Activity 3.2.
2 Loads
Introduction
Once an architectural program has been devised and a preliminary structural system has been
chosen, the structural engineer may begin the process to design the structural elements. This
requires that the engineer identify the design loads for each structural element. Once the applied
design loads have been determined, the loads must be traced through the structure so that the
load(s) are included in the design of each element through which the load(s) will travel.
In this activity you will determine design loads for the roof of a high school in a suburb of Chicago,
Illinois (enrollment 2500) and select appropriate roof joists (beams) based on the loads transferred
to the joists.
ROOF PLAN
BEAM
GIRDER
Procedure
You will calculate the design loads for the roof and choose a steel roof deck to carry the loads.
You will then chase the loads to the roof beams (joists). Based on the uniform beam loading, you
will then choose an open web steel joist that can carry the applied beam loading.
Criteria
The low-slope roof will be constructed of a steel roof deck, five inches of rigid insulation,
and a built-up roof (BUR).
The roof will support a mineral fiberboard suspended ceiling.
The roof will support mechanical, electrical, and plumbing equipment (MEP). Assume 10
psf for all of this equipment.
The building is located in an urban environment surrounded by other buildings.
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CEA – Unit 3 – Lesson 3.2 – Activity 3.2.2– Loads – Page 1
1. Calculate the snow load for the low-slope (flat) roof. For this structure we will use the following
coefficients.
Ce = 1.0 assuming an urban or suburban area surrounded by other buildings such that the
building is partially exposed to the wind.
Ct = 1.0 assuming the building is heated and loses some heat through the roof.
Cs = 1.0 because the roof is low-slope.
Ps > Is (20 psf) = (1.1)(20 psf) = 22 psf
2. Find the roof live loading required by the IBC.
20
3. Estimate the dead load of the roofing system by estimating the weight of all of the roof
components. Note that typically the ceiling and PEM will be supported by the roof framing (and
not the roof deck), but conservatively include these loads for all roof calculations.
Insulation-2.6psf x 9 Metal Deck- 3 psf x 2.75
4. Determine the total design load for the roof system. 47psf
5. Select a steel roof deck to support the required loads. Type 20 steel deck
6. Determine the roof beam loading for both the interior and exterior beams .interior- 7ft exterior
3.5 interior uniform = 329lb/ft exterior uniform= 164.5lb./ft
7. Choose open web steel joists to act as the roof beams. For now, use only the top load values
in the table and ignore the live load deflection load values. Assume the roof deck will be
installed with a triple span. Column Lines 8-9, span= 16ft Interior Beam 12K1
Exterior Beam 8K1 will work however, or 12K1 (similar depth)
Column line 8 and 9 =20 ft
Interior beam 16K2 or 12K5 (similar depth)
Exterior beam -10K1 or 12K1( same weight) or 16K2(same depth)
8. Select an appropriate Type F roof deck and open web steel joist for the Keystone Library
Renovation. Show all of your work and record all of your assumptions.
Roof Dead Load (not including deck or framing)
Per the weight of Materials Table
Built up Roof (or EDPM) 6.5 psf
Metal Deck 3 psf
5 in. Rigid Insulation 3.75 psf
Suspended plaster ceiling w/lath 11 psf
PEM (plumbing, electrical, mechanical) 10 psf (assumed)
Total Dead Load 34 psf
Roof Live Load 20 psf
Roof Snow Load Is= 1.0 Ce= 1.0 Ct= 1.0 Cs= 1.0
Ps > Is (20 psf) = (1.0)(20 psf) = 20 psf
Total Roof Design load 54 psf
Interior Beam Tributary width is 5ft
Exterior Beam Tributary width is 2.5 psf
Total is 135 lb./ft.
Choose open web steel joints to act as the roof beams. For now, use only the top load values
in the values in the table and ignore the live load deflection load values.
Beams span = 24ft
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CEA – Unit 3 – Lesson 3.2 – Activity 3.2.2– Loads – Page 2
9. Revisit your Keystone Library Renovation 3D model and make appropriate changes to the roof
system and roof framing to reflect your choices. Tag the roof joists and create a roof framing
plan.
Conclusion
1. Why are dead load and live load considered separately? How do they affect the structure
differently?
They are two separate variables to consider.
2. What is the justification for requiring engineers to consider many load combinations when
designing a structure rather than just one or two?
3. How would the size of the roof decking and roof beams change if the spacing of the beams
was increased? Why?
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CEA – Unit 3 – Lesson 3.2 – Activity 3.2.2– Loads – Page 3
