- An analyst applies the method of joints to a truss under ideal assumptions and finds that at one joint, the equilibrium equations yield a unique solution where one member force is negative. What does the negative value indicate about this member?
Answer: The member is in compression when the assumed direction was tension in the free body diagram
When analyzing trusses using the method of joints, you assume a direction for each unknown member force (typically tension, pulling away from the joint) and then solve the equilibrium equations. The sign of your solution tells you whether your assumption was correct.
A negative force result means your initial assumption about the force direction was wrong. If you assumed tension (force pulling away from the joint), a negative answer indicates the member is actually in compression (pushing toward the joint). This is completely normal and expected in truss analysis - it's simply how the math communicates the actual force direction to you.
Let's examine why the other options are incorrect:
Option A is wrong because negative forces are perfectly valid in structural analysis. They're a mathematical tool that corrects your directional assumptions, not an error.
Option C misunderstands ideal truss behavior. Members in compression are essential to truss function and don't violate any assumptions. Ideal trusses can have both tension and compression members.
Option D suggests repeating the analysis with different assumptions, but this is unnecessary. The negative result already gives you the correct answer - you don't need to redo the calculation just because the sign wasn't what you initially assumed.
Study tip: When solving truss problems, always assume a direction for unknown forces (conventionally tension), then let the math correct you. Negative results aren't mistakes - they're the solution telling you the actual direction. Embrace the negative signs rather than fighting them.
- When applying ideal truss assumptions, an engineer models all joints as pins and assumes members are straight and prismatic. During analysis of a long compression member, what additional consideration becomes important that may conflict with ideal assumptions?
Answer: The member's potential for elastic instability (buckling) which depends on length and end conditions
When analyzing trusses, you start with ideal assumptions: pin joints, straight prismatic members, and axial forces only. However, real structural behavior introduces complications that can override these simplifications, especially for long compression members.
The critical issue with long compression members is elastic instability, commonly known as buckling. Unlike material failure where stress reaches the yield strength, buckling occurs when a slender member under compression suddenly deflects laterally at a load much lower than its material capacity. This buckling load depends heavily on the member's length, cross-sectional properties, and end conditions (pinned, fixed, etc.), following Euler's buckling formula: Pcr=(KL)2π2EI, where the effective length KL varies based on end restraints.
This phenomenon directly conflicts with ideal truss assumptions because buckling involves lateral deflection and bending moments - violating the "axial forces only" assumption. For short, stocky members, you can safely ignore buckling, but as length increases, it becomes the governing failure mode.
Option A describes lateral-torsional buckling under combined loading, but ideal trusses have axial forces only, not combined loading scenarios. Option C mentions plastic hinges, which relate to ultimate strength design and material yielding - not a geometric instability issue. Option D addresses fatigue from repeated loading, which is a material deterioration concern unrelated to the geometric instability that contradicts truss assumptions.
Study tip: Remember that buckling is fundamentally different from strength failure - it's a stability problem that depends on geometry (length, shape) rather than just material properties, making it the key exception to simple axial-force truss analysis. - A truss designer applies ideal assumptions and finds that a particular member alternates between tension and compression as different load cases are considered. The client questions whether this member should be designed for tension only, compression only, or both. What guidance should be provided?
Answer: Design for both tension and compression capacities since both conditions occur under ideal analysis
When analyzing truss members under ideal assumptions, you're dealing with axial forces that can vary significantly depending on loading conditions. The key principle here is that real structures must handle all possible loading scenarios they'll encounter during their service life.
Under ideal truss analysis, members experience pure axial forces (tension or compression) with no bending moments. When different load cases produce different force directions in the same member, this isn't a design error—it's a realistic reflection of how structures respond to varying loads like live loads, wind, seismic forces, or different load combinations.
Answer C is correct because the member must be designed to safely carry both tension and compression forces. Each loading condition represents a legitimate scenario the structure will face, so the member needs adequate capacity for both tension strength and compression stability (including buckling considerations).
Answer A is wrong because alternating between tension and compression doesn't violate ideal truss behavior—it demonstrates proper analysis of multiple load cases. Answer B incorrectly suggests designing only for compression; while compression often governs due to buckling, you can't ignore tension capacity entirely if tension forces occur. Answer D is flawed because structural design must account for all critical load cases, not just the most frequent ones—rare but severe loading conditions often control design.
Remember this fundamental design principle: structures must be safe under all anticipated loading conditions, not just typical ones. When your analysis shows different force types in a member, design for the most demanding requirements of each type.
- A truss is designed with pinned connections at all joints, but during construction, some pins become loose due to wear, creating gaps that allow slight rotation before load transfer occurs. When analyzing this truss using ideal assumptions, the calculated member forces will most likely:
Answer: Overestimate the actual forces because the gaps reduce load transfer efficiency between members
Loose pins with gaps create a situation where load transfer is delayed and less efficient than ideal pinned connections. This typically results in load redistribution where some members carry higher forces to compensate for the reduced effectiveness of loose connections. The ideal analysis assumes perfect load transfer, so it will overestimate what the loose connections can actually transmit. Choice A ignores the mechanical effects of gaps. Choice C suggests the wrong direction of error. Choice D is too absolute—small deviations don't invalidate the entire approach.
- An analyst applies ideal truss assumptions to a structure where members have significant self-weight compared to applied loads. The analysis shows several members in tension, but field observations indicate some of these members are actually in compression. What most likely explains this discrepancy?
Answer: The ideal assumption of negligible member weight is inappropriate for this structure
When analyzing trusses, you must carefully evaluate whether the ideal truss assumptions are valid for your specific structure. These assumptions include pin connections, loads applied only at joints, and negligible member self-weight. When field observations contradict your analysis, the most likely culprit is a violated assumption.
The correct answer is B because when member self-weight is significant compared to applied loads, neglecting it fundamentally changes the internal force distribution. Self-weight creates distributed loads along each member, which can reverse the direction of forces from what you'd calculate ignoring weight. A member that appears to be in tension under applied loads alone might actually be in compression when its own weight is properly considered. This is especially common in long-span or heavy timber trusses where member weight rivals the external loading.
Option A is wrong because members can absolutely change from tension to compression when conditions change - there's no physical law preventing this. Option C incorrectly identifies the issue as connection stiffness, but rotational restraint typically causes secondary moments rather than complete force reversals. Option D suggests frame behavior, but the question specifically states the structure is being analyzed as a truss and the discrepancy involves force directions, not unexpected moments.
Remember this pattern: when your truss analysis doesn't match reality, first question whether all assumptions are valid. Self-weight is the most commonly overlooked factor, especially in structures with heavy or long members relative to the applied loads.
- A structural engineer is analyzing a truss where members are connected using welded gusset plates that create rigid joints capable of transmitting moments. Additionally, some members have distributed loads from attached floor systems. If the engineer chooses to apply ideal truss assumptions during the analysis, what is the most significant consequence?
Answer: The analysis will underestimate member forces because distributed loads and moment transmission are ignored in the idealization
When ideal truss assumptions are applied to a structure that violates these assumptions (rigid joints and distributed loads), the analysis typically underestimates member forces. Real rigid joints create additional load paths through moment transfer, and distributed loads create bending effects that aren't captured in pin-connected, axially-loaded member assumptions. Choice A incorrectly suggests overestimation. Choice C is wrong because ideal assumptions aren't universally valid. Choice D is incorrect because neglecting these effects usually leads to non-conservative (lower) force estimates.
- A truss analysis under ideal assumptions shows that a particular member has a force exactly equal to zero under dead load, but develops significant force under live load. During construction sequencing, this member will be installed after dead loads are applied but before live loads. How does this affect the member design?
Answer: The member must be designed for live load forces and installed before live loads are applied
When analyzing truss construction sequences, you need to consider both structural behavior and practical construction requirements. The key insight is that members must be physically present to carry loads, regardless of when those loads are applied.
Even though this member carries zero force under dead load in the theoretical analysis, it will experience significant forces once live loads are applied. Since the construction sequence has the member installed after dead loads but before live loads, it will be in place when needed to resist those live load forces. The member must therefore be designed for the full live load forces it will experience, making option B correct.
Option A is dangerously wrong because it suggests omitting a structurally necessary member. Just because a member has zero force under one load case doesn't mean it's unnecessary—it's critical for resisting live loads, and omitting it would lead to structural failure.
Option C misunderstands truss analysis principles. The construction sequence described doesn't violate ideal truss assumptions. These assumptions (pinned joints, loads at joints, etc.) relate to how forces are calculated, not when members are installed.
Option D incorrectly suggests that construction timing affects load superposition. The principle of superposition remains valid regardless of construction sequence—the total force is still the sum of dead load force (zero) plus live load force, giving the same result as simultaneous analysis.
Remember: construction sequence affects when members must be in place, but doesn't change the forces they must resist. Always ensure critical load-carrying members are installed before the loads they're designed to carry are applied.
- An engineer applies ideal truss assumptions to a space truss (3D) and finds that the method of joints requires solving six equilibrium equations at certain joints. A colleague suggests this violates ideal assumptions because planar trusses only require two equations per joint. How should this situation be resolved?
Answer: Three force equilibrium equations (ΣFx = ΣFy = ΣFz = 0) are appropriate for space truss joints under ideal assumptions
When analyzing space trusses under ideal assumptions, you need to understand how equilibrium principles extend from 2D to 3D analysis. Ideal truss assumptions still apply in three dimensions: members carry only axial forces (tension or compression), joints are pinned connections, and loads are applied only at joints.
The key difference between planar and space trusses is the number of force components that must be balanced. In 3D space, forces can act in three directions (x, y, and z), so equilibrium requires that the sum of forces in each direction equals zero: ∑Fx=0, ∑Fy=0, and ∑Fz=0. This gives you three equilibrium equations per joint, not six.
Option A is incorrect because space trusses don't violate ideal assumptions—they simply extend them to three dimensions. Frame analysis methods are unnecessary when ideal truss conditions are met.
Option B is wrong because decomposing a 3D structure into planar components would ignore the three-dimensional force interactions that make the structure stable and would not represent the actual structural behavior.
Option C incorrectly suggests using moment equilibrium equations. Under ideal truss assumptions, joints are pinned connections that cannot transfer moments, so moment equilibrium equations are not needed at joints—only force equilibrium matters.
Remember this pattern: the number of equilibrium equations equals the number of spatial dimensions. Planar trusses need two force equations (x and y), while space trusses need three force equations (x, y, and z). Moments don't come into play at ideal truss joints. - An engineering student applies ideal truss assumptions to analyze a roof truss but finds that the calculated deflections are significantly smaller than field measurements. Which assumption is most likely causing this discrepancy?
Answer: The assumption that joints behave as perfect pin connections with no rotational stiffness
When analyzing truss behavior, understanding the gap between theoretical predictions and real-world performance is crucial for engineering judgment. Ideal truss theory relies on several simplifying assumptions that can significantly affect deflection calculations.
The correct answer is C because joint stiffness has the most dramatic impact on structural deflections. In ideal truss analysis, you assume joints are perfect pins that allow free rotation with zero moment resistance. However, real connections—whether bolted, welded, or riveted—always provide some rotational stiffness. This partial fixity reduces member rotations and significantly decreases overall deflections. When your calculations predict deflections much smaller than field measurements, it typically indicates that real joints are behaving more like pins than your analysis assumed, allowing more rotation and flexibility than theoretical perfect pins would.
Option A is incorrect because member self-weight, while it adds load, rarely causes the magnitude of discrepancy described. The effect is usually predictable and proportional.
Option B represents a fundamental assumption of truss theory that generally holds well in practice for properly designed trusses with axially loaded members.
Option D relates to beam theory (Bernoulli-Euler assumptions) rather than truss analysis, and violations of this assumption typically affect stress distribution more than overall deflections.
Remember this pattern: when field deflections exceed calculated values in truss analysis, suspect that real joints are more flexible than assumed. Conversely, when calculated deflections exceed measurements, real joints likely provide more stiffness than perfect pin assumptions predict.
- Under ideal truss assumptions, a zero-force member is identified in a structure. Subsequently, an additional load is applied to the same truss at a different location. What can be concluded about this member's behavior under the new loading?
Answer: The member may carry a non-zero force depending on how the new loading affects force distribution
When analyzing zero-force members in trusses, you need to understand that these members are identified based on specific loading conditions and joint equilibrium. A zero-force member exists when the geometry and current loads create a situation where no internal force is needed in that member to maintain equilibrium.
The correct answer is C because zero-force members are load-dependent, not permanent structural characteristics. When you apply a new load at a different location, the entire force distribution throughout the truss changes. The member that was previously carrying zero force may now be required to carry load to maintain equilibrium under the new loading pattern. Whether this happens depends on how the new load affects the force paths through the structure.
Option A is wrong because geometric position alone doesn't determine force state – the loading pattern is equally important. A member can be geometrically capable of carrying load but simply not required to under specific loading conditions. Option B incorrectly assumes all members must contribute under multiple loads, but some members may still carry zero force even with additional loading, depending on the load locations and magnitudes. Option D treats zero-force members as if they're structurally irrelevant, but they're actually important backup elements that can activate when loading changes.
Remember this key principle: zero-force members are identified for specific loading conditions. When loads change, you must re-analyze the entire truss. Don't assume previous zero-force members will remain inactive – always check how new loads affect the complete force distribution.
- In ideal truss analysis, members are assumed to be connected only at their ends. A student argues that this assumption is violated when analyzing a truss where some members are continuous through joints and connect to multiple other members. How should this situation be addressed?
Answer: The assumption remains valid if forces are considered to act at joint centers regardless of physical continuity
When analyzing trusses, you need to understand that the fundamental assumptions of truss theory are conceptual tools, not strict physical requirements. The key principle is that all forces at a joint must be in equilibrium, and members carry only axial forces (tension or compression).
The correct approach is D because truss analysis works by treating joints as points where forces converge, regardless of the physical configuration of the members. Whether a member is continuous through a joint or stops and starts at the joint doesn't change the force equilibrium at that point. The analysis considers the internal forces acting at the joint center, making the physical continuity irrelevant to the mathematical model.
A is wrong because continuity doesn't invalidate truss analysis. Frame analysis is only needed when members carry bending moments, which doesn't occur in ideal trusses regardless of physical continuity.
B is incorrect because artificially breaking continuous members into segments creates unnecessary complexity without changing the underlying physics. The forces in a continuous member are the same whether you model it as one piece or multiple segments.
C is wrong because physical continuity doesn't create additional unknown forces or indeterminate conditions. The same equilibrium equations apply, and the degree of static determinacy depends on the overall geometry and support conditions, not member continuity.
Study tip: Remember that truss assumptions are about force behavior, not physical construction details. Focus on joint equilibrium and axial forces—the rest is just structural detailing that doesn't affect your analysis.
- In applying the method of sections to analyze a truss under ideal assumptions, an engineer cuts through four members and attempts to solve for their forces using three equilibrium equations. What does this situation indicate?
Answer: The cutting plane should be repositioned to intersect exactly three members for solvability
The method of sections is a powerful tool for analyzing trusses, but it has a fundamental limitation: you can only solve for as many unknowns as you have independent equilibrium equations available. For a single section cut through a truss, you have exactly three equilibrium equations: ∑Fx=0, ∑Fy=0, and ∑M=0.
When you cut through four members, you create four unknown forces but still only have three equilibrium equations. This creates an indeterminate system where you have more unknowns than equations - a classic case of "too many unknowns, too few equations." The solution is straightforward: reposition your cutting plane to intersect exactly three members, giving you three unknowns and three equations for a solvable system.
Choice A is incorrect because the method of sections is still applicable - you just need to choose your cut more carefully. The method of joints isn't required. Choice B confuses this situation with structural indeterminacy. The truss itself may be statically determinate, but your section cut has created a locally indeterminate problem. Choice D reflects a common misconception. While you can write moment equations about different points, these aren't independent equations - they're just different forms of the same equilibrium requirements and won't provide additional information to solve for the fourth unknown.
Study tip: Before applying the method of sections, always count the members your cutting plane intersects. If it's more than three, reposition the cut. This simple check will save you from unsolvable systems. - An ideal truss analysis assumes members are straight and undergo only axial deformation. In a real truss, members may have initial curvature from manufacturing or installation. If a slightly curved member is analyzed as straight, the calculated force will:
Answer: Depend on whether the member is in tension or compression, with different effects for each
Initial curvature affects tension and compression members differently. In tension, the member tends to straighten under load, and axial forces dominate. In compression, initial curvature can lead to amplified bending effects and potential buckling at lower loads than a straight member. The ideal analysis doesn't capture these different behaviors. Choices A, B, and C assume the same effect regardless of force type, missing this crucial distinction.
- A truss member is found to be in compression under one loading condition and in tension under a different loading condition. Based on ideal truss assumptions, which statement best explains this behavior?
Answer: This is consistent with ideal truss assumptions because members are axial force-only elements that respond to different loading patterns
When analyzing truss behavior, you need to understand that ideal trusses are designed to carry only axial forces (tension or compression) along their centerlines. The key insight is that these forces depend entirely on the applied loading conditions.
Under ideal truss assumptions, each member responds purely to the force equilibrium at the joints. When loads change, the internal force distribution throughout the entire truss changes accordingly. A member that experiences compression under one loading pattern can absolutely experience tension under a different loading pattern - this is exactly what we expect from a properly functioning truss system.
Looking at the wrong answers: A is incorrect because truss members routinely carry both tension and compression forces depending on the loading. There's no restriction to tension-only forces. B misunderstands truss behavior - internal forces are never constant across different loading conditions; they must change as the external loads change to maintain equilibrium. D incorrectly suggests that changing from compression to tension indicates bending moments, but this force reversal actually confirms pure axial behavior responding to different load cases.
The correct answer is C because this behavior perfectly exemplifies ideal truss action: members carrying only axial forces that vary appropriately with different loading patterns.
Study tip: Remember that truss analysis is all about equilibrium under specific loading conditions. When you change the loads, you expect the internal forces to change too - including potential reversals from tension to compression or vice versa. This adaptability is a feature, not a flaw, of ideal trusses.
- A roof truss supports uniformly distributed snow loads along its top chord. The engineer models this by applying equivalent concentrated loads at the panel points. If the actual distributed load creates a maximum bending moment of M=8wL2 in the top chord between panel points, what is the primary limitation of the idealized analysis?
Answer: The idealized analysis ignores local bending stresses that must be superimposed on axial stresses
When analyzing roof trusses, engineers face a key decision: model the distributed snow load as equivalent concentrated forces at panel points (the idealized approach) or account for the actual distributed loading. This question tests your understanding of what gets lost in the idealization process.
The correct answer is A because when you replace a distributed load with equivalent concentrated loads, you eliminate the local bending moments that occur between panel points. The given maximum moment M=8wL2 represents real bending stress in the top chord members between joints. In the idealized truss analysis, these members are assumed to carry only axial forces (tension or compression), completely ignoring this local bending. In reality, the top chord experiences both axial compression from the overall truss action AND bending from the distributed load between panel points.
Answer B is incorrect because idealized truss analysis absolutely can and does account for compression forces in top chord members - that's a primary output of the analysis. Answer C gets the deflection relationship backwards; idealized analysis typically underestimates deflections because it ignores member flexibility and local deformations. Answer D is wrong because distributed loads don't violate truss analysis principles - engineers routinely convert them to equivalent joint loads using statics principles.
The key insight for truss problems: idealized analysis gives you the "big picture" forces but misses local effects. Always remember that real truss members experience both the calculated axial forces AND local bending from loads applied between joints - both must be considered in design. - A truss analysis under ideal assumptions yields a member force of exactly zero. However, the structural drawings show this member has the same cross-section as other load-carrying members. From a design perspective, what does this suggest?
Answer: The member serves as a stabilizing element for other loading conditions or construction phases
When analyzing trusses, you'll often encounter members that show zero force under your specific loading conditions. This doesn't mean these members are useless - it reveals an important distinction between analysis and real-world design considerations.
Answer B is correct because structural design must account for multiple scenarios beyond your single analysis case. A member showing zero force under one loading condition likely serves critical functions during construction phases (before the structure is complete), wind loads from different directions, live load patterns you didn't analyze, or provides lateral stability to prevent buckling of other members. Designers intentionally include these "redundant" members for structural robustness and safety.
Answer A is wrong because removing zero-force members would compromise structural integrity under other loading conditions and violate building codes requiring redundancy.
Answer C reflects a misconception - zero-force members are perfectly normal in truss analysis. Many members will show zero force under specific loading patterns, and this indicates correct analysis, not errors.
Answer D misunderstands redundancy - while this member does provide redundancy, this doesn't violate ideal truss assumptions. Ideal assumptions (pinned joints, axial loads only) are analysis tools, not design restrictions. Real structures intentionally incorporate redundancy for safety.
Key takeaway: When you encounter zero-force members in analysis, think beyond the single loading case. Real structures must handle multiple scenarios, so apparent "extra" members usually serve important functions you haven't analyzed yet. Always consider the bigger design picture, not just your specific analysis results.
- A truss analysis under ideal assumptions shows that forces at a particular joint satisfy equilibrium exactly. However, when the same joint is analyzed considering actual connection details with finite-size gusset plates, small additional forces appear. What principle explains this difference?
Answer: Real connections have eccentricities and finite dimensions that create secondary force effects
When analyzing trusses, you're working with two different levels of idealization that can produce different results even when both analyses are correct.
In ideal truss analysis, we assume members are connected by frictionless pins at mathematical points, with forces acting along member centerlines. This creates a statically determinate system where joint equilibrium gives exact force solutions. However, real truss connections use gusset plates with finite dimensions, and member centerlines rarely intersect at perfect points.
The correct answer is B because these geometric realities create eccentricities - small distances between where forces actually act and where we assumed they act in our ideal analysis. When forces don't act through the assumed joint center, they create additional moments that must be balanced by secondary forces. These aren't errors; they're legitimate structural effects that our simplified model couldn't capture.
Option A is incorrect because while gusset plates can introduce some bending, the primary issue is geometric eccentricity, not a fundamental violation of the pin assumption. Option C misidentifies the cause - the additional forces aren't due to stiffness changes altering the load path, but rather geometric effects at the connections themselves. Option D wrongly suggests the original analysis contained errors, when in fact both analyses are valid for their respective assumptions.
Remember this pattern: when "ideal" and "real" structural analyses differ, look for geometric effects like eccentricities or finite member sizes as the explanation. The math isn't wrong - the modeling assumptions are simply different levels of approximation.
- A truss designer must decide whether ideal truss assumptions are appropriate for a structure where joint rigidity varies significantly throughout the structure. Some joints are pinned, others are semi-rigid, and some are fully rigid. The most appropriate approach is to:
Answer: Avoid ideal truss analysis because mixed joint conditions create unpredictable force distributions
Mixed joint rigidity conditions create complex and unpredictable load redistribution patterns that can't be reliably approximated by uniform ideal assumptions. The force distribution becomes highly dependent on the specific locations and degrees of joint rigidity, making simplified analysis unreliable. Choice A incorrectly suggests averaging effects. Choice B provides an arbitrary threshold without technical basis. Choice D understates the complexity—even preliminary analysis may be misleading with mixed conditions.
- Under ideal truss assumptions, an analysis shows that three members meeting at an interior joint have forces of 50 kN, 75 kN, and 100 kN. Without knowing the member orientations, what can be definitively concluded about this joint?
Answer: The joint equilibrium cannot be verified without knowing the angular orientations of the members
When analyzing joints in trusses, you need to apply equilibrium equations: the sum of forces in both x and y directions must equal zero. This means ∑Fx=0 and ∑Fy=0 must both be satisfied simultaneously.
The key insight here is that force equilibrium depends entirely on both the magnitudes AND directions of the forces. Given only three force magnitudes (50 kN, 75 kN, and 100 kN), you cannot determine whether equilibrium is satisfied without knowing how these forces are oriented in space. The vector sum depends on the angles between the members.
Answer C is correct because the angular orientations of the members are essential information. For example, if the three forces were arranged at 120° angles from each other with appropriate magnitudes, they could sum to zero. Alternatively, if they were nearly parallel, they definitely would not balance.
Answer A incorrectly assumes that having three members automatically ensures equilibrium. While three members can provide static determinacy for a joint, this doesn't guarantee the forces satisfy equilibrium - that depends on the specific geometry and loading.
Answer B makes the opposite error, assuming these magnitudes cannot possibly sum to zero vectorially. With proper angular arrangements, various force combinations can indeed balance.
Answer D incorrectly suggests you can determine tension/compression states and relative relationships without knowing member orientations. The direction of forces (tension vs. compression) and equilibrium requirements both depend on geometry.
Remember: In truss analysis, both force magnitudes and member orientations are always required to verify joint equilibrium. Never attempt equilibrium checks with incomplete geometric information. - A bridge truss is analyzed using ideal assumptions, but during construction, thermal effects cause differential expansion between the top and bottom chords. This creates internal stresses before any live loads are applied. When live loads are subsequently applied, the member forces will:
Answer: Be different from ideal analysis because thermal effects alter the initial geometry and stress state
Thermal differential expansion creates initial stresses and potentially changes the geometry of the structure before live loads are applied. This altered initial state affects how subsequently applied loads are distributed through the structure. The ideal analysis assumes no initial stresses or geometric changes. Choice A ignores the interaction between thermal and mechanical effects. Choice B oversimplifies by assuming direct addition. Choice D incorrectly assumes thermal effects are always beneficial.
