Structural Loads Define the Forces the Building Must Resist - Understanding Them Is Fundamental to Structural System Design
Every building is subject to multiple types of forces - gravity forces (dead and live loads) acting downward, and lateral forces (wind and seismic) acting horizontally. The structural system is designed to resist all of these forces, both individually and in combinations that represent realistic scenarios of simultaneous loading. While the structural engineer is responsible for load calculation and structural design, the architect must understand load types, their sources, and their order of magnitude to participate meaningfully in structural system selection, to understand why the structural engineer specifies certain member sizes and connection types, and to recognize when architectural program changes will materially affect structural loads. The ARE PPD tests structural load fundamentals as part of the building systems and structural engineering coordination content area.
Dead Loads
Dead loads are the permanent, static weights of building materials and permanent equipment - the structural frame, floor systems, roofing, facade cladding, mechanical equipment, permanent partitions. Dead loads are constant and predictable. Representative values: concrete slab (6 inches): 75 psf; steel roof deck + roofing system: 15-25 psf; curtain wall: 10-20 psf; typical commercial floor (structure + finishes): 80-120 psf total. Dead loads are determined by the actual weights of specified materials - changes in floor or roof assembly specifications change the dead load and may require structural redesign.
Live Loads
Live loads are the variable, movable loads from building occupancy - people, furniture, equipment, stored materials. Live loads are tabulated in ASCE 7 Table 4.3-1 by occupancy type: Office floors: 50 psf; Assembly areas: 100 psf; Retail: 100 psf at floors, 125 psf at heavy storage; Residential: 40 psf; Hospital patient rooms: 60 psf; Mechanical equipment rooms: 150 psf. Live loads can be reduced on larger tributary areas (live load reduction) because the probability that the entire tributary area is simultaneously loaded to the maximum is low for larger areas.
Snow, Wind, and Seismic Loads
Snow loads: Determined from ground snow loads in ASCE 7 maps, modified for roof slope, exposure, and thermal conditions. Roofs with slopes above 30 degrees see reduced snow loads; flat roofs in cold climates can accumulate significant snow. Wind loads: Determined from design wind speed maps in ASCE 7 by geographic location, risk category, and exposure category (how open or sheltered the site). Higher-risk buildings (risk category III and IV) use higher design wind speeds. Seismic loads: Based on mapped spectral acceleration values (Ss and S1 in ASCE 7) modified for site class (soil type) and risk category. Seismic design category (A through F) determines the level of seismic detailing required for the structural system.
Load Combinations
Structural elements are designed for the governing combination of loads per ASCE 7: 1.2D + 1.6L (dead + live - usually governs for floor design); 1.2D + 1.0L + 1.0W (dead + live + wind - lateral system design); 0.9D + 1.0W (when wind uplift could overcome gravity dead loads). Load combinations include safety factors (the 1.2, 1.6, 1.0 factors) that account for uncertainty in load magnitudes.
Key Exam Points
- Dead loads: permanent; structural materials, finishes, cladding, mechanical equipment.
- Live loads: variable; ASCE 7 Table 4.3-1 by occupancy; office 50 psf, assembly 100 psf, residential 40 psf.
- Snow loads: ASCE 7 ground snow load maps; modified for slope and exposure.
- Wind loads: ASCE 7 maps; higher for risk category III/IV buildings.
- Seismic loads: spectral acceleration maps; site class modification; seismic design category determines detailing requirements.
AREprep's PPD structural systems content covers all structural load types, ASCE 7 references, and how loads combine in design scenarios - giving ARE PPD candidates the structural loading knowledge the exam tests in building performance questions that require understanding which loads govern structural design for different building types, locations, and risk categories.
Study PPD on AREprep
AREprep's PPD flashcards and practice exams are built around the actual NCARB exam guide - so every study session targets what the exam tests.