).
Sufficient voids to allow a slight amount of added compaction under traffic
loading without bleeding and loss of stability. However, the volume of voids
should be low enough to keep out harmful air and moisture.
46
Enough workability to permit placement and proper compaction without
segregation.
The Superpave design method for hot mix asphalt (HMA) consists of three phases: (1)
materials selection for the asphalt binder and aggregate, (2) volumetric proportioning of
aggregate and binder, and (3) evaluation of the compacted mixture based on specimens
compacted using the Superpave gyratory compactor (SGC). The SGC compacts the
asphalt mixture into a mold using a gyratory motion that causes a kneading action. The
appropriate number of gyrations varies for different highway projects, and is determined
based on traffic and project site high temperature conditions. As traffic and temperature
increase, the number of required gyrations at which the asphalt mixture is evaluated also
increases. There is no general strength test to complement the volumetric mixture design
method. Industry has expressed the need for a simple strength test to complement the
Superpave volumetric mix design method and ensure reliable mixture performance over a
wide range of traffic and climatic conditions. So far, no simple strength test has been
adopted for use with the Superpave design method.
D.
Materials:
Several samples of HMA mixtures compacted in a Superpave Gyratory Compactor
(SGC). The instructor will provide the theoretical maximum specific gravity (Gmm) for
each sample. Gmm is the ratio of the weight of a given volume of voidless (no air voids)
HMA at a given temperature to the weight of an equal volume of water at the same
temperature.
E.
Equipment:
5 kg balance or scale fitted with a suspension apparatus and holder to permit weighing
the specimen while suspended in water, water bath equipped with overflow outlet for
maintaining a constant water level, oven for drying specimens.
F.
Procedure: (based on AASHTO T166 Test Method A)
For each specimen
1. Weigh and record the dry mass. Designate this mass as “A”.
2. Fill the water bath to overflow level with water at 77o F (25o C) and immerse the
specimen for 4 minutes.
3. Weigh and record the submerged weight, with the specimen in the water bath and
using a suspension apparatus and holder. Designate the submerged weight as “C”.
4. Remove the sample from the water and quickly surface dry with a damp towel.
5. Weigh and record the mass of the saturated surface dry (SSD) specimen.
Designate this mass as “B”. Any water that seeps from the specimen during the
weighing operation is considered part of the saturated specimen. Because the
47
SSD mass is more difficult to properly measure, repeat this measurement several
times until you get readings that are in reasonable agreement with each other.
G.
Measurements and Calculations:
1. Prepare a data table to record the following information for all specimens tested;
data tables will be shared among all groups by posting to shared files on the
Campus Cruiser course web page.
Group ID:
Specimen ID:
Base or Top course;
Dry Mass, “A” (kg):
Saturated Surface Dry Mass, “B” (kg):
Submerged Weight, “C” (kg):
2. Perform the following calculations for each specimen tested in lab (include data
from all lab groups so that means and standard deviations can be calculated).
a. Calculate the Bulk Specific Gravity Gmb of the asphalt mixture, which is defined
as the ratio of the weight in air of a unit volume of a permeable material at a given
temperature relative to the weight in air of an equal volume of water at the same
temperature. The Bulk Specific Gravity can be calculated from
Gmb = A/(B-C)
where
Gmb =
A=
B=
C=
Bulk Specific Gravity
Mass of dry specimen in air, g
Mass of SSD specimen in air, g
Weight of specimen in water, g
b. Calculate the Percent Water Absorbed (by volume) = 100 x (B-A)/(B-C)
If the percent water absorbed is greater than 3 percent, Bulk Specific Gravity
should be calculated using paraffin-coated specimens. Indicate whether or not
your specimens are acceptable for percent water absorbed, or if they should have
been paraffin-coated.
c. Calculate the Percent Compaction and Percent Air Voids for each sample
Percent Compaction = Bulk Sp. Gravity/Max. Th. Sp. Gravity = 100 x Gmb/Gmm
Percent Air Voids = 100 – Percent Compaction
3. Calculate averages and standard deviations using data from all samples of the same
mix design. Compare average results from different design mixes. Do the samples
fall within PennDOT’s acceptance criteria?
48
EXECUTIVE SUMMARY FORMAT
The executive summary is a standalone section in a formal report that
provides a shortened version of the report and summarizes the key facts
and conclusions contained in the report. Sometimes the executive
summary is filed separately from the formal report. We will be using a
modified form of the executive summary as the report you will turn in
for some of the experiments we do this semester. Which experiments
require full lab reports and which require executive summaries is shown
on the syllabus. Your executive summaries will be “modified” in that
you must provide a full discussion of the results and also include an
appendix with all data sheets and appropriate calculations. Your
executive summary will be similar to the requirements for the lab reports
but will leave out the abstract, table of contents, background, and
methods and procedures sections from the report. Your executive
summary should NOT contain separate sections as is done in the lab
report. Instead, it should be written in paragraph format with each
section starting in a new paragraph. The executive summary shall
contain:
a.
Title Page
b.
Background Statement: The background statement (also called
the context) connects the lab to real world applications to show
you understand the problem and its relevance from an
engineering perspective.
c.
Objectives: A statement of what you are trying to accomplish.
Similar to the objectives in a lab report, this section should only
contain the technical objectives.
d.
Scope of Work: A brief description of what you were required to
do in the lab. Include general processes but not the details. For
example, you should state that strain gage data was collected but
you don’t need to provide details about where the strain gages
were located or how the data was collected.
e.
Results and Discussion: This is where you introduce, present and
describe the results you obtained in the laboratory. Summarize the
data collected and the analysis that is done with that data, giving
sufficient detail to justify your conclusions. As in the Results and
Discussion section of the full laboratory report, tables and graphs
are to be used where necessary to present your
F.
1.
data, calculations, and results. Remember that discussion must be
provided to describe and explain the data and the significance of the
information in the tables and graphs. The purpose of the discussion is to
interpret and compare the results. Point out the features and limitation of
your work and relate your results to the technical objectives of the lab.
Compare your results with theory or accepted formulas and discuss the
comparison. Sources of error should be discussed with respect to your
findings and the significance of the errors with respect to the objectives
of the lab. The Results and Discussion section will follow the same
format that was used in the lab report format.
f. Conclusions: Unlike in the full lab report, in the executive summary’s
conclusions you do NOT repeat the objectives or significant
results, as the information was presented in earlier paragraphs of
the executive summary. This is where you should write about the
“lessons learned” from the laboratory. What were your expected
results? Were those results achieved? If not, why not? Have you
resolved the problem? Briefly state the logical implications of your
results. Suggest further study or applications if appropriate. If you
had different constraints in the laboratory, could you have gotten
better results? If so, how?
g.
Appendix: While a typical executive summary would not include
an appendix, you are required to submit the original data sheets
from lab, derivations, calculations (include at least one complete
set of sample calculations), …
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