Wind, Seismic & Environmental Loads · Study · PE Civil: Structural · FE → PE Prep
Wind, Seismic & Environmental Loads
8% of exam
Wind pressures and forces, seismic base shear and vertical distribution of lateral force, snow, rain, and ice loads, impact and moving (vehicular and crane) loads, and their delivery to the lateral system.
5 concepts
D. Wind loads
Wind Velocity Pressure & MWFRS
The ASCE 7-16 velocity pressure qz = 0.00256·Kz·Kzt·Kd·V², the MWFRS design pressure p = q·G·Cp, exposure categories, and the directional procedure for enclosed buildings.
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Wind Loads on Components & Cladding
The ASCE 7-16 C&C design pressure p = qh[(GCp) − (GCpi)], effective wind area, the wall/roof pressure zones, and why local C&C pressures exceed the MWFRS.
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E. Seismic loads
Seismic Base Shear (Equivalent Lateral Force)
The ASCE 7-16 equivalent lateral force base shear V = Cs·W, the seismic response coefficient and its caps, the approximate period, and how SDS, SD1, R and Ie set the demand.
Seismic design on the PE Civil exam almost always starts at the same place: the total lateral force a building must be able to develop at its base when the ground shakes. The Equivalent Lateral Force (ELF) procedure boils the entire dynamic problem down to one static force, the base shear V=CsW, that you then push up the structure. It looks like one multiplication, but the points live in the details — picking the right seismic response coefficient Cs, knowing which of its three caps governs, estimating the period correctly, and using the supplied seismic parameters without mixing up SDS and SD1. This is a NARRATIVE topic: none of these relations appear in the NCEES PE Civil Reference Handbook (Ch 4 Structural carries no ASCE 7 load equations). Teach and cite the governing code, ASCE/SEI 7-16 §12.8 Equivalent Lateral Force Procedure — not a handbook page.
The base shear and the effective seismic weight
The base shear is the seismic response coefficient times the effective seismic weight, V=CsW. The weight W is the total dead load plus the prescribed portions of other loads that ride with the structure during an earthquake — ASCE 7-16 §12.7.2 adds storage live load, a partition allowance (at least 10psf where partitions are present), permanent equipment, and a fraction of flat-roof snow where pg
The seismic response coefficient
The seismic response coefficient is, in its base form, the design spectral acceleration reduced by the response modification factor and scaled by importance, Cs=SDS/(R/Ie)
The upper-limit cap at long period
Real buildings are not all short-period. As the fundamental period T lengthens, the design spectrum falls off as SD1/T, so ASCE 7-16 caps Cs by the velocity-region value Cs=SD1/[T(R/Ie)]
The lower-limit floor
There is also a floor: Cs may not drop below 0.044SDSIe≥0.01
The approximate fundamental period
The cap depends on the period, and the exam expects the approximate period from building height, Ta=Cthnx, with hn
The design spectral accelerations
The parameters SDS and SD1 are themselves derived: from the mapped SS
Exam strategy
Work it as a short checklist. First get the period Ta=Cthnx with the right framing coefficients. Then compute three candidate Cs
Key equations
Seismic base shearV=CsW
V = total design base shear (kip), Cs
Worked examples
Base shear for a steel moment-frame building
Problem. A 9-story special steel moment frame has hn=120ft and an effective seismic weight W=12,000kip. The site gives SDS=1.0g
Common pitfalls
•Citing the NCEES PE Civil Handbook (§4 Structural) for the base-shear method — it contains NO ASCE 7 seismic equations. The governing source is ASCE/SEI 7-16 §12.8; cite it and the edition.
•Reporting the base value SDS/(R/Ie) for a flexible building and skipping the long-period cap SD1/[T(R/Ie)]
References
ASCE/SEI 7-16, Minimum Design Loads and Associated Criteria for Buildings and Other Structures — §12.8 Equivalent Lateral Force Procedure
Vertical distribution Fx = Cvx·V with the period-dependent k-exponent, story shears, amplified story drift against the allowable, and accidental/torsional eccentricity.
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G. Snow, rain, and ice
Snow, Rain & Ice Loads
The ASCE 7-16 flat-roof snow pf = 0.7·Ce·Ct·Is·pg, sloped-roof and drift/unbalanced snow, rain ponding load, and atmospheric ice on the exposure factors.
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is large. Note what is excluded: ordinary floor live load and roof live load do not count, because they are not reliably present when the ground moves. Getting
W
right is half the problem; a base shear is only as good as the mass it is built on.
V=CsW
. The numerator
SDS
is the design short-period spectral acceleration (the plateau of the design spectrum). The response modification coefficient
R
rewards ductility — a special moment frame (
R=8
) is allowed far less force than an ordinary cantilever column (
R=1.25
) because it can yield and dissipate energy. The seismic importance factor
Ie
(1.0, 1.25, or 1.5 by Risk Category) raises the demand for buildings that must survive. This base value applies in the constant-acceleration (short-period) region of the spectrum.
Cs=R/IeSDS
for
T≤TL
(and by
SD1TL/[T2(R/Ie)]
for
T>TL
, the long-period corner). This upper limit is what governs for most moment-frame and taller buildings — the base equation
SDS/(R/Ie)
is a ceiling that only controls for stiff, short-period structures. If you blindly use
SDS/(R/Ie)
for a flexible building you will badly over-state the force; always compute the period-dependent cap and take the smaller.
the height above base in feet. The coefficients depend on the framing: steel moment frames use
Ct=0.028,x=0.8
; concrete moment frames
Ct=0.016,x=0.9
; eccentrically braced and buckling-restrained frames
Ct=0.03,x=0.75
; all other systems
Ct=0.02,x=0.75
. If a rational (modal) analysis gives a computed period
T
, ASCE 7-16 lets you use it only up to
CuTa
— you may not take credit for an arbitrarily long, low-force period. Absent an analysis, use
Ta
directly.
Ta=Cthnx(hnin ft)
and
S1
, apply site coefficients
Fa
and
Fv
for the site class to get
SMS=FaSS
and
SM1=FvS1
(the maximum-considered-earthquake values), then take two-thirds:
SDS=32SMS
and
SD1=32SM1
. The two-thirds factor converts the MCE (a rare, large event) to the design basis. The exam usually supplies
SDS
and
SD1
directly — but if it hands you
SS
,
S1
, and a site class, this is the chain you walk down first.
SDS=32FaSS,SD1=32FvS1
values: the base
SDS/(R/Ie)
, the cap
SD1/[T(R/Ie)]
(use the
T>TL
form only if told
TL
is exceeded), and the floor
0.044SDSIe
. Take the base value but no greater than the cap and no less than the floor. Multiply by
W
— and confirm
W
excludes ordinary live load. The single most common error is forgetting the long-period cap and reporting the (too large) base value for a flexible building; the second is mixing
SDS
(no
T
) with
SD1
(divided by
T
). Always cite ASCE 7-16 §12.8, never a handbook section.
= seismic response coefficient (dimensionless),
W
= effective seismic weight (kip) per ASCE 7-16 §12.7.2.
A rationally computed period may be used for the cap only up to CuTa (Cu from ASCE 7-16 Table 12.8-1, ~1.4–1.7).
,
SD1=0.6g
, and
S1=0.4g
. Assume
T≤TL
(the mapped long-period transition
TL
exceeds the building period). Use
R=8
,
Ie=1.0
. Find the design base shear by the ELF procedure.
Solution. Period (steel MF, Ct=0.028, x=0.8): Ta=0.028(120)0.8=0.028(46.08)=1.29s.
Base Cs=R/IeSDS=8/1.01.0=0.125.
Upper-limit cap (T≤TL): Cs,max=T(R/Ie)SD1=1.29(8)0.6=10.320.6=0.0582.
Lower limit: 0.044SDSIe=0.044(1.0)(1.0)=0.044. (And S1=0.4<0.6, so the near-fault floor does not apply.)
Governing: the cap 0.0582 is below the base 0.125 and above the floor 0.044, so Cs=0.0582.
V=CsW=0.0582(12,000)=698kip.
**Answer: V≈698kip (using Cs=0.0582).** Sanity check: a 9-story frame is flexible, so the long-period cap should govern over the short-period base value — it does, cutting the coefficient from 0.125 to 0.0582. Reporting 0.125×12,000=1,500kip would be the classic over-statement from ignoring the cap.
Problem. A 2-story bearing-wall building ('other' system, R=4) has hn=24ft, W=3,200kip, SDS=0.9g, SD1=0.35g, Ie=1.25 (Risk Category III). Find Cs and V.
Solution. Period ('other', Ct=0.02, x=0.75): Ta=0.02(24)0.75=0.02(10.84)=0.217s
Problem. A tall, flexible special steel moment frame in a low-to-moderate seismic region has hn=200ft and W=18,000kip. The site gives SDS=0.5g, SD1=0.18g, and S1=0.12g (T≤TL). Use R=8, Ie=1.0. Show that the minimum Cs floor governs, and find V.
Solution. Period (steel MF, Ct=0.028, x=0.8): Ta=0.028(200)0.8=0.028(69.31)=1.94s
. For most moment frames the cap governs and gives a much smaller (correct)
Cs
.
•Confusing SDS and SD1: SDS (short-period) appears alone; SD1 (one-second) is always divided by the period T. Swapping them changes the answer by a factor of T.
•Forgetting the lower-limit floor 0.044SDSIe (and the S1≥0.6g near-fault floor). A Cs below the floor is not a smaller safe answer — it is simply non-compliant.
•Using the wrong Ct, x for the framing type, or putting hn in the wrong units. Ta=Cthnx takes hn in FEET; mismatching the framing coefficients (e.g., steel MF vs. 'other') shifts the period and the governing cap.
•Citing the wrong code edition — ASCE 7-22 or the 2015 NEHRP — when NCEES supplies ASCE 7-16. The Cs caps and period coefficients are edition-specific.
•Including ordinary floor or roof live load in the effective seismic weight W. ASCE 7-16 §12.7.2 includes dead load, storage/partition allowances, permanent equipment, and heavy snow — but not transient live load.
).** Sanity check: for a stiff, short-period building the cap does NOT govern — exactly the opposite of the tall-frame case — so here the short-period base equation controls, which is the correct behavior. Note